From 9ffa3cd3e50747fadbf724bb3b3cff5649832a87 Mon Sep 17 00:00:00 2001
From: Yui T <yuisu@microsoft.com>
Date: Wed, 2 Dec 2015 13:42:43 -0800
Subject: [PATCH 1/4] Check grammar for let/const declaration for all targets

---
 src/compiler/checker.ts | 37 +++++++++++++++++++++++--------------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e737cd58bd5..1431a91ff43 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15973,18 +15973,27 @@ namespace ts {
                             : Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_of_statement;
                         return grammarErrorOnFirstToken(variableList.declarations[1], diagnostic);
                     }
-                    const firstDeclaration = variableList.declarations[0];
-                    if (firstDeclaration.initializer) {
-                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
-                            ? Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer
-                            : Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;
-                        return grammarErrorOnNode(firstDeclaration.name, diagnostic);
-                    }
-                    if (firstDeclaration.type) {
-                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
-                            ? Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_use_a_type_annotation
-                            : Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_use_a_type_annotation;
-                        return grammarErrorOnNode(firstDeclaration, diagnostic);
+                    const firstDeclaration = variableList.declarations[0]
+
+                    // firstDeclaration can be undefined if there is variable declaration in for-of or for-in
+                    // See http://www.ecma-international.org/ecma-262/6.0/#sec-for-in-and-for-of-statements for details
+                    // For example:
+                    //      var let = 10;
+                    //      for (let of [1,2,3]) {} // this is invalid ES6 syntax
+                    //      for (let in [1,2,3]) {} // this is invalid ES6 syntax
+                    if (firstDeclaration) {
+                        if (firstDeclaration.initializer) {
+                            const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
+                                ? Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer
+                                : Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;
+                            return grammarErrorOnNode(firstDeclaration.name, diagnostic);
+                        }
+                        if (firstDeclaration.type) {
+                            const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
+                                ? Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_use_a_type_annotation
+                                : Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_use_a_type_annotation;
+                            return grammarErrorOnNode(firstDeclaration, diagnostic);
+                        }
                     }
                 }
             }
@@ -16172,7 +16181,7 @@ namespace ts {
                 }
             }
 
-            const checkLetConstNames = languageVersion >= ScriptTarget.ES6 && (isLet(node) || isConst(node));
+            const checkLetConstNames = (isLet(node) || isConst(node));
 
             // 1. LexicalDeclaration : LetOrConst BindingList ;
             // It is a Syntax Error if the BoundNames of BindingList contains "let".
@@ -16186,7 +16195,7 @@ namespace ts {
 
         function checkGrammarNameInLetOrConstDeclarations(name: Identifier | BindingPattern): boolean {
             if (name.kind === SyntaxKind.Identifier) {
-                if ((<Identifier>name).text === "let") {
+                if ((<Identifier>name).originalKeywordKind === SyntaxKind.LetKeyword) {
                     return grammarErrorOnNode(name, Diagnostics.let_is_not_allowed_to_be_used_as_a_name_in_let_or_const_declarations);
                 }
             }

From 63ac3cda711541989e2648fe5c1e53046a8baaea Mon Sep 17 00:00:00 2001
From: Yui T <yuisu@microsoft.com>
Date: Wed, 2 Dec 2015 13:43:06 -0800
Subject: [PATCH 2/4] All tests for using let

---
 .../invalidLetInForOfAndForIn_ES5.errors.txt  | 22 +++++++++
 .../invalidLetInForOfAndForIn_ES5.js          | 18 +++++++
 .../invalidLetInForOfAndForIn_ES6.errors.txt  | 22 +++++++++
 .../invalidLetInForOfAndForIn_ES6.js          | 18 +++++++
 tests/baselines/reference/letAsIdentifier2.js |  6 +++
 .../reference/letAsIdentifier2.symbols        |  5 ++
 .../reference/letAsIdentifier2.types          |  5 ++
 .../letInConstDeclarations_ES5.errors.txt     | 16 +++++++
 .../reference/letInConstDeclarations_ES5.js   | 15 ++++++
 .../letInConstDeclarations_ES6.errors.txt     | 16 +++++++
 .../reference/letInConstDeclarations_ES6.js   | 15 ++++++
 ...LetConstDeclOfForOfAndForIn_ES5.errors.txt | 48 +++++++++++++++++++
 .../letInLetConstDeclOfForOfAndForIn_ES5.js   | 43 +++++++++++++++++
 ...LetConstDeclOfForOfAndForIn_ES6.errors.txt | 48 +++++++++++++++++++
 .../letInLetConstDeclOfForOfAndForIn_ES6.js   | 35 ++++++++++++++
 .../letInLetDeclarations_ES5.errors.txt       | 16 +++++++
 .../reference/letInLetDeclarations_ES5.js     | 15 ++++++
 .../letInLetDeclarations_ES6.errors.txt       | 16 +++++++
 .../reference/letInLetDeclarations_ES6.js     | 15 ++++++
 .../letInLetOrConstDeclarations.errors.txt    | 23 ---------
 .../reference/letInLetOrConstDeclarations.js  | 25 ----------
 .../reference/letInVarDeclOfForIn_ES5.js      | 16 +++++++
 .../reference/letInVarDeclOfForIn_ES5.symbols | 11 +++++
 .../reference/letInVarDeclOfForIn_ES5.types   | 19 ++++++++
 .../reference/letInVarDeclOfForIn_ES6.js      | 16 +++++++
 .../reference/letInVarDeclOfForIn_ES6.symbols | 11 +++++
