diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 2c0416c0711..8ad12a14011 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1291,7 +1291,7 @@ namespace ts {
                 args[i] = arguments[i];
             }
 
-            return t => reduceLeft<(t: T) => T, T>(args, (u, f) => f(u), t);
+            return t => reduceLeft(args, (u, f) => f(u), t);
         }
         else if (d) {
             return t => d(c(b(a(t))));
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index e30d5dbe1e4..b0782b6707a 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -2237,7 +2237,8 @@ namespace ts {
             return token() === SyntaxKind.DotDotDotToken ||
                 isIdentifierOrPattern() ||
                 isModifierKind(token()) ||
-                token() === SyntaxKind.AtToken || isStartOfType();
+                token() === SyntaxKind.AtToken ||
+                isStartOfType(/*inStartOfParameter*/ true);
         }
 
         function parseParameter(): ParameterDeclaration {
@@ -2698,7 +2699,7 @@ namespace ts {
             }
         }
 
-        function isStartOfType(): boolean {
+        function isStartOfType(inStartOfParameter?: boolean): boolean {
             switch (token()) {
                 case SyntaxKind.AnyKeyword:
                 case SyntaxKind.StringKeyword:
@@ -2728,11 +2729,11 @@ namespace ts {
                 case SyntaxKind.DotDotDotToken:
                     return true;
                 case SyntaxKind.MinusToken:
-                    return lookAhead(nextTokenIsNumericLiteral);
+                    return !inStartOfParameter && lookAhead(nextTokenIsNumericLiteral);
                 case SyntaxKind.OpenParenToken:
                     // Only consider '(' the start of a type if followed by ')', '...', an identifier, a modifier,
                     // or something that starts a type. We don't want to consider things like '(1)' a type.
-                    return lookAhead(isStartOfParenthesizedOrFunctionType);
+                    return !inStartOfParameter && lookAhead(isStartOfParenthesizedOrFunctionType);
                 default:
                     return isIdentifier();
             }
diff --git a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.errors.txt b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.errors.txt
index 51db9c7391e..b435e7773f5 100644
--- a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.errors.txt
+++ b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.errors.txt
@@ -1,7 +1,10 @@
-tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,15): error TS1003: Identifier expected.
+tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,12): error TS2304: Cannot find name 'a'.
+tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,12): error TS2695: Left side of comma operator is unused and has no side effects.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,16): error TS2304: Cannot find name 'b'.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,16): error TS2695: Left side of comma operator is unused and has no side effects.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,19): error TS2304: Cannot find name 'c'.
+tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,23): error TS1005: ';' expected.
+tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,26): error TS2304: Cannot find name 'a'.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,28): error TS2304: Cannot find name 'b'.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(1,30): error TS2304: Cannot find name 'c'.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(2,12): error TS2695: Left side of comma operator is unused and has no side effects.
@@ -18,16 +21,22 @@ tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(4,17): error TS1005
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts(4,20): error TS2304: Cannot find name 'a'.
 
 
-==== tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts (18 errors) ====
+==== tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors2.ts (21 errors) ====
     var tt1 = (a, (b, c)) => a+b+c;
-                  ~
-!!! error TS1003: Identifier expected.
+               ~
+!!! error TS2304: Cannot find name 'a'.
+               ~
+!!! error TS2695: Left side of comma operator is unused and has no side effects.
                    ~
 !!! error TS2304: Cannot find name 'b'.
                    ~
 !!! error TS2695: Left side of comma operator is unused and has no side effects.
                       ~
 !!! error TS2304: Cannot find name 'c'.
+                          ~~
+!!! error TS1005: ';' expected.
+                             ~
+!!! error TS2304: Cannot find name 'a'.
                                ~
 !!! error TS2304: Cannot find name 'b'.
                                  ~
diff --git a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.js b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.js
index a1c68ea4476..b51003b62df 100644
--- a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.js
+++ b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors2.js
@@ -5,10 +5,8 @@ var tt2 = ((a), b, c) => a+b+c;
 var tt3 = ((a)) => a;
 
