Merge pull request #32460 from microsoft/fix32434

Improve type inference for types like 'T | Promise<T>'

Author: ahejlsberg

src/compiler/checker.ts

@@ -20,7 +20,7 @@ var e1: number | string | boolean;
 >e1 : string | number | boolean
 
 f1(a1); // string
->f1(a1) : string
+>f1(a1) : unknown
 >f1 : <T>(x: string | T) => T
 >a1 : string
 

tests/baselines/reference/unionAndIntersectionInference2.types

@@ -1,38 +1,61 @@
-tests/cases/conformance/types/typeRelationships/typeInference/unionTypeInference.ts(9,15): error TS2345: Argument of type '2' is not assignable to parameter of type 'string | 1'.
+tests/cases/conformance/types/typeRelationships/typeInference/unionTypeInference.ts(13,24): error TS2345: Argument of type '1' is not assignable to parameter of type 'string'.
+tests/cases/conformance/types/typeRelationships/typeInference/unionTypeInference.ts(31,15): error TS2345: Argument of type '42' is not assignable to parameter of type 'never'.
 
 
-==== tests/cases/conformance/types/typeRelationships/typeInference/unionTypeInference.ts (1 errors) ====
-    // Verify that inferences made *to* a type parameter in a union type are secondary
-    // to inferences made directly to that type parameter
+==== tests/cases/conformance/types/typeRelationships/typeInference/unionTypeInference.ts (2 errors) ====
+    declare const b: boolean;
+    declare const s: string;
+    declare const sn: string | number;
 
-    function f<T>(x: T, y: string|T): T {
-        return x;
+    declare function f1<T>(x: T, y: string | T): T;
+    
+    const a1 = f1(1, 2);  // 1 | 2
+    const a2 = f1(1, "hello");  // 1
+    const a3 = f1(1, sn);  // number
+    const a4 = f1(undefined, "abc");  // undefined
+    const a5 = f1("foo", "bar");  // "foo"
+    const a6 = f1(true, false);  // boolean
+    const a7 = f1("hello", 1);  // Error
+                           ~
+!!! error TS2345: Argument of type '1' is not assignable to parameter of type 'string'.
+    
+    declare function f2<T>(value: [string, T]): T;
+    
+    var b1 = f2(["string", true]);  // boolean
+    
+    declare function f3<T>(x: string | false | T): T;
+    
+    const c1 = f3(5);  // 5
+    const c2 = f3(sn);  // number
+    const c3 = f3(true);  // true
+    const c4 = f3(b);  // true
+    const c5 = f3("abc");  // never
+    
+    declare function f4<T>(x: string & T): T;
+    
+    const d1 = f4("abc");
+    const d2 = f4(s);
+    const d3 = f4(42);  // Error
+                  ~~
+!!! error TS2345: Argument of type '42' is not assignable to parameter of type 'never'.
+    
+    export interface Foo<T> {
+        then<U>(f: (x: T) => U | Foo<U>, g: U): Foo<U>;
+    }
+    export interface Bar<T> {
+        then<S>(f: (x: T) => S | Bar<S>, g: S): Bar<S>;
     }
 
-    var a1: number;
-    var a1 = f(1, 2);
-                  ~
-!!! error TS2345: Argument of type '2' is not assignable to parameter of type 'string | 1'.
-    var a2: number;
-    var a2 = f(1, "hello");
-    var a3: number;
-    var a3 = f(1, a1 || "hello");
-    var a4: any;
-    var a4 = f(undefined, "abc");
-    
-    function g<T>(value: [string, T]): T {
-        return value[1];
+    function qux(p1: Foo<void>, p2: Bar<void>) {
+        p1 = p2;
     }
 
-    var b1: boolean;
-    var b1 = g(["string", true]);
+    // Repros from #32434
 
-    function h<T>(x: string|boolean|T): T {
-        return typeof x === "string" || typeof x === "boolean" ? undefined : x;
-    }
+    declare function foo<T>(x: T | Promise<T>): void;
+    declare let x: false | Promise<true>;
+    foo(x);
 