 .../reference/letInVarDeclOfForIn_ES6.types   | 19 ++++++++
 .../reference/letInVarDeclOfForOf_ES5.js      | 20 ++++++++
 .../reference/letInVarDeclOfForOf_ES5.symbols | 11 +++++
 .../reference/letInVarDeclOfForOf_ES5.types   | 19 ++++++++
 .../reference/letInVarDeclOfForOf_ES6.js      | 16 +++++++
 .../reference/letInVarDeclOfForOf_ES6.symbols | 11 +++++
 .../reference/letInVarDeclOfForOf_ES6.types   | 19 ++++++++
 .../strictModeReservedWord.errors.txt         |  8 +++-
 .../compiler/invalidLetInForOfAndForIn_ES5.ts | 10 ++++
 .../compiler/invalidLetInForOfAndForIn_ES6.ts | 10 ++++
 tests/cases/compiler/letAsIdentifier2.ts      |  3 ++
 .../compiler/letInConstDeclarations_ES5.ts    |  8 ++++
 .../compiler/letInConstDeclarations_ES6.ts    |  8 ++++
 .../letInLetConstDeclOfForOfAndForIn_ES5.ts   | 21 ++++++++
 .../letInLetConstDeclOfForOfAndForIn_ES6.ts   | 21 ++++++++
 .../compiler/letInLetDeclarations_ES5.ts      |  8 ++++
 .../compiler/letInLetDeclarations_ES6.ts      |  8 ++++
 .../compiler/letInLetOrConstDeclarations.ts   | 12 -----
 .../cases/compiler/letInVarDeclOfForIn_ES5.ts |  8 ++++
 .../cases/compiler/letInVarDeclOfForIn_ES6.ts |  8 ++++
 .../cases/compiler/letInVarDeclOfForOf_ES5.ts |  8 ++++
 .../cases/compiler/letInVarDeclOfForOf_ES6.ts |  8 ++++
 48 files changed, 718 insertions(+), 61 deletions(-)
 create mode 100644 tests/baselines/reference/invalidLetInForOfAndForIn_ES5.errors.txt
 create mode 100644 tests/baselines/reference/invalidLetInForOfAndForIn_ES5.js
 create mode 100644 tests/baselines/reference/invalidLetInForOfAndForIn_ES6.errors.txt
 create mode 100644 tests/baselines/reference/invalidLetInForOfAndForIn_ES6.js
 create mode 100644 tests/baselines/reference/letAsIdentifier2.js
 create mode 100644 tests/baselines/reference/letAsIdentifier2.symbols
 create mode 100644 tests/baselines/reference/letAsIdentifier2.types
 create mode 100644 tests/baselines/reference/letInConstDeclarations_ES5.errors.txt
 create mode 100644 tests/baselines/reference/letInConstDeclarations_ES5.js
 create mode 100644 tests/baselines/reference/letInConstDeclarations_ES6.errors.txt
 create mode 100644 tests/baselines/reference/letInConstDeclarations_ES6.js
 create mode 100644 tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.errors.txt
 create mode 100644 tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.js
 create mode 100644 tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.errors.txt
 create mode 100644 tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.js
 create mode 100644 tests/baselines/reference/letInLetDeclarations_ES5.errors.txt
 create mode 100644 tests/baselines/reference/letInLetDeclarations_ES5.js
 create mode 100644 tests/baselines/reference/letInLetDeclarations_ES6.errors.txt
 create mode 100644 tests/baselines/reference/letInLetDeclarations_ES6.js
 delete mode 100644 tests/baselines/reference/letInLetOrConstDeclarations.errors.txt
 delete mode 100644 tests/baselines/reference/letInLetOrConstDeclarations.js
 create mode 100644 tests/baselines/reference/letInVarDeclOfForIn_ES5.js
 create mode 100644 tests/baselines/reference/letInVarDeclOfForIn_ES5.symbols
 create mode 100644 tests/baselines/reference/letInVarDeclOfForIn_ES5.types
 create mode 100644 tests/baselines/reference/letInVarDeclOfForIn_ES6.js
 create mode 100644 tests/baselines/reference/letInVarDeclOfForIn_ES6.symbols
 create mode 100644 tests/baselines/reference/letInVarDeclOfForIn_ES6.types
 create mode 100644 tests/baselines/reference/letInVarDeclOfForOf_ES5.js
 create mode 100644 tests/baselines/reference/letInVarDeclOfForOf_ES5.symbols
 create mode 100644 tests/baselines/reference/letInVarDeclOfForOf_ES5.types
 create mode 100644 tests/baselines/reference/letInVarDeclOfForOf_ES6.js
 create mode 100644 tests/baselines/reference/letInVarDeclOfForOf_ES6.symbols
 create mode 100644 tests/baselines/reference/letInVarDeclOfForOf_ES6.types
 create mode 100644 tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts
 create mode 100644 tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts
 create mode 100644 tests/cases/compiler/letAsIdentifier2.ts
 create mode 100644 tests/cases/compiler/letInConstDeclarations_ES5.ts
 create mode 100644 tests/cases/compiler/letInConstDeclarations_ES6.ts
 create mode 100644 tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts
 create mode 100644 tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts
 create mode 100644 tests/cases/compiler/letInLetDeclarations_ES5.ts
 create mode 100644 tests/cases/compiler/letInLetDeclarations_ES6.ts
 delete mode 100644 tests/cases/compiler/letInLetOrConstDeclarations.ts
 create mode 100644 tests/cases/compiler/letInVarDeclOfForIn_ES5.ts
 create mode 100644 tests/cases/compiler/letInVarDeclOfForIn_ES6.ts
 create mode 100644 tests/cases/compiler/letInVarDeclOfForOf_ES5.ts
 create mode 100644 tests/cases/compiler/letInVarDeclOfForOf_ES6.ts

diff --git a/tests/baselines/reference/invalidLetInForOfAndForIn_ES5.errors.txt b/tests/baselines/reference/invalidLetInForOfAndForIn_ES5.errors.txt
new file mode 100644
index 00000000000..c31b03bae4c
--- /dev/null
+++ b/tests/baselines/reference/invalidLetInForOfAndForIn_ES5.errors.txt
@@ -0,0 +1,22 @@
+tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts(5,13): error TS1005: '=' expected.
+tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts(5,20): error TS1005: ',' expected.
+tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts(7,1): error TS1005: ';' expected.
+
+
+==== tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts (3 errors) ====
+    // This should be an error
+    // More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+    
+    var let = 10;
+    for (let of [1,2,3]) {}
+                ~
+!!! error TS1005: '=' expected.
+                       ~
+!!! error TS1005: ',' expected.
+    
+    for (let in [1,2,3]) {}
+    ~~~
+!!! error TS1005: ';' expected.
