Improve inference to union and intersection types

src/compiler/checker.ts

@@ -14473,26 +14473,15 @@ namespace ts {
                     inferFromTypes(source, getUnionType([getTrueTypeFromConditionalType(<ConditionalType>target), getFalseTypeFromConditionalType(<ConditionalType>target)]));
                 }
                 else if (target.flags & TypeFlags.UnionOrIntersection) {
-                    const targetTypes = (<UnionOrIntersectionType>target).types;
-                    let typeVariableCount = 0;
-                    let typeVariable: TypeParameter | IndexedAccessType | undefined;
-                    // First infer to each type in union or intersection that isn't a type variable
-                    for (const t of targetTypes) {
+                    for (const t of (<UnionOrIntersectionType>target).types) {
+                        const savePriority = priority;
+                        // Inferences directly to naked type variables are given lower priority as they are
+                        // less specific. For example, when inferring from Promise<string> to T | Promise<T>,
+                        // we want to infer string for T, not Promise<string> | string.
                         if (getInferenceInfoForType(t)) {
-                            typeVariable = <InstantiableType>t;
-                            typeVariableCount++;
-                        }
-                        else {
-                            inferFromTypes(source, t);
+                            priority |= InferencePriority.NakedTypeVariable;
                         }
-                    }
-                    // Next, if target containings a single naked type variable, make a secondary inference to that type
-                    // variable. This gives meaningful results for union types in co-variant positions and intersection
-                    // types in contra-variant positions (such as callback parameters).
-                    if (typeVariableCount === 1) {
-                        const savePriority = priority;
-                        priority |= InferencePriority.NakedTypeVariable;
-                        inferFromTypes(source, typeVariable!);
+                        inferFromTypes(source, t);
                         priority = savePriority;
                     }
                 }

tests/baselines/reference/conditionalTypeDoesntSpinForever.types

@@ -120,9 +120,9 @@ export enum PubSubRecordIsStoredInRedisAsA {
 
         buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString})) as
 >buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString})) as          BuildPubSubRecordType<SO_FAR & {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString}> : BuildPubSubRecordType<SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString; }>
->buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString})) : BuildPubSubRecordType<SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA; }>
+>buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString})) : BuildPubSubRecordType<SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString; }>
 >buildPubSubRecordType : <SO_FAR>(soFar: SO_FAR) => BuildPubSubRecordType<SO_FAR>
->Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString}) : SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA; }
+>Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString}) : SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA.jsonEncodedRedisString; }
 >Object.assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
 >Object : ObjectConstructor
 >assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
@@ -144,9 +144,9 @@ export enum PubSubRecordIsStoredInRedisAsA {
 
         buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash})) as
 >buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash})) as          BuildPubSubRecordType<SO_FAR & {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash}> : BuildPubSubRecordType<SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA.redisHash; }>
->buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash})) : BuildPubSubRecordType<SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA; }>
+>buildPubSubRecordType(Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash})) : BuildPubSubRecordType<SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA.redisHash; }>
 >buildPubSubRecordType : <SO_FAR>(soFar: SO_FAR) => BuildPubSubRecordType<SO_FAR>
->Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash}) : SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA; }
+>Object.assign({}, soFar, {storedAs: PubSubRecordIsStoredInRedisAsA.redisHash}) : SO_FAR & { storedAs: PubSubRecordIsStoredInRedisAsA.redisHash; }
 >Object.assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
 >Object : ObjectConstructor
 >assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
@@ -337,16 +337,16 @@ export enum PubSubRecordIsStoredInRedisAsA {
 
         buildPubSubRecordType(Object.assign({}, soFar, {maxMsToWaitBeforePublishing: 0})) as BuildPubSubRecordType<SO_FAR & {maxMsToWaitBeforePublishing: 0}>,
 >buildPubSubRecordType(Object.assign({}, soFar, {maxMsToWaitBeforePublishing: 0})) as BuildPubSubRecordType<SO_FAR & {maxMsToWaitBeforePublishing: 0}> : BuildPubSubRecordType<SO_FAR & { maxMsToWaitBeforePublishing: 0; }>
->buildPubSubRecordType(Object.assign({}, soFar, {maxMsToWaitBeforePublishing: 0})) : BuildPubSubRecordType<SO_FAR & { maxMsToWaitBeforePublishing: number; }>
+>buildPubSubRecordType(Object.assign({}, soFar, {maxMsToWaitBeforePublishing: 0})) : BuildPubSubRecordType<SO_FAR & { maxMsToWaitBeforePublishing: 0; }>
 >buildPubSubRecordType : <SO_FAR>(soFar: SO_FAR) => BuildPubSubRecordType<SO_FAR>
->Object.assign({}, soFar, {maxMsToWaitBeforePublishing: 0}) : SO_FAR & { maxMsToWaitBeforePublishing: number; }
+>Object.assign({}, soFar, {maxMsToWaitBeforePublishing: 0}) : SO_FAR & { maxMsToWaitBeforePublishing: 0; }
 >Object.assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
 >Object : ObjectConstructor
 >assign : { <T, U>(target: T, source: U): T & U; <T, U, V>(target: T, source1: U, source2: V): T & U & V; <T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W; (target: object, ...sources: any[]): any; }
 >{} : {}
 >soFar : SO_FAR
->{maxMsToWaitBeforePublishing: 0} : { maxMsToWaitBeforePublishing: number; }
->maxMsToWaitBeforePublishing : number
+>{maxMsToWaitBeforePublishing: 0} : { maxMsToWaitBeforePublishing: 0; }
+>maxMsToWaitBeforePublishing : 0
 >0 : 0
 >maxMsToWaitBeforePublishing : 0
     }