-    var c1: number;
-    var c1 = h(5);
-    var c2: string;
-    var c2 = h("abc");
+    declare function bar<T>(x: T, y: string | T): T;
+    const y = bar(1, 2);
 

tests/baselines/reference/unionTypeInference.errors.txt

@@ -1,60 +1,78 @@
 //// [unionTypeInference.ts]
-// Verify that inferences made *to* a type parameter in a union type are secondary
-// to inferences made directly to that type parameter
+declare const b: boolean;
+declare const s: string;
+declare const sn: string | number;
 
-function f<T>(x: T, y: string|T): T {
-    return x;
+declare function f1<T>(x: T, y: string | T): T;
+
+const a1 = f1(1, 2);  // 1 | 2
+const a2 = f1(1, "hello");  // 1
+const a3 = f1(1, sn);  // number
+const a4 = f1(undefined, "abc");  // undefined
+const a5 = f1("foo", "bar");  // "foo"
+const a6 = f1(true, false);  // boolean
+const a7 = f1("hello", 1);  // Error
+
+declare function f2<T>(value: [string, T]): T;
+
+var b1 = f2(["string", true]);  // boolean
+
+declare function f3<T>(x: string | false | T): T;
+
+const c1 = f3(5);  // 5
+const c2 = f3(sn);  // number
+const c3 = f3(true);  // true
+const c4 = f3(b);  // true
+const c5 = f3("abc");  // never
+
+declare function f4<T>(x: string & T): T;
+
+const d1 = f4("abc");
+const d2 = f4(s);
+const d3 = f4(42);  // Error
+
+export interface Foo<T> {
+    then<U>(f: (x: T) => U | Foo<U>, g: U): Foo<U>;
+}
+export interface Bar<T> {
+    then<S>(f: (x: T) => S | Bar<S>, g: S): Bar<S>;
 }
 
-var a1: number;
-var a1 = f(1, 2);
-var a2: number;
-var a2 = f(1, "hello");
-var a3: number;
-var a3 = f(1, a1 || "hello");
-var a4: any;
-var a4 = f(undefined, "abc");
-
-function g<T>(value: [string, T]): T {
-    return value[1];
+function qux(p1: Foo<void>, p2: Bar<void>) {
+    p1 = p2;
 }
 
-var b1: boolean;
-var b1 = g(["string", true]);
+// Repros from #32434
 
-function h<T>(x: string|boolean|T): T {
-    return typeof x === "string" || typeof x === "boolean" ? undefined : x;
-}
+declare function foo<T>(x: T | Promise<T>): void;
+declare let x: false | Promise<true>;
+foo(x);
 
-var c1: number;
-var c1 = h(5);
-var c2: string;
-var c2 = h("abc");
+declare function bar<T>(x: T, y: string | T): T;
+const y = bar(1, 2);
 
 
 //// [unionTypeInference.js]
-// Verify that inferences made *to* a type parameter in a union type are secondary
-// to inferences made directly to that type parameter
-function f(x, y) {
-    return x;
-}
-var a1;
-var a1 = f(1, 2);
-var a2;
-var a2 = f(1, "hello");
-var a3;
-var a3 = f(1, a1 || "hello");
-var a4;
-var a4 = f(undefined, "abc");
-function g(value) {
-    return value[1];
-}
-var b1;
-var b1 = g(["string", true]);
-function h(x) {
-    return typeof x === "string" || typeof x === "boolean" ? undefined : x;
+"use strict";
+exports.__esModule = true;
+var a1 = f1(1, 2); // 1 | 2
+var a2 = f1(1, "hello"); // 1
+var a3 = f1(1, sn); // number
+var a4 = f1(undefined, "abc"); // undefined
+var a5 = f1("foo", "bar"); // "foo"
+var a6 = f1(true, false); // boolean
+var a7 = f1("hello", 1); // Error
+var b1 = f2(["string", true]); // boolean
+var c1 = f3(5); // 5
+var c2 = f3(sn); // number
+var c3 = f3(true); // true
+var c4 = f3(b); // true
+var c5 = f3("abc"); // never
+var d1 = f4("abc");
+var d2 = f4(s);
+var d3 = f4(42); // Error
+function qux(p1, p2) {
+    p1 = p2;
 }
-var c1;
-var c1 = h(5);
-var c2;
-var c2 = h("abc");
+foo(x);
+var y = bar(1, 2);