+     
+    
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/invalidLetInForOfAndForIn_ES5.js b/tests/baselines/reference/invalidLetInForOfAndForIn_ES5.js
new file mode 100644
index 00000000000..729cb246a85
--- /dev/null
+++ b/tests/baselines/reference/invalidLetInForOfAndForIn_ES5.js
@@ -0,0 +1,18 @@
+//// [invalidLetInForOfAndForIn_ES5.ts]
+// This should be an error
+// More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+
+var let = 10;
+for (let of [1,2,3]) {}
+
+for (let in [1,2,3]) {}
+ 
+
+
+
+//// [invalidLetInForOfAndForIn_ES5.js]
+// This should be an error
+// More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+var let = 10;
+for (let of = [1, 2, 3], {  }; ; )
+    for ( in [1, 2, 3]) { }
diff --git a/tests/baselines/reference/invalidLetInForOfAndForIn_ES6.errors.txt b/tests/baselines/reference/invalidLetInForOfAndForIn_ES6.errors.txt
new file mode 100644
index 00000000000..c126e8b7443
--- /dev/null
+++ b/tests/baselines/reference/invalidLetInForOfAndForIn_ES6.errors.txt
@@ -0,0 +1,22 @@
+tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts(5,13): error TS1005: '=' expected.
+tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts(5,20): error TS1005: ',' expected.
+tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts(7,1): error TS1005: ';' expected.
+
+
+==== tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts (3 errors) ====
+    // This should be an error
+    // More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+    
+    var let = 10;
+    for (let of [1,2,3]) {}
+                ~
+!!! error TS1005: '=' expected.
+                       ~
+!!! error TS1005: ',' expected.
+    
+    for (let in [1,2,3]) {}
+    ~~~
+!!! error TS1005: ';' expected.
+     
+    
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/invalidLetInForOfAndForIn_ES6.js b/tests/baselines/reference/invalidLetInForOfAndForIn_ES6.js
new file mode 100644
index 00000000000..93258d6b225
--- /dev/null
+++ b/tests/baselines/reference/invalidLetInForOfAndForIn_ES6.js
@@ -0,0 +1,18 @@
+//// [invalidLetInForOfAndForIn_ES6.ts]
+// This should be an error
+// More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+
+var let = 10;
+for (let of [1,2,3]) {}
+
+for (let in [1,2,3]) {}
+ 
+
+
+
+//// [invalidLetInForOfAndForIn_ES6.js]
+// This should be an error
+// More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+var let = 10;
+for (let of = [1, 2, 3], {  }; ; )
+    for ( in [1, 2, 3]) { }
diff --git a/tests/baselines/reference/letAsIdentifier2.js b/tests/baselines/reference/letAsIdentifier2.js
new file mode 100644
index 00000000000..62c8461bfc8
--- /dev/null
+++ b/tests/baselines/reference/letAsIdentifier2.js
@@ -0,0 +1,6 @@
+//// [letAsIdentifier2.ts]
+
+function let() {}
+
+//// [letAsIdentifier2.js]
+function let() { }
diff --git a/tests/baselines/reference/letAsIdentifier2.symbols b/tests/baselines/reference/letAsIdentifier2.symbols
new file mode 100644
index 00000000000..211aae05316
--- /dev/null
+++ b/tests/baselines/reference/letAsIdentifier2.symbols
@@ -0,0 +1,5 @@
+=== tests/cases/compiler/letAsIdentifier2.ts ===
+
+function let() {}
+>let : Symbol(let, Decl(letAsIdentifier2.ts, 0, 0))
+
diff --git a/tests/baselines/reference/letAsIdentifier2.types b/tests/baselines/reference/letAsIdentifier2.types
new file mode 100644
index 00000000000..dc1416fdae9
--- /dev/null
+++ b/tests/baselines/reference/letAsIdentifier2.types
@@ -0,0 +1,5 @@
+=== tests/cases/compiler/letAsIdentifier2.ts ===
+
+function let() {}
+>let : () => void
+
diff --git a/tests/baselines/reference/letInConstDeclarations_ES5.errors.txt b/tests/baselines/reference/letInConstDeclarations_ES5.errors.txt
new file mode 100644
index 00000000000..8b694c35dbb
--- /dev/null
+++ b/tests/baselines/reference/letInConstDeclarations_ES5.errors.txt
@@ -0,0 +1,16 @@
+tests/cases/compiler/letInConstDeclarations_ES5.ts(3,15): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInConstDeclarations_ES5.ts(6,19): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+
+
+==== tests/cases/compiler/letInConstDeclarations_ES5.ts (2 errors) ====
+    
+    // All use of let in const declaration should be an error
+    const x = 50, let = 5;
+                  ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    {
+        const x = 10, let = 20;
+                      ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/letInConstDeclarations_ES5.js b/tests/baselines/reference/letInConstDeclarations_ES5.js
new file mode 100644
index 00000000000..f76d49765f0
--- /dev/null
+++ b/tests/baselines/reference/letInConstDeclarations_ES5.js
@@ -0,0 +1,15 @@
+//// [letInConstDeclarations_ES5.ts]
+
+// All use of let in const declaration should be an error
+const x = 50, let = 5;
+
+{
+    const x = 10, let = 20;
+}
+
+//// [letInConstDeclarations_ES5.js]
+// All use of let in const declaration should be an error
+var x = 50, let = 5;
+{
+    var x_1 = 10, let_1 = 20;
+}
diff --git a/tests/baselines/reference/letInConstDeclarations_ES6.errors.txt b/tests/baselines/reference/letInConstDeclarations_ES6.errors.txt
new file mode 100644
index 00000000000..d29f78be014
--- /dev/null
+++ b/tests/baselines/reference/letInConstDeclarations_ES6.errors.txt
@@ -0,0 +1,16 @@
+tests/cases/compiler/letInConstDeclarations_ES6.ts(3,15): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInConstDeclarations_ES6.ts(6,19): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+
+
+==== tests/cases/compiler/letInConstDeclarations_ES6.ts (2 errors) ====
+    
+    // All use of let in const declaration should be an error
+    const x = 50, let = 5;
+                  ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    {
+        const x = 10, let = 20;
+                      ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/letInConstDeclarations_ES6.js b/tests/baselines/reference/letInConstDeclarations_ES6.js
new file mode 100644
index 00000000000..f209ef1dede
--- /dev/null
+++ b/tests/baselines/reference/letInConstDeclarations_ES6.js
@@ -0,0 +1,15 @@
+//// [letInConstDeclarations_ES6.ts]
+
+// All use of let in const declaration should be an error
+const x = 50, let = 5;
+
+{
+    const x = 10, let = 20;
+}
+
+//// [letInConstDeclarations_ES6.js]
+// All use of let in const declaration should be an error
