Fix for https://github.com/Microsoft/TypeScript/issues/6428

src/compiler/checker.ts

@@ -2326,7 +2326,9 @@ namespace ts {
                         const firstIdentifier = getFirstIdentifier(internalModuleReference);
                         const importSymbol = resolveName(declaration, firstIdentifier.text, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace,
                             Diagnostics.Cannot_find_name_0, firstIdentifier);
-                        buildVisibleNodeList(importSymbol.declarations);
+                        if (importSymbol) {
+                            buildVisibleNodeList(importSymbol.declarations);
+                        }
                     }
                 });
             }

tests/baselines/reference/declarationEmit_UnknownImport.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/compiler/declarationEmit_UnknownImport.ts(2,1): error TS2304: Cannot find name 'SomeNonExistingName'.
+tests/cases/compiler/declarationEmit_UnknownImport.ts(2,14): error TS2503: Cannot find namespace 'SomeNonExistingName'.
+tests/cases/compiler/declarationEmit_UnknownImport.ts(2,14): error TS4000: Import declaration 'Foo' is using private name 'SomeNonExistingName'.
+
+
+==== tests/cases/compiler/declarationEmit_UnknownImport.ts (3 errors) ====
+    
+    import Foo = SomeNonExistingName
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'SomeNonExistingName'.
+                 ~~~~~~~~~~~~~~~~~~~
+!!! error TS2503: Cannot find namespace 'SomeNonExistingName'.
+                 ~~~~~~~~~~~~~~~~~~~
+!!! error TS4000: Import declaration 'Foo' is using private name 'SomeNonExistingName'.
+    export {Foo}

tests/baselines/reference/declarationEmit_UnknownImport.js

@@ -0,0 +1,7 @@
+//// [declarationEmit_UnknownImport.ts]
+
+import Foo = SomeNonExistingName
+export {Foo}
+
+//// [declarationEmit_UnknownImport.js]
+"use strict";

tests/baselines/reference/declarationEmit_UnknownImport2.errors.txt

@@ -0,0 +1,21 @@
+tests/cases/compiler/declarationEmit_UnknownImport2.ts(2,1): error TS2304: Cannot find name 'From'.
+tests/cases/compiler/declarationEmit_UnknownImport2.ts(2,12): error TS1005: '=' expected.
+tests/cases/compiler/declarationEmit_UnknownImport2.ts(2,12): error TS2503: Cannot find namespace 'From'.
+tests/cases/compiler/declarationEmit_UnknownImport2.ts(2,12): error TS4000: Import declaration 'Foo' is using private name 'From'.
+tests/cases/compiler/declarationEmit_UnknownImport2.ts(2,17): error TS1005: ';' expected.
+
+
+==== tests/cases/compiler/declarationEmit_UnknownImport2.ts (5 errors) ====
+    
+    import Foo From './Foo'; // Syntax error
+    ~~~~~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'From'.
+               ~~~~
+!!! error TS1005: '=' expected.
+               ~~~~
+!!! error TS2503: Cannot find namespace 'From'.
+               ~~~~
+!!! error TS4000: Import declaration 'Foo' is using private name 'From'.
+                    ~~~~~~~
+!!! error TS1005: ';' expected.
+    export default Foo

tests/baselines/reference/declarationEmit_UnknownImport2.js

@@ -0,0 +1,8 @@
+//// [declarationEmit_UnknownImport2.ts]
+
+import Foo From './Foo'; // Syntax error
+export default Foo
+
+//// [declarationEmit_UnknownImport2.js]
+"use strict";
+'./Foo'; // Syntax error

tests/cases/compiler/declarationEmit_UnknownImport.ts

@@ -0,0 +1,6 @@
+// @target: es5
+// @module: commonjs
+// @declaration: true
+
+import Foo = SomeNonExistingName
+export {Foo}

tests/cases/compiler/declarationEmit_UnknownImport2.ts

@@ -0,0 +1,6 @@
+// @target: es5
+// @module: commonjs
+// @declaration: true
+
+import Foo From './Foo'; // Syntax error
+export default Foo