 //// [fatarrowfunctionsOptionalArgsErrors2.js]
-var tt1 = function (a, ) {
-    if ( === void 0) {  = (b, c); }
-    return a + b + c;
-};
+var tt1 = (a, (b, c));
+a + b + c;
 var tt2 = ((a), b, c);
 a + b + c;
 var tt3 = ((a));
diff --git a/tests/baselines/reference/parser512325.errors.txt b/tests/baselines/reference/parser512325.errors.txt
index f54d9f2110a..e6a47fbf226 100644
--- a/tests/baselines/reference/parser512325.errors.txt
+++ b/tests/baselines/reference/parser512325.errors.txt
@@ -1,21 +1,30 @@
-tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,14): error TS1003: Identifier expected.
+tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,11): error TS2304: Cannot find name 'a'.
+tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,11): error TS2695: Left side of comma operator is unused and has no side effects.
 tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,15): error TS2304: Cannot find name 'b'.
 tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,15): error TS2695: Left side of comma operator is unused and has no side effects.
 tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,18): error TS2304: Cannot find name 'c'.
+tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,22): error TS1005: ';' expected.
+tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,25): error TS2304: Cannot find name 'a'.
 tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,27): error TS2304: Cannot find name 'b'.
 tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts(1,29): error TS2304: Cannot find name 'c'.
 
 
-==== tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts (6 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/RegressionTests/parser512325.ts (9 errors) ====
     var tt = (a, (b, c)) => a+b+c;
-                 ~
-!!! error TS1003: Identifier expected.
+              ~
+!!! error TS2304: Cannot find name 'a'.
+              ~
+!!! error TS2695: Left side of comma operator is unused and has no side effects.
                   ~
 !!! error TS2304: Cannot find name 'b'.
                   ~
 !!! error TS2695: Left side of comma operator is unused and has no side effects.
                      ~
 !!! error TS2304: Cannot find name 'c'.
+                         ~~
+!!! error TS1005: ';' expected.
+                            ~
+!!! error TS2304: Cannot find name 'a'.
                               ~
 !!! error TS2304: Cannot find name 'b'.
                                 ~
diff --git a/tests/baselines/reference/parser512325.js b/tests/baselines/reference/parser512325.js
index 75af6b9f39a..14cbcddd86b 100644
--- a/tests/baselines/reference/parser512325.js
+++ b/tests/baselines/reference/parser512325.js
@@ -2,7 +2,5 @@
 var tt = (a, (b, c)) => a+b+c;
 
 //// [parser512325.js]
-var tt = function (a, ) {
-    if ( === void 0) {  = (b, c); }
-    return a + b + c;
-};
+var tt = (a, (b, c));
+a + b + c;
diff --git a/tests/baselines/reference/parserArrowFunctionExpression5.errors.txt b/tests/baselines/reference/parserArrowFunctionExpression5.errors.txt
new file mode 100644
index 00000000000..220c4d42279
--- /dev/null
+++ b/tests/baselines/reference/parserArrowFunctionExpression5.errors.txt
@@ -0,0 +1,15 @@
+tests/cases/conformance/parser/ecmascript5/ArrowFunctionExpressions/parserArrowFunctionExpression5.ts(1,2): error TS2304: Cannot find name 'bar'.
+tests/cases/conformance/parser/ecmascript5/ArrowFunctionExpressions/parserArrowFunctionExpression5.ts(1,6): error TS2304: Cannot find name 'x'.
+
+
+==== tests/cases/conformance/parser/ecmascript5/ArrowFunctionExpressions/parserArrowFunctionExpression5.ts (2 errors) ====
+    (bar(x,
+     ~~~
+!!! error TS2304: Cannot find name 'bar'.
+         ~
+!!! error TS2304: Cannot find name 'x'.
+        () => {},
+        () => {}
+      )
+    )
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/parserArrowFunctionExpression5.js b/tests/baselines/reference/parserArrowFunctionExpression5.js
new file mode 100644
index 00000000000..b25d77a4b02
--- /dev/null
+++ b/tests/baselines/reference/parserArrowFunctionExpression5.js
@@ -0,0 +1,10 @@
+//// [parserArrowFunctionExpression5.ts]
+(bar(x,
+    () => {},
+    () => {}
+  )
+)
+
+
+//// [parserArrowFunctionExpression5.js]
+(bar(x, function () { }, function () { }));
diff --git a/tests/cases/conformance/parser/ecmascript5/ArrowFunctionExpressions/parserArrowFunctionExpression5.ts b/tests/cases/conformance/parser/ecmascript5/ArrowFunctionExpressions/parserArrowFunctionExpression5.ts
new file mode 100644
index 00000000000..d4ab2adefba
--- /dev/null
+++ b/tests/cases/conformance/parser/ecmascript5/ArrowFunctionExpressions/parserArrowFunctionExpression5.ts
@@ -0,0 +1,5 @@
+(bar(x,
+    () => {},
+    () => {}
+  )
+)