+const x = 50, let = 5;
+{
+    const x = 10, let = 20;
+}
diff --git a/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.errors.txt b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.errors.txt
new file mode 100644
index 00000000000..e335a9652f4
--- /dev/null
+++ b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.errors.txt
@@ -0,0 +1,48 @@
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(3,10): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(5,12): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(7,10): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(9,12): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(12,11): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(14,13): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(16,11): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts(18,13): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+
+
+==== tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts (8 errors) ====
+    
+    // Should be an error
+    for (let let of [1,2,3]) {}
+             ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    for (const let of [1,2,3]) {}
+               ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    for (let let in [1,2,3]) {}
+             ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    for (const let in [1,2,3]) {}
+               ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    {
+    	for (let let of [1,2,3]) {}
+    	         ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    	for (const let of [1,2,3]) {}
+    	           ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    	
+    	for (let let in [1,2,3]) {}
+    	         ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    	for (const let in [1,2,3]) {}
+    	           ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    }
+    
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.js b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.js
new file mode 100644
index 00000000000..4c467176492
--- /dev/null
+++ b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES5.js
@@ -0,0 +1,43 @@
+//// [letInLetConstDeclOfForOfAndForIn_ES5.ts]
+
+// Should be an error
+for (let let of [1,2,3]) {}
+
+for (const let of [1,2,3]) {}
+
+for (let let in [1,2,3]) {}
+
+for (const let in [1,2,3]) {}
+
+{
+	for (let let of [1,2,3]) {}
+
+	for (const let of [1,2,3]) {}
+	
+	for (let let in [1,2,3]) {}
+
+	for (const let in [1,2,3]) {}
+}
+
+
+
+//// [letInLetConstDeclOfForOfAndForIn_ES5.js]
+// Should be an error
+for (var _i = 0, _a = [1, 2, 3]; _i < _a.length; _i++) {
+    var let = _a[_i];
+}
+for (var _b = 0, _c = [1, 2, 3]; _b < _c.length; _b++) {
+    var let = _c[_b];
+}
+for (var let in [1, 2, 3]) { }
+for (var let in [1, 2, 3]) { }
+{
+    for (var _d = 0, _e = [1, 2, 3]; _d < _e.length; _d++) {
+        var let = _e[_d];
+    }
+    for (var _f = 0, _g = [1, 2, 3]; _f < _g.length; _f++) {
+        var let = _g[_f];
+    }
+    for (var let in [1, 2, 3]) { }
+    for (var let in [1, 2, 3]) { }
+}
diff --git a/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.errors.txt b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.errors.txt
new file mode 100644
index 00000000000..1a5af892b74
--- /dev/null
+++ b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.errors.txt
@@ -0,0 +1,48 @@
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(3,10): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(5,12): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(7,10): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(9,12): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(12,11): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(14,13): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(16,11): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts(18,13): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+
+
+==== tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts (8 errors) ====
+    
+    // Should be an error
+    for (let let of [1,2,3]) {}
+             ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    for (const let of [1,2,3]) {}
+               ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    for (let let in [1,2,3]) {}
+             ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    for (const let in [1,2,3]) {}
+               ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    {
+    	for (let let of [1,2,3]) {}
+    	         ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    	for (const let of [1,2,3]) {}
+    	           ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    	
+    	for (let let in [1,2,3]) {}
+    	         ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    	for (const let in [1,2,3]) {}
+    	           ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    }
+    
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.js b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.js
new file mode 100644
index 00000000000..583dc936871
--- /dev/null
+++ b/tests/baselines/reference/letInLetConstDeclOfForOfAndForIn_ES6.js
@@ -0,0 +1,35 @@
+//// [letInLetConstDeclOfForOfAndForIn_ES6.ts]
+
+// Should be an error
+for (let let of [1,2,3]) {}
+
+for (const let of [1,2,3]) {}
+
+for (let let in [1,2,3]) {}
+
+for (const let in [1,2,3]) {}
+
+{
+	for (let let of [1,2,3]) {}
+
+	for (const let of [1,2,3]) {}
+	
+	for (let let in [1,2,3]) {}
+
+	for (const let in [1,2,3]) {}
+}
+
+
+
+//// [letInLetConstDeclOfForOfAndForIn_ES6.js]
+// Should be an error
+for (let let of [1, 2, 3]) { }
+for (const let of [1, 2, 3]) { }
+for (let let in [1, 2, 3]) { }
+for (const let in [1, 2, 3]) { }
+{
+    for (let let of [1, 2, 3]) { }
+    for (const let of [1, 2, 3]) { }
+    for (let let in [1, 2, 3]) { }
+    for (const let in [1, 2, 3]) { }
+}
diff --git a/tests/baselines/reference/letInLetDeclarations_ES5.errors.txt b/tests/baselines/reference/letInLetDeclarations_ES5.errors.txt
new file mode 100644
index 00000000000..73f824dd7de
--- /dev/null
+++ b/tests/baselines/reference/letInLetDeclarations_ES5.errors.txt
@@ -0,0 +1,16 @@
+tests/cases/compiler/letInLetDeclarations_ES5.ts(3,13): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetDeclarations_ES5.ts(6,17): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+
