parse less than token rather than left shift in context of type argum…

src/compiler/parser.ts

@@ -1093,6 +1093,10 @@ namespace ts {
             return currentToken = scanner.reScanTemplateToken();
         }
 
+        function reScanLessThanToken(): SyntaxKind {
+            return currentToken = scanner.reScanLessThanToken();
+        }
+
         function scanJsxIdentifier(): SyntaxKind {
             return currentToken = scanner.scanJsxIdentifier();
         }
@@ -2276,7 +2280,7 @@ namespace ts {
         function parseTypeReference(): TypeReferenceNode {
             const node = <TypeReferenceNode>createNode(SyntaxKind.TypeReference);
             node.typeName = parseEntityName(/*allowReservedWords*/ true, Diagnostics.Type_expected);
-            if (!scanner.hasPrecedingLineBreak() && token() === SyntaxKind.LessThanToken) {
+            if (!scanner.hasPrecedingLineBreak() && reScanLessThanToken() === SyntaxKind.LessThanToken) {
                 node.typeArguments = parseBracketedList(ParsingContext.TypeArguments, parseType, SyntaxKind.LessThanToken, SyntaxKind.GreaterThanToken);
             }
             return finishNode(node);
@@ -4523,7 +4527,8 @@ namespace ts {
         function parseCallExpressionRest(expression: LeftHandSideExpression): LeftHandSideExpression {
             while (true) {
                 expression = parseMemberExpressionRest(expression);
-                if (token() === SyntaxKind.LessThanToken) {
+                // handle 'foo<<T>()'
+                if (token() === SyntaxKind.LessThanToken || token() === SyntaxKind.LessThanLessThanToken) {
                     // See if this is the start of a generic invocation.  If so, consume it and
                     // keep checking for postfix expressions.  Otherwise, it's just a '<' that's
                     // part of an arithmetic expression.  Break out so we consume it higher in the
@@ -4565,9 +4570,10 @@ namespace ts {
         }
 
         function parseTypeArgumentsInExpression() {
-            if (!parseOptional(SyntaxKind.LessThanToken)) {
+            if (reScanLessThanToken() !== SyntaxKind.LessThanToken) {
                 return undefined;
             }
+            nextToken();
 
             const typeArguments = parseDelimitedList(ParsingContext.TypeArguments, parseType);
             if (!parseExpected(SyntaxKind.GreaterThanToken)) {

src/compiler/scanner.ts

@@ -31,6 +31,7 @@ namespace ts {
         scanJsxIdentifier(): SyntaxKind;
         scanJsxAttributeValue(): SyntaxKind;
         reScanJsxToken(): JsxTokenSyntaxKind;
+        reScanLessThanToken(): SyntaxKind;
         scanJsxToken(): JsxTokenSyntaxKind;
         scanJSDocToken(): JsDocSyntaxKind;
         scan(): SyntaxKind;
@@ -874,6 +875,7 @@ namespace ts {
             scanJsxIdentifier,
             scanJsxAttributeValue,
             reScanJsxToken,
+            reScanLessThanToken,
             scanJsxToken,
             scanJSDocToken,
             scan,
@@ -1939,6 +1941,14 @@ namespace ts {
             return token = scanJsxToken();
         }
 
+        function reScanLessThanToken(): SyntaxKind {
+            if (token === SyntaxKind.LessThanLessThanToken) {
+                pos = tokenPos + 1;
+                return token = SyntaxKind.LessThanToken;
+            }
+            return token;
+        }
+
         function scanJsxToken(): JsxTokenSyntaxKind {
             startPos = tokenPos = pos;
 

