Add tests

tests/cases/conformance/types/literal/literalTypeWidening.ts

@@ -0,0 +1,97 @@
+// Widening vs. non-widening literal types
+
+function f1() {
+    const c1 = "hello";  // Widening type "hello"
+    let v1 = c1;  // Type string
+    const c2 = c1;  // Widening type "hello"
+    let v2 = c2;  // Type string
+    const c3: "hello" = "hello";  // Type "hello"
+    let v3 = c3;  // Type "hello"
+    const c4: "hello" = c1;  // Type "hello"
+    let v4 = c4;  // Type "hello"
+}
+
+function f2(cond: boolean) {
+    const c1 = cond ? "foo" : "bar";  // widening "foo" | widening "bar"
+    const c2: "foo" | "bar" = c1;  // "foo" | "bar"
+    const c3 = cond ? c1 : c2;  // "foo" | "bar"
+    const c4 = cond ? c3 : "baz";  // "foo" | "bar" | widening "baz"
+    const c5: "foo" | "bar" | "baz" = c4; // "foo" | "bar" | "baz"
+    let v1 = c1;  // string
+    let v2 = c2;  // "foo" | "bar"
+    let v3 = c3;  // "foo" | "bar"
+    let v4 = c4;  // string
+    let v5 = c5;  // "foo" | "bar" | "baz"
+}
+
+function f3() {
+    const c1 = 123;  // Widening type 123
+    let v1 = c1;  // Type number
+    const c2 = c1;  // Widening type 123
+    let v2 = c2;  // Type number
+    const c3: 123 = 123;  // Type 123
+    let v3 = c3;  // Type 123
+    const c4: 123 = c1;  // Type 123
+    let v4 = c4;  // Type 123
+}
+
+function f4(cond: boolean) {
+    const c1 = cond ? 123 : 456;  // widening 123 | widening 456
+    const c2: 123 | 456 = c1;  // 123 | 456
+    const c3 = cond ? c1 : c2;  // 123 | 456
+    const c4 = cond ? c3 : 789;  // 123 | 456 | widening 789
+    const c5: 123 | 456 | 789 = c4; // 123 | 456 | 789
+    let v1 = c1;  // number
+    let v2 = c2;  // 123 | 456
+    let v3 = c3;  // 123 | 456
+    let v4 = c4;  // number
+    let v5 = c5;  // 123 | 456 | 789
+}
+
+function f5() {
+    const c1 = "foo";
+    let v1 = c1;
+    const c2: "foo" = "foo";
+    let v2 = c2;
+    const c3 = "foo" as "foo";
+    let v3 = c3;
+    const c4 = <"foo">"foo";
+    let v4 = c4;
+}
+
+// Repro from #10898
+
+type FAILURE = "FAILURE";
+const FAILURE = "FAILURE";
+
+type Result<T> = T | FAILURE;
+
+function doWork<T>(): Result<T> {
+  return FAILURE;
+}
+
+function isSuccess<T>(result: Result<T>): result is T {
+  return !isFailure(result);
+}
+
+function isFailure<T>(result: Result<T>): result is FAILURE {
+  return result === FAILURE;
+}
+
+function increment(x: number): number {
+  return x + 1;
+}
+
+let result = doWork<number>();
+
+if (isSuccess(result)) {
+  increment(result);
+}
+
+// Repro from #10898
+
+type TestEvent = "onmouseover" | "onmouseout";
+
+function onMouseOver(): TestEvent { return "onmouseover"; }
+
+let x = onMouseOver();