+
+==== tests/cases/compiler/letInLetDeclarations_ES5.ts (2 errors) ====
+    
+    // All use of let in const declaration should be an error
+    let x = 50, let = 5;
+                ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    {
+        let x = 10, let = 20;
+                    ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/letInLetDeclarations_ES5.js b/tests/baselines/reference/letInLetDeclarations_ES5.js
new file mode 100644
index 00000000000..0b7f02150a4
--- /dev/null
+++ b/tests/baselines/reference/letInLetDeclarations_ES5.js
@@ -0,0 +1,15 @@
+//// [letInLetDeclarations_ES5.ts]
+
+// All use of let in const declaration should be an error
+let x = 50, let = 5;
+
+{
+    let x = 10, let = 20;
+}
+
+//// [letInLetDeclarations_ES5.js]
+// All use of let in const declaration should be an error
+var x = 50, let = 5;
+{
+    var x_1 = 10, let_1 = 20;
+}
diff --git a/tests/baselines/reference/letInLetDeclarations_ES6.errors.txt b/tests/baselines/reference/letInLetDeclarations_ES6.errors.txt
new file mode 100644
index 00000000000..0fe1aa882e9
--- /dev/null
+++ b/tests/baselines/reference/letInLetDeclarations_ES6.errors.txt
@@ -0,0 +1,16 @@
+tests/cases/compiler/letInLetDeclarations_ES6.ts(3,13): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+tests/cases/compiler/letInLetDeclarations_ES6.ts(6,17): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+
+
+==== tests/cases/compiler/letInLetDeclarations_ES6.ts (2 errors) ====
+    
+    // All use of let in const declaration should be an error
+    let x = 50, let = 5;
+                ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    
+    {
+        let x = 10, let = 20;
+                    ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/letInLetDeclarations_ES6.js b/tests/baselines/reference/letInLetDeclarations_ES6.js
new file mode 100644
index 00000000000..7c4385cd4d9
--- /dev/null
+++ b/tests/baselines/reference/letInLetDeclarations_ES6.js
@@ -0,0 +1,15 @@
+//// [letInLetDeclarations_ES6.ts]
+
+// All use of let in const declaration should be an error
+let x = 50, let = 5;
+
+{
+    let x = 10, let = 20;
+}
+
+//// [letInLetDeclarations_ES6.js]
+// All use of let in const declaration should be an error
+let x = 50, let = 5;
+{
+    let x = 10, let = 20;
+}
diff --git a/tests/baselines/reference/letInLetOrConstDeclarations.errors.txt b/tests/baselines/reference/letInLetOrConstDeclarations.errors.txt
deleted file mode 100644
index fbcee276583..00000000000
--- a/tests/baselines/reference/letInLetOrConstDeclarations.errors.txt
+++ /dev/null
@@ -1,23 +0,0 @@
-tests/cases/compiler/letInLetOrConstDeclarations.ts(2,9): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
-tests/cases/compiler/letInLetOrConstDeclarations.ts(3,14): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
-tests/cases/compiler/letInLetOrConstDeclarations.ts(6,11): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
-
-
-==== tests/cases/compiler/letInLetOrConstDeclarations.ts (3 errors) ====
-    {
-        let let = 1; // should error
-            ~~~
-!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
-        for (let let in []) { } // should error
-                 ~~~
-!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
-    }
-    {
-        const let = 1; // should error
-              ~~~
-!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
-    }
-    {
-        function let() { // should be ok
-        }
-    }
\ No newline at end of file
diff --git a/tests/baselines/reference/letInLetOrConstDeclarations.js b/tests/baselines/reference/letInLetOrConstDeclarations.js
deleted file mode 100644
index ee23abc4e4f..00000000000
--- a/tests/baselines/reference/letInLetOrConstDeclarations.js
+++ /dev/null
@@ -1,25 +0,0 @@
-//// [letInLetOrConstDeclarations.ts]
-{
-    let let = 1; // should error
-    for (let let in []) { } // should error
-}
-{
-    const let = 1; // should error
-}
-{
-    function let() { // should be ok
-    }
-}
-
-//// [letInLetOrConstDeclarations.js]
-{
-    let let = 1; // should error
-    for (let let in []) { } // should error
-}
-{
-    const let = 1; // should error
-}
-{
-    function let() {
-    }
-}
diff --git a/tests/baselines/reference/letInVarDeclOfForIn_ES5.js b/tests/baselines/reference/letInVarDeclOfForIn_ES5.js
new file mode 100644
index 00000000000..2cef7610199
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForIn_ES5.js
@@ -0,0 +1,16 @@
+//// [letInVarDeclOfForIn_ES5.ts]
+
+// should not be an error
+for (var let in [1,2,3]) {}
+
+{
+	for (var let in [1,2,3]) {}
+}
+
+
+//// [letInVarDeclOfForIn_ES5.js]
+// should not be an error
+for (var let in [1, 2, 3]) { }
+{
+    for (var let in [1, 2, 3]) { }
+}
diff --git a/tests/baselines/reference/letInVarDeclOfForIn_ES5.symbols b/tests/baselines/reference/letInVarDeclOfForIn_ES5.symbols
new file mode 100644
index 00000000000..cbfd0f58086
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForIn_ES5.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/letInVarDeclOfForIn_ES5.ts ===
+
+// should not be an error
+for (var let in [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForIn_ES5.ts, 2, 8), Decl(letInVarDeclOfForIn_ES5.ts, 5, 9))
+
+{
+	for (var let in [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForIn_ES5.ts, 2, 8), Decl(letInVarDeclOfForIn_ES5.ts, 5, 9))
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForIn_ES5.types b/tests/baselines/reference/letInVarDeclOfForIn_ES5.types
new file mode 100644
index 00000000000..ac1ca3f27c9
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForIn_ES5.types
@@ -0,0 +1,19 @@
+=== tests/cases/compiler/letInVarDeclOfForIn_ES5.ts ===
+
+// should not be an error
+for (var let in [1,2,3]) {}
+>let : any
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+
+{
+	for (var let in [1,2,3]) {}
+>let : any
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForIn_ES6.js b/tests/baselines/reference/letInVarDeclOfForIn_ES6.js
new file mode 100644
index 00000000000..1cf6850f588
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForIn_ES6.js
@@ -0,0 +1,16 @@
+//// [letInVarDeclOfForIn_ES6.ts]
+
+// should not be an error
+for (var let in [1,2,3]) {}
+
+{
+	for (var let in [1,2,3]) {}
+}
+
+
+//// [letInVarDeclOfForIn_ES6.js]