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -3097,6 +3097,7 @@ declare namespace ts {
         scanJsxIdentifier(): SyntaxKind;
         scanJsxAttributeValue(): SyntaxKind;
         reScanJsxToken(): JsxTokenSyntaxKind;
+        reScanLessThanToken(): SyntaxKind;
         scanJsxToken(): JsxTokenSyntaxKind;
         scanJSDocToken(): JsDocSyntaxKind;
         scan(): SyntaxKind;

tests/baselines/reference/api/typescript.d.ts

@@ -3097,6 +3097,7 @@ declare namespace ts {
         scanJsxIdentifier(): SyntaxKind;
         scanJsxAttributeValue(): SyntaxKind;
         reScanJsxToken(): JsxTokenSyntaxKind;
+        reScanLessThanToken(): SyntaxKind;
         scanJsxToken(): JsxTokenSyntaxKind;
         scanJSDocToken(): JsDocSyntaxKind;
         scan(): SyntaxKind;

tests/baselines/reference/parseGenericArrowRatherThanLeftShift.js

@@ -0,0 +1,11 @@
+//// [parseGenericArrowRatherThanLeftShift.ts]
+type Bar = ReturnType<<T>(x: T) => number>;
+declare const a: Bar;
+
+function foo<T>(_x: T) {}
+const b = foo<<T>(x: T) => number>(() => 1);
+
+
+//// [parseGenericArrowRatherThanLeftShift.js]
+function foo(_x) { }
+var b = foo(function () { return 1; });

tests/baselines/reference/parseGenericArrowRatherThanLeftShift.symbols

@@ -0,0 +1,25 @@
+=== tests/cases/compiler/parseGenericArrowRatherThanLeftShift.ts ===
+type Bar = ReturnType<<T>(x: T) => number>;
+>Bar : Symbol(Bar, Decl(parseGenericArrowRatherThanLeftShift.ts, 0, 0))
+>ReturnType : Symbol(ReturnType, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(parseGenericArrowRatherThanLeftShift.ts, 0, 23))
+>x : Symbol(x, Decl(parseGenericArrowRatherThanLeftShift.ts, 0, 26))
+>T : Symbol(T, Decl(parseGenericArrowRatherThanLeftShift.ts, 0, 23))
+
+declare const a: Bar;
+>a : Symbol(a, Decl(parseGenericArrowRatherThanLeftShift.ts, 1, 13))
+>Bar : Symbol(Bar, Decl(parseGenericArrowRatherThanLeftShift.ts, 0, 0))
+
+function foo<T>(_x: T) {}
+>foo : Symbol(foo, Decl(parseGenericArrowRatherThanLeftShift.ts, 1, 21))
+>T : Symbol(T, Decl(parseGenericArrowRatherThanLeftShift.ts, 3, 13))
+>_x : Symbol(_x, Decl(parseGenericArrowRatherThanLeftShift.ts, 3, 16))
+>T : Symbol(T, Decl(parseGenericArrowRatherThanLeftShift.ts, 3, 13))
+
+const b = foo<<T>(x: T) => number>(() => 1);
+>b : Symbol(b, Decl(parseGenericArrowRatherThanLeftShift.ts, 4, 5))
+>foo : Symbol(foo, Decl(parseGenericArrowRatherThanLeftShift.ts, 1, 21))
+>T : Symbol(T, Decl(parseGenericArrowRatherThanLeftShift.ts, 4, 15))
+>x : Symbol(x, Decl(parseGenericArrowRatherThanLeftShift.ts, 4, 18))
+>T : Symbol(T, Decl(parseGenericArrowRatherThanLeftShift.ts, 4, 15))
+

tests/baselines/reference/parseGenericArrowRatherThanLeftShift.types

@@ -0,0 +1,20 @@
+=== tests/cases/compiler/parseGenericArrowRatherThanLeftShift.ts ===
+type Bar = ReturnType<<T>(x: T) => number>;
+>Bar : number
+>x : T
+
+declare const a: Bar;
+>a : number
+
+function foo<T>(_x: T) {}
+>foo : <T>(_x: T) => void
+>_x : T
+
+const b = foo<<T>(x: T) => number>(() => 1);
+>b : void
+>foo<<T>(x: T) => number>(() => 1) : void
+>foo : <T>(_x: T) => void
+>x : T
+>() => 1 : () => number
+>1 : 1
+

tests/cases/compiler/parseGenericArrowRatherThanLeftShift.ts

@@ -0,0 +1,5 @@
+type Bar = ReturnType<<T>(x: T) => number>;
+declare const a: Bar;
+
+function foo<T>(_x: T) {}
+const b = foo<<T>(x: T) => number>(() => 1);