tests/baselines/reference/jqueryInference.js

@@ -11,7 +11,7 @@ declare function shouldBeIdentity<T, U>(p: DoNothingAlias<T, U>): MyPromise<T, U
 
 declare const p1: MyPromise<boolean, any>;
 var p2 = shouldBeIdentity(p1);
-var p2: MyPromise<boolean, {}>;
+var p2: MyPromise<boolean, any>;
 
 
 //// [jqueryInference.js]

tests/baselines/reference/jqueryInference.symbols

@@ -48,7 +48,7 @@ var p2 = shouldBeIdentity(p1);
 >shouldBeIdentity : Symbol(shouldBeIdentity, Decl(jqueryInference.ts, 6, 58))
 >p1 : Symbol(p1, Decl(jqueryInference.ts, 10, 13))
 
-var p2: MyPromise<boolean, {}>;
+var p2: MyPromise<boolean, any>;
 >p2 : Symbol(p2, Decl(jqueryInference.ts, 11, 3), Decl(jqueryInference.ts, 12, 3))
 >MyPromise : Symbol(MyPromise, Decl(jqueryInference.ts, 0, 0))
 

tests/baselines/reference/jqueryInference.types

@@ -22,11 +22,11 @@ declare const p1: MyPromise<boolean, any>;
 >p1 : MyPromise<boolean, any>
 
 var p2 = shouldBeIdentity(p1);
->p2 : MyPromise<boolean, {}>
->shouldBeIdentity(p1) : MyPromise<boolean, {}>
+>p2 : MyPromise<boolean, any>
+>shouldBeIdentity(p1) : MyPromise<boolean, any>
 >shouldBeIdentity : <T, U>(p: DoNothingAlias<T, U>) => MyPromise<T, U>
 >p1 : MyPromise<boolean, any>
 
-var p2: MyPromise<boolean, {}>;
->p2 : MyPromise<boolean, {}>
+var p2: MyPromise<boolean, any>;
+>p2 : MyPromise<boolean, any>
 

tests/baselines/reference/objectSpread.types

@@ -602,7 +602,7 @@ let exclusive: { id: string, a: number, b: string, c: string, d: boolean } =
 >d : boolean
 
     f({ a: 1, b: 'yes' }, { c: 'no', d: false })
->f({ a: 1, b: 'yes' }, { c: 'no', d: false }) : { a: number; b: string; } & { c: string; d: boolean; } & { id: string; }
+>f({ a: 1, b: 'yes' }, { c: 'no', d: false }) : { a: number; b: string; } & { c: string; d: false; } & { id: string; }
 >f : <T, U>(t: T, u: U) => T & U & { id: string; }
 >{ a: 1, b: 'yes' } : { a: number; b: string; }
 >a : number

tests/baselines/reference/restTupleElements1.types

@@ -173,7 +173,7 @@ f0([]);  // Error
 >[] : never[]
 
 f0([1]);
->f0([1]) : [number, {}]
+>f0([1]) : [number, number]
 >f0 : <T, U>(x: [T, ...U[]]) => [T, U]
 >[1] : [number]
 >1 : 1

tests/baselines/reference/unionAndIntersectionInference1.js

@@ -71,6 +71,24 @@ declare var mbp: Man & Bear;
 
 pigify(mbp).oinks; // OK, mbp is treated as Pig
 pigify(mbp).walks; // Ok, mbp is treated as Man
+
+// Repros from #29815
+
+interface ITest {
+  name: 'test'
+}
+
+const createTestAsync = (): Promise<ITest> => Promise.resolve().then(() => ({ name: 'test' }))
+
+const createTest = (): ITest => {
+  return { name: 'test' }
+}
+
+declare function f1<T, U>(x: T | U): T | U;
+declare function f2<T, U>(x: T & U): T & U;
+
+let x1: string = f1('a');
+let x2: string = f2('a');
 
