allow trailing comman in type arguments list

src/compiler/checker.ts

@@ -27614,8 +27614,7 @@ namespace ts {
         }
 
         function checkGrammarTypeArguments(node: Node, typeArguments: NodeArray<TypeNode> | undefined): boolean {
-            return checkGrammarForDisallowedTrailingComma(typeArguments) ||
-                checkGrammarForAtLeastOneTypeArgument(node, typeArguments);
+            return checkGrammarForAtLeastOneTypeArgument(node, typeArguments);
         }
 
         function checkGrammarForOmittedArgument(args: NodeArray<Expression> | undefined): boolean {

tests/baselines/reference/allowTrailingCommasInTypeArguments.js

@@ -0,0 +1,17 @@
+//// [allowTrailingCommasInTypeArguments.ts]
+class FooClass<A, B, C,> {
+	a: A;
+	b: B;
+	c: C;
+}
+
+var a = new FooClass<number, number, number,>();
+
+
+//// [allowTrailingCommasInTypeArguments.js]
+var FooClass = /** @class */ (function () {
+    function FooClass() {
+    }
+    return FooClass;
+}());
+var a = new FooClass();

tests/baselines/reference/allowTrailingCommasInTypeArguments.symbols

@@ -0,0 +1,24 @@
+=== tests/cases/compiler/allowTrailingCommasInTypeArguments.ts ===
+class FooClass<A, B, C,> {
+>FooClass : Symbol(FooClass, Decl(allowTrailingCommasInTypeArguments.ts, 0, 0))
+>A : Symbol(A, Decl(allowTrailingCommasInTypeArguments.ts, 0, 15))
+>B : Symbol(B, Decl(allowTrailingCommasInTypeArguments.ts, 0, 17))
+>C : Symbol(C, Decl(allowTrailingCommasInTypeArguments.ts, 0, 20))
+
+	a: A;
+>a : Symbol(FooClass.a, Decl(allowTrailingCommasInTypeArguments.ts, 0, 26))
+>A : Symbol(A, Decl(allowTrailingCommasInTypeArguments.ts, 0, 15))
+
+	b: B;
+>b : Symbol(FooClass.b, Decl(allowTrailingCommasInTypeArguments.ts, 1, 6))
+>B : Symbol(B, Decl(allowTrailingCommasInTypeArguments.ts, 0, 17))
+
+	c: C;
+>c : Symbol(FooClass.c, Decl(allowTrailingCommasInTypeArguments.ts, 2, 6))
+>C : Symbol(C, Decl(allowTrailingCommasInTypeArguments.ts, 0, 20))
+}
+
+var a = new FooClass<number, number, number,>();
+>a : Symbol(a, Decl(allowTrailingCommasInTypeArguments.ts, 6, 3))
+>FooClass : Symbol(FooClass, Decl(allowTrailingCommasInTypeArguments.ts, 0, 0))
+

tests/baselines/reference/allowTrailingCommasInTypeArguments.types

@@ -0,0 +1,25 @@
+=== tests/cases/compiler/allowTrailingCommasInTypeArguments.ts ===
+class FooClass<A, B, C,> {
+>FooClass : FooClass<A, B, C>
+>A : A
+>B : B
+>C : C
+
+	a: A;
+>a : A
+>A : A
+
+	b: B;
+>b : B
+>B : B
+
+	c: C;
+>c : C
+>C : C
+}
+
+var a = new FooClass<number, number, number,>();
+>a : FooClass<number, number, number>
+>new FooClass<number, number, number,>() : FooClass<number, number, number>
+>FooClass : typeof FooClass
+

tests/baselines/reference/syntaxErrors.errors.txt

@@ -1,17 +1,14 @@
 tests/cases/conformance/jsdoc/badTypeArguments.js(1,15): error TS1099: Type argument list cannot be empty.
-tests/cases/conformance/jsdoc/badTypeArguments.js(2,22): error TS1009: Trailing comma not allowed.
 
 
 ==== tests/cases/conformance/jsdoc/dummyType.d.ts (0 errors) ====
     declare class C<T> { t: T }
 
-==== tests/cases/conformance/jsdoc/badTypeArguments.js (2 errors) ====
+==== tests/cases/conformance/jsdoc/badTypeArguments.js (1 errors) ====
     /** @param {C.<>} x */
                   ~~
 !!! error TS1099: Type argument list cannot be empty.
     /** @param {C.<number,>} y */
-                         ~
-!!! error TS1009: Trailing comma not allowed.
     // @ts-ignore
     /** @param {C.<number,>} skipped */
     function f(x, y, skipped) {

tests/cases/compiler/allowTrailingCommasInTypeArguments.ts

@@ -0,0 +1,7 @@
+class FooClass<A, B, C,> {
+	a: A;
+	b: B;
+	c: C;
+}
+
+var a = new FooClass<number, number, number,>();