add use-case: type substraction / assertion (!)

Author: KiaraGrouwstra

tests/baselines/reference/typeCall.js

@@ -107,6 +107,12 @@ let strBool: isBool(string); // 0
 let anyBool: isBool(any); // 0
 let neverBool: isBool(never); // 0
 
+type Assert<T> = (<U>(v: U | null | undefined) => U)(T);
+let assert: Assert<string | undefined>; // string
+
+type Minus<A, B> = (<U>(v: U | B) => U)(A);
+let noNumbers: Minus<string | number, number>; // string
+
 interface ObjectHasStringIndex {
   // <T extends { [k: string]: any }>(o: T): T[string];
   (o: { [k: string]: any }): '1';
@@ -152,8 +158,8 @@ type T19 = Strip2<() => Obj>; // fails, unresolved, want { x: number, z: { kind:
 
 let a1: () => string;
 let b1: typeof a1();
-type Assert<T extends () => any> = T();
-let c1: Assert<typeof a1>;
+type Abc<T extends () => any> = T();
+let c1: Abc<typeof a1>;
 
 declare function infer1<T extends () => any>(x: T): T();
 infer1(null! as () => number);
@@ -221,6 +227,8 @@ var trueBool; // 1
 var strBool; // 0
 var anyBool; // 0
 var neverBool; // 0
+var assert; // string
+var noNumbers; // string
 var ObjectHasStringIndexTestT; // '1'
 var ObjectHasStringIndexTestF; // wanted '0', got '1'... so can't match for index, and erroring RHS yields `any`. ouch.
 var a1;