+// should not be an error
+for (var let in [1, 2, 3]) { }
+{
+    for (var let in [1, 2, 3]) { }
+}
diff --git a/tests/baselines/reference/letInVarDeclOfForIn_ES6.symbols b/tests/baselines/reference/letInVarDeclOfForIn_ES6.symbols
new file mode 100644
index 00000000000..3b3e68e3040
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForIn_ES6.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/letInVarDeclOfForIn_ES6.ts ===
+
+// should not be an error
+for (var let in [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForIn_ES6.ts, 2, 8), Decl(letInVarDeclOfForIn_ES6.ts, 5, 9))
+
+{
+	for (var let in [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForIn_ES6.ts, 2, 8), Decl(letInVarDeclOfForIn_ES6.ts, 5, 9))
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForIn_ES6.types b/tests/baselines/reference/letInVarDeclOfForIn_ES6.types
new file mode 100644
index 00000000000..3c508bd8e2b
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForIn_ES6.types
@@ -0,0 +1,19 @@
+=== tests/cases/compiler/letInVarDeclOfForIn_ES6.ts ===
+
+// should not be an error
+for (var let in [1,2,3]) {}
+>let : any
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+
+{
+	for (var let in [1,2,3]) {}
+>let : any
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForOf_ES5.js b/tests/baselines/reference/letInVarDeclOfForOf_ES5.js
new file mode 100644
index 00000000000..cdc374fbea0
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForOf_ES5.js
@@ -0,0 +1,20 @@
+//// [letInVarDeclOfForOf_ES5.ts]
+
+// should not be an error
+for (var let of [1,2,3]) {}
+
+{
+	for (var let of [1,2,3]) {}
+}
+
+
+//// [letInVarDeclOfForOf_ES5.js]
+// should not be an error
+for (var _i = 0, _a = [1, 2, 3]; _i < _a.length; _i++) {
+    var let = _a[_i];
+}
+{
+    for (var _b = 0, _c = [1, 2, 3]; _b < _c.length; _b++) {
+        var let = _c[_b];
+    }
+}
diff --git a/tests/baselines/reference/letInVarDeclOfForOf_ES5.symbols b/tests/baselines/reference/letInVarDeclOfForOf_ES5.symbols
new file mode 100644
index 00000000000..f5411cfca2d
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForOf_ES5.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/letInVarDeclOfForOf_ES5.ts ===
+
+// should not be an error
+for (var let of [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForOf_ES5.ts, 2, 8), Decl(letInVarDeclOfForOf_ES5.ts, 5, 9))
+
+{
+	for (var let of [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForOf_ES5.ts, 2, 8), Decl(letInVarDeclOfForOf_ES5.ts, 5, 9))
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForOf_ES5.types b/tests/baselines/reference/letInVarDeclOfForOf_ES5.types
new file mode 100644
index 00000000000..a1000df8bed
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForOf_ES5.types
@@ -0,0 +1,19 @@
+=== tests/cases/compiler/letInVarDeclOfForOf_ES5.ts ===
+
+// should not be an error
+for (var let of [1,2,3]) {}
+>let : number
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+
+{
+	for (var let of [1,2,3]) {}
+>let : number
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForOf_ES6.js b/tests/baselines/reference/letInVarDeclOfForOf_ES6.js
new file mode 100644
index 00000000000..fca365c4b0d
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForOf_ES6.js
@@ -0,0 +1,16 @@
+//// [letInVarDeclOfForOf_ES6.ts]
+
+// should not be an error
+for (var let of [1,2,3]) {}
+
+{
+	for (var let of [1,2,3]) {}
+}
+
+
+//// [letInVarDeclOfForOf_ES6.js]
+// should not be an error
+for (var let of [1, 2, 3]) { }
+{
+    for (var let of [1, 2, 3]) { }
+}
diff --git a/tests/baselines/reference/letInVarDeclOfForOf_ES6.symbols b/tests/baselines/reference/letInVarDeclOfForOf_ES6.symbols
new file mode 100644
index 00000000000..d4c14eff0b0
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForOf_ES6.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/letInVarDeclOfForOf_ES6.ts ===
+
+// should not be an error
+for (var let of [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForOf_ES6.ts, 2, 8), Decl(letInVarDeclOfForOf_ES6.ts, 5, 9))
+
+{
+	for (var let of [1,2,3]) {}
+>let : Symbol(let, Decl(letInVarDeclOfForOf_ES6.ts, 2, 8), Decl(letInVarDeclOfForOf_ES6.ts, 5, 9))
+}
+
diff --git a/tests/baselines/reference/letInVarDeclOfForOf_ES6.types b/tests/baselines/reference/letInVarDeclOfForOf_ES6.types
new file mode 100644
index 00000000000..1515895feda
--- /dev/null
+++ b/tests/baselines/reference/letInVarDeclOfForOf_ES6.types
@@ -0,0 +1,19 @@
+=== tests/cases/compiler/letInVarDeclOfForOf_ES6.ts ===
+
+// should not be an error
+for (var let of [1,2,3]) {}
+>let : number
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+
+{
+	for (var let of [1,2,3]) {}
+>let : number
+>[1,2,3] : number[]
+>1 : number
+>2 : number
+>3 : number
+}
+
diff --git a/tests/baselines/reference/strictModeReservedWord.errors.txt b/tests/baselines/reference/strictModeReservedWord.errors.txt
index 6f6a387429d..02ece81a1d9 100644
--- a/tests/baselines/reference/strictModeReservedWord.errors.txt
+++ b/tests/baselines/reference/strictModeReservedWord.errors.txt
@@ -1,6 +1,8 @@
+tests/cases/compiler/strictModeReservedWord.ts(1,5): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
 tests/cases/compiler/strictModeReservedWord.ts(5,9): error TS1212: Identifier expected. 'public' is a reserved word in strict mode
 tests/cases/compiler/strictModeReservedWord.ts(6,9): error TS1212: Identifier expected. 'static' is a reserved word in strict mode
 tests/cases/compiler/strictModeReservedWord.ts(7,9): error TS1212: Identifier expected. 'let' is a reserved word in strict mode
+tests/cases/compiler/strictModeReservedWord.ts(7,9): error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
 tests/cases/compiler/strictModeReservedWord.ts(8,9): error TS1212: Identifier expected. 'package' is a reserved word in strict mode
 tests/cases/compiler/strictModeReservedWord.ts(8,9): error TS2300: Duplicate identifier 'package'.
 tests/cases/compiler/strictModeReservedWord.ts(9,14): error TS1212: Identifier expected. 'package' is a reserved word in strict mode
@@ -41,8 +43,10 @@ tests/cases/compiler/strictModeReservedWord.ts(24,5): error TS1212: Identifier e
 tests/cases/compiler/strictModeReservedWord.ts(24,5): error TS2349: Cannot invoke an expression whose type lacks a call signature.
