Added tests for string literal types used as generic constraints.

tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTypeParameterConstraint01.ts

@@ -0,0 +1,19 @@
+// @declaration: true
+
+function foo<T extends "foo">(f: (x: T) => T) {
+    return f;
+}
+
+function bar<T extends "foo" | "bar">(f: (x: T) => T) {
+    return f;
+}
+
+let f = foo(x => x);
+let fResult = f("foo");
+
+let g = foo((x => x));
+let gResult = g("foo");
+
+let h = bar(x => x);
+let hResult = h("foo");
+hResult = h("bar");

tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTypeParameterConstraint02.ts

@@ -0,0 +1,8 @@
+// @declaration: true
+
+function foo<T extends "foo">(f: (x: T) => T) {
+    return f;
+}
+
+let f = foo((y: "foo" | "bar") => y === "foo" ? y : "foo");
+let fResult = f("foo");