Add tests

tests/cases/compiler/immediateCallbacks.ts

@@ -178,6 +178,56 @@ function ff4() {
     z;  // string | number
 }
 
+// Subtype reduction considers (cb: () => void) => a subtype of (immediate cb: () => void) => void
+
+declare function fa1(cb1: () => void): void;
+declare function fa2(immediate cb2: () => void): void;
+
+const fa = Math.random() > 0.5 ? fa1 : fa2;
+
+function fta() {
+    let x: string | number = "OK";
+    fa(() => {
+        x = 10;
+    });
+    x;  // string | number
+}
+
+declare function fb1(cb1: () => void): void;
+declare function fb2(immediate cb2: () => void): void;
+
+const fb = Math.random() > 0.5 ? fb2 : fb1;
+
+function ftb() {
+    let x: string | number = "OK";
+    fb(() => {
+        x = 10;
+    });
+    x;  // string | number
+}
+
+// A union signature parameter is immediate if any underlying parameter in same position is immediate
+
+declare const fc: ((immediate cb: () => void) => void) | ((cb: () => void) => void);
+
+function ftc() {
+    let x: string | number = "OK";
+    fc(() => {
+        x = 10;
+    });
+    x;  // string | number
+}
+
+declare const fd: ((cb: () => void) => void) | ((immediate cb: () => void) => void);
+
+function ftd() {
+    let x: string | number = "OK";
+    fd(() => {
+        x = 10;
+    });
+    x;  // string | number
+}
+
 // https://github.com/microsoft/TypeScript/issues/11498
 
 declare function mystery(immediate cb: () => void): void;