 
 
-==== tests/cases/compiler/strictModeReservedWord.ts (41 errors) ====
+==== tests/cases/compiler/strictModeReservedWord.ts (43 errors) ====
     let let = 10;
+        ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
     
     function foo() {
         "use strict"
@@ -55,6 +59,8 @@ tests/cases/compiler/strictModeReservedWord.ts(24,5): error TS2349: Cannot invok
         let let = "blah";
             ~~~
 !!! error TS1212: Identifier expected. 'let' is a reserved word in strict mode
+            ~~~
+!!! error TS2480: 'let' is not allowed to be used as a name in 'let' or 'const' declarations.
         var package = "hello"
             ~~~~~~~
 !!! error TS1212: Identifier expected. 'package' is a reserved word in strict mode
diff --git a/tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts b/tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts
new file mode 100644
index 00000000000..03a01204462
--- /dev/null
+++ b/tests/cases/compiler/invalidLetInForOfAndForIn_ES5.ts
@@ -0,0 +1,10 @@
+// @target: es6
+// This should be an error
+// More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+
+var let = 10;
+for (let of [1,2,3]) {}
+
+for (let in [1,2,3]) {}
+ 
+
diff --git a/tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts b/tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts
new file mode 100644
index 00000000000..03a01204462
--- /dev/null
+++ b/tests/cases/compiler/invalidLetInForOfAndForIn_ES6.ts
@@ -0,0 +1,10 @@
+// @target: es6
+// This should be an error
+// More details: http://www.ecma-international.org/ecma-262/6.0/#sec-iteration-statements
+
+var let = 10;
+for (let of [1,2,3]) {}
+
+for (let in [1,2,3]) {}
+ 
+
diff --git a/tests/cases/compiler/letAsIdentifier2.ts b/tests/cases/compiler/letAsIdentifier2.ts
new file mode 100644
index 00000000000..7ebbeb0704b
--- /dev/null
+++ b/tests/cases/compiler/letAsIdentifier2.ts
@@ -0,0 +1,3 @@
+// @target: es6
+
+function let() {}
\ No newline at end of file
diff --git a/tests/cases/compiler/letInConstDeclarations_ES5.ts b/tests/cases/compiler/letInConstDeclarations_ES5.ts
new file mode 100644
index 00000000000..2898abd1e2a
--- /dev/null
+++ b/tests/cases/compiler/letInConstDeclarations_ES5.ts
@@ -0,0 +1,8 @@
+// @target: es5
+
+// All use of let in const declaration should be an error
+const x = 50, let = 5;
+
+{
+    const x = 10, let = 20;
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/letInConstDeclarations_ES6.ts b/tests/cases/compiler/letInConstDeclarations_ES6.ts
new file mode 100644
index 00000000000..aac2a592d11
--- /dev/null
+++ b/tests/cases/compiler/letInConstDeclarations_ES6.ts
@@ -0,0 +1,8 @@
+// @target: es6
+
+// All use of let in const declaration should be an error
+const x = 50, let = 5;
+
+{
+    const x = 10, let = 20;
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts b/tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts
new file mode 100644
index 00000000000..7e8b6824329
--- /dev/null
+++ b/tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES5.ts
@@ -0,0 +1,21 @@
+// @target: es5
+
+// Should be an error
+for (let let of [1,2,3]) {}
+
+for (const let of [1,2,3]) {}
+
+for (let let in [1,2,3]) {}
+
+for (const let in [1,2,3]) {}
+
+{
+	for (let let of [1,2,3]) {}
+
+	for (const let of [1,2,3]) {}
+	
+	for (let let in [1,2,3]) {}
+
+	for (const let in [1,2,3]) {}
+}
+
diff --git a/tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts b/tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts
new file mode 100644
index 00000000000..68ba1716199
--- /dev/null
+++ b/tests/cases/compiler/letInLetConstDeclOfForOfAndForIn_ES6.ts
@@ -0,0 +1,21 @@
+// @target: es6
+
+// Should be an error
+for (let let of [1,2,3]) {}
+
+for (const let of [1,2,3]) {}
+
+for (let let in [1,2,3]) {}
+
+for (const let in [1,2,3]) {}
+
+{
+	for (let let of [1,2,3]) {}
+
+	for (const let of [1,2,3]) {}
+	
+	for (let let in [1,2,3]) {}
+
+	for (const let in [1,2,3]) {}
+}
+
diff --git a/tests/cases/compiler/letInLetDeclarations_ES5.ts b/tests/cases/compiler/letInLetDeclarations_ES5.ts
new file mode 100644
index 00000000000..8d4a71b85c8
--- /dev/null
+++ b/tests/cases/compiler/letInLetDeclarations_ES5.ts
@@ -0,0 +1,8 @@
+// @target: es5
+
+// All use of let in const declaration should be an error
+let x = 50, let = 5;
+
+{
+    let x = 10, let = 20;
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/letInLetDeclarations_ES6.ts b/tests/cases/compiler/letInLetDeclarations_ES6.ts
new file mode 100644
index 00000000000..8c4650dd938
--- /dev/null
+++ b/tests/cases/compiler/letInLetDeclarations_ES6.ts
@@ -0,0 +1,8 @@
+// @target: es6
+
+// All use of let in const declaration should be an error
+let x = 50, let = 5;
+
+{
+    let x = 10, let = 20;
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/letInLetOrConstDeclarations.ts b/tests/cases/compiler/letInLetOrConstDeclarations.ts
deleted file mode 100644
index c622759a459..00000000000
--- a/tests/cases/compiler/letInLetOrConstDeclarations.ts
+++ /dev/null
@@ -1,12 +0,0 @@
-// @target: es6
-{
-    let let = 1; // should error
-    for (let let in []) { } // should error
-}
-{
-    const let = 1; // should error
-}
-{
-    function let() { // should be ok
-    }
-}
\ No newline at end of file
diff --git a/tests/cases/compiler/letInVarDeclOfForIn_ES5.ts b/tests/cases/compiler/letInVarDeclOfForIn_ES5.ts
new file mode 100644
index 00000000000..684dc3b2e9f
--- /dev/null
+++ b/tests/cases/compiler/letInVarDeclOfForIn_ES5.ts
@@ -0,0 +1,8 @@
+// @target: es5
+
+// should not be an error
+for (var let in [1,2,3]) {}
+
+{
+	for (var let in [1,2,3]) {}
+}
diff --git a/tests/cases/compiler/letInVarDeclOfForIn_ES6.ts b/tests/cases/compiler/letInVarDeclOfForIn_ES6.ts
new file mode 100644
index 00000000000..8d79f41713e
--- /dev/null
+++ b/tests/cases/compiler/letInVarDeclOfForIn_ES6.ts
@@ -0,0 +1,8 @@
+// @target: es6
+
+// should not be an error
+for (var let in [1,2,3]) {}
+
+{
+	for (var let in [1,2,3]) {}
+}
diff --git a/tests/cases/compiler/letInVarDeclOfForOf_ES5.ts b/tests/cases/compiler/letInVarDeclOfForOf_ES5.ts
new file mode 100644
index 00000000000..2d369aa162e
