"let" should only be considered a keyword in strict mode, or when it starts an actual construct

Handles #1301

src/compiler/parser.ts

@@ -3699,7 +3699,6 @@ module ts {
                 case SyntaxKind.OpenBraceToken:
                     return parseBlock(SyntaxKind.Block, /* ignoreMissingOpenBrace */ false, /*checkForStrictMode*/ false);
                 case SyntaxKind.VarKeyword:
-                case SyntaxKind.LetKeyword:
                 case SyntaxKind.ConstKeyword:
                     // const here should always be parsed as const declaration because of check in 'isStatement' 
                     return parseVariableStatement(scanner.getStartPos(), /*modifiers:*/ undefined);
@@ -3734,6 +3733,12 @@ module ts {
                     return parseTryStatement();
                 case SyntaxKind.DebuggerKeyword:
                     return parseDebuggerStatement();
+                case SyntaxKind.LetKeyword:
+                    // If let follows identifier on the same line, it is declaration parse it as variable statement
+                    if (isLetDeclaration()) {
+                        return parseVariableStatement(scanner.getStartPos(), /*modifiers:*/ undefined);
+                    }
+                    // Else parse it like identifier - fall through
                 default:
                     return isLabel()
                         ? parseLabeledStatement()
@@ -4166,14 +4171,21 @@ module ts {
             parseSemicolon();
             return finishNode(node);
         }
+        
+        function isLetDeclaration() {
+            // It is let declaration if in strict mode or next token is identifier on same line.
+            // otherwise it needs to be treated like identifier
+            return inStrictModeContext() || lookAhead(nextTokenIsIdentifierOnSameLine);
+        }
 
         function isDeclarationStart(): boolean {
             switch (token) {
                 case SyntaxKind.VarKeyword:
-                case SyntaxKind.LetKeyword:
                 case SyntaxKind.ConstKeyword:
                 case SyntaxKind.FunctionKeyword:
                     return true;
+                case SyntaxKind.LetKeyword:
+                    return isLetDeclaration(); 
                 case SyntaxKind.ClassKeyword:
                 case SyntaxKind.InterfaceKeyword:
                 case SyntaxKind.EnumKeyword:

tests/baselines/reference/VariableDeclaration6_es6.errors.txt

@@ -1,7 +1,7 @@
-tests/cases/conformance/es6/variableDeclarations/VariableDeclaration6_es6.ts(1,4): error TS1123: Variable declaration list cannot be empty.
+tests/cases/conformance/es6/variableDeclarations/VariableDeclaration6_es6.ts(1,1): error TS2304: Cannot find name 'let'.
 
 
 ==== tests/cases/conformance/es6/variableDeclarations/VariableDeclaration6_es6.ts (1 errors) ====
     let
-       
-!!! error TS1123: Variable declaration list cannot be empty.
+    ~~~
+!!! error TS2304: Cannot find name 'let'.

tests/baselines/reference/VariableDeclaration6_es6.js

@@ -0,0 +1,5 @@
+//// [VariableDeclaration6_es6.ts]
+let
+
+//// [VariableDeclaration6_es6.js]
+let;

tests/baselines/reference/letAsIdentifier.js

@@ -0,0 +1,19 @@
+//// [letAsIdentifier.ts]
+
+var let = 10;
+var a = 10;
+let = 30;
+let
+a;
+
+//// [letAsIdentifier.js]
+var let = 10;
+var a = 10;
+let = 30;
+let;
+a;
+
+
+//// [letAsIdentifier.d.ts]
+declare var let: number;
+declare var a: number;

tests/baselines/reference/letAsIdentifier.types

@@ -0,0 +1,18 @@
+=== tests/cases/compiler/letAsIdentifier.ts ===
+
+var let = 10;
+>let : number
+
+var a = 10;
+>a : number
+
+let = 30;
+>let = 30 : number
+>let : number
+
+let
+>let : number
+
+a;
+>a : number
+

tests/baselines/reference/letAsIdentifierInStrictMode.errors.txt

@@ -0,0 +1,30 @@
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(2,5): error TS1134: Variable declaration expected.
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(2,9): error TS1134: Variable declaration expected.
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(2,11): error TS1134: Variable declaration expected.
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(4,5): error TS1134: Variable declaration expected.
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(4,7): error TS1134: Variable declaration expected.
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(3,5): error TS2300: Duplicate identifier 'a'.
+tests/cases/compiler/letAsIdentifierInStrictMode.ts(6,1): error TS2300: Duplicate identifier 'a'.
+
+
+==== tests/cases/compiler/letAsIdentifierInStrictMode.ts (7 errors) ====
+    "use strict";
+    var let = 10;
+        ~~~
+!!! error TS1134: Variable declaration expected.
+            ~
+!!! error TS1134: Variable declaration expected.
+              ~~
+!!! error TS1134: Variable declaration expected.
+    var a = 10;
+        ~
+!!! error TS2300: Duplicate identifier 'a'.
+    let = 30;
+        ~
+!!! error TS1134: Variable declaration expected.
+          ~~
+!!! error TS1134: Variable declaration expected.
+    let
+    a;
+    ~
+!!! error TS2300: Duplicate identifier 'a'.

tests/cases/compiler/letAsIdentifier.ts

@@ -0,0 +1,7 @@
+// @declaration: true
+
+var let = 10;
+var a = 10;
+let = 30;
+let
+a;

tests/cases/compiler/letAsIdentifierInStrictMode.ts

@@ -0,0 +1,6 @@
+"use strict";
+var let = 10;
+var a = 10;
+let = 30;
+let
+a;