Add tests

tests/cases/compiler/deferredLookupTypeResolution.ts

@@ -0,0 +1,28 @@
+// @strict: true
+// @declaration: true
+
+// Repro from #17456
+
+type StringContains<S extends string, L extends string> = (
+    { [K in S]:      'true' } &
+    { [key: string]: 'false' }
+  )[L]
+
+type ObjectHasKey<O, L extends string> = StringContains<keyof O, L>
+
+type First<T> = ObjectHasKey<T, '0'>;  // Should be deferred
+
+type T1 = ObjectHasKey<{ a: string }, 'a'>;  // 'true'
+type T2 = ObjectHasKey<{ a: string }, 'b'>;  // 'false'
+
+// Verify that mapped type isn't eagerly resolved in type-to-string operation
+
+declare function f1<A extends string, B extends string>(a: A, b: B): { [P in A | B]: any };
+
+function f2<A extends string>(a: A) {
+    return f1(a, 'x');
+}
+
+function f3(x: 'a' | 'b') {
+    return f2(x);
+}