--- /dev/null
+++ b/tests/cases/compiler/letInVarDeclOfForOf_ES5.ts
@@ -0,0 +1,8 @@
+// @target: es5
+
+// should not be an error
+for (var let of [1,2,3]) {}
+
+{
+	for (var let of [1,2,3]) {}
+}
diff --git a/tests/cases/compiler/letInVarDeclOfForOf_ES6.ts b/tests/cases/compiler/letInVarDeclOfForOf_ES6.ts
new file mode 100644
index 00000000000..9e343a45f4b
--- /dev/null
+++ b/tests/cases/compiler/letInVarDeclOfForOf_ES6.ts
@@ -0,0 +1,8 @@
+// @target: es6
+
+// should not be an error
+for (var let of [1,2,3]) {}
+
+{
+	for (var let of [1,2,3]) {}
+}

From 58a83f1e45ddaa7a289c24bc84f870b7b0500203 Mon Sep 17 00:00:00 2001
From: Yui T <yuisu@microsoft.com>
Date: Wed, 2 Dec 2015 13:57:10 -0800
Subject: [PATCH 3/4] Fix linter error

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1431a91ff43..5063cb6dea2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15973,7 +15973,7 @@ namespace ts {
                             : Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_of_statement;
                         return grammarErrorOnFirstToken(variableList.declarations[1], diagnostic);
                     }
-                    const firstDeclaration = variableList.declarations[0]
+                    const firstDeclaration = variableList.declarations[0];
 
                     // firstDeclaration can be undefined if there is variable declaration in for-of or for-in
                     // See http://www.ecma-international.org/ecma-262/6.0/#sec-for-in-and-for-of-statements for details

From 0784ea169fd480d558fcda4b04584036cb482f97 Mon Sep 17 00:00:00 2001
From: Yui T <yuisu@microsoft.com>
Date: Thu, 3 Dec 2015 13:17:57 -0800
Subject: [PATCH 4/4] Check for lenght of declaration instead

---
 src/compiler/checker.ts | 47 +++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 21 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5063cb6dea2..5f9636f8f73 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15967,33 +15967,38 @@ namespace ts {
             if (forInOrOfStatement.initializer.kind === SyntaxKind.VariableDeclarationList) {
                 const variableList = <VariableDeclarationList>forInOrOfStatement.initializer;
                 if (!checkGrammarVariableDeclarationList(variableList)) {
-                    if (variableList.declarations.length > 1) {
-                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
-                            ? Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_in_statement
-                            : Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_of_statement;
-                        return grammarErrorOnFirstToken(variableList.declarations[1], diagnostic);
-                    }
-                    const firstDeclaration = variableList.declarations[0];
+                    const declarations = variableList.declarations;
 
-                    // firstDeclaration can be undefined if there is variable declaration in for-of or for-in
+                    // declarations.length can be zero if there is an error in variable declaration in for-of or for-in
                     // See http://www.ecma-international.org/ecma-262/6.0/#sec-for-in-and-for-of-statements for details
                     // For example:
                     //      var let = 10;
                     //      for (let of [1,2,3]) {} // this is invalid ES6 syntax
                     //      for (let in [1,2,3]) {} // this is invalid ES6 syntax
-                    if (firstDeclaration) {
-                        if (firstDeclaration.initializer) {
-                            const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
-                                ? Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer
-                                : Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;
-                            return grammarErrorOnNode(firstDeclaration.name, diagnostic);
-                        }
-                        if (firstDeclaration.type) {
-                            const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
-                                ? Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_use_a_type_annotation
-                                : Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_use_a_type_annotation;
-                            return grammarErrorOnNode(firstDeclaration, diagnostic);
-                        }
+                    // We will then want to skip on grammar checking on variableList declaration
+                    if (!declarations.length) {
+                        return false;
+                    }
+
+                    if (declarations.length > 1) {
+                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
+                            ? Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_in_statement
+                            : Diagnostics.Only_a_single_variable_declaration_is_allowed_in_a_for_of_statement;
+                        return grammarErrorOnFirstToken(variableList.declarations[1], diagnostic);
+                    }
+                    const firstDeclaration = declarations[0];
+
+                    if (firstDeclaration.initializer) {
+                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
+                            ? Diagnostics.The_variable_declaration_of_a_for_in_statement_cannot_have_an_initializer
+                            : Diagnostics.The_variable_declaration_of_a_for_of_statement_cannot_have_an_initializer;
+                        return grammarErrorOnNode(firstDeclaration.name, diagnostic);
+                    }
+                    if (firstDeclaration.type) {
+                        const diagnostic = forInOrOfStatement.kind === SyntaxKind.ForInStatement
+                            ? Diagnostics.The_left_hand_side_of_a_for_in_statement_cannot_use_a_type_annotation
+                            : Diagnostics.The_left_hand_side_of_a_for_of_statement_cannot_use_a_type_annotation;
+                        return grammarErrorOnNode(firstDeclaration, diagnostic);
                     }
                 }
             }