 
 //// [unionAndIntersectionInference1.js]
@@ -80,7 +98,7 @@ function destructure(something, haveValue, haveY) {
     return something === y ? haveY(y) : haveValue(something);
 }
 var value = Math.random() > 0.5 ? 'hey!' : undefined;
-var result = destructure(value, function (text) { return 'string'; }, function (y) { return 'other one'; }); // text: string, y: Y
+var result = destructure(value, text => 'string', y => 'other one'); // text: string, y: Y
 // Repro from #4212
 function isVoid(value) {
     return undefined;
@@ -107,7 +125,13 @@ function baz1(value) {
 function get(x) {
     return null; // just an example
 }
-var foo;
+let foo;
 get(foo).toUpperCase(); // Ok
 pigify(mbp).oinks; // OK, mbp is treated as Pig
 pigify(mbp).walks; // Ok, mbp is treated as Man
+const createTestAsync = () => Promise.resolve().then(() => ({ name: 'test' }));
+const createTest = () => {
+    return { name: 'test' };
+};
+let x1 = f1('a');
+let x2 = f2('a');

tests/baselines/reference/unionAndIntersectionInference1.symbols

@@ -50,7 +50,7 @@ function destructure<a, r>(
 var value = Math.random() > 0.5 ? 'hey!' : <Y>undefined;
 >value : Symbol(value, Decl(unionAndIntersectionInference1.ts, 12, 3))
 >Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
->Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 >random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
 >Y : Symbol(Y, Decl(unionAndIntersectionInference1.ts, 0, 0))
 >undefined : Symbol(undefined)
@@ -201,3 +201,59 @@ pigify(mbp).walks; // Ok, mbp is treated as Man
 >mbp : Symbol(mbp, Decl(unionAndIntersectionInference1.ts, 68, 11))
 >walks : Symbol(Man.walks, Decl(unionAndIntersectionInference1.ts, 55, 15))
 
+// Repros from #29815
+
+interface ITest {
+>ITest : Symbol(ITest, Decl(unionAndIntersectionInference1.ts, 71, 18))
+
+  name: 'test'
+>name : Symbol(ITest.name, Decl(unionAndIntersectionInference1.ts, 75, 17))
+}
+
+const createTestAsync = (): Promise<ITest> => Promise.resolve().then(() => ({ name: 'test' }))
+>createTestAsync : Symbol(createTestAsync, Decl(unionAndIntersectionInference1.ts, 79, 5))
+>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
+>ITest : Symbol(ITest, Decl(unionAndIntersectionInference1.ts, 71, 18))
+>Promise.resolve().then : Symbol(Promise.then, Decl(lib.es5.d.ts, --, --))
+>Promise.resolve : Symbol(PromiseConstructor.resolve, Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --))
+>Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
+>resolve : Symbol(PromiseConstructor.resolve, Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --))
+>then : Symbol(Promise.then, Decl(lib.es5.d.ts, --, --))
+>name : Symbol(name, Decl(unionAndIntersectionInference1.ts, 79, 77))
+
+const createTest = (): ITest => {
+>createTest : Symbol(createTest, Decl(unionAndIntersectionInference1.ts, 81, 5))
+>ITest : Symbol(ITest, Decl(unionAndIntersectionInference1.ts, 71, 18))
+
+  return { name: 'test' }
+>name : Symbol(name, Decl(unionAndIntersectionInference1.ts, 82, 10))
+}
+
+declare function f1<T, U>(x: T | U): T | U;
+>f1 : Symbol(f1, Decl(unionAndIntersectionInference1.ts, 83, 1))
+>T : Symbol(T, Decl(unionAndIntersectionInference1.ts, 85, 20))
+>U : Symbol(U, Decl(unionAndIntersectionInference1.ts, 85, 22))
+>x : Symbol(x, Decl(unionAndIntersectionInference1.ts, 85, 26))
+>T : Symbol(T, Decl(unionAndIntersectionInference1.ts, 85, 20))
+>U : Symbol(U, Decl(unionAndIntersectionInference1.ts, 85, 22))
+>T : Symbol(T, Decl(unionAndIntersectionInference1.ts, 85, 20))
+>U : Symbol(U, Decl(unionAndIntersectionInference1.ts, 85, 22))
+
+declare function f2<T, U>(x: T & U): T & U;
+>f2 : Symbol(f2, Decl(unionAndIntersectionInference1.ts, 85, 43))
+>T : Symbol(T, Decl(unionAndIntersectionInference1.ts, 86, 20))
+>U : Symbol(U, Decl(unionAndIntersectionInference1.ts, 86, 22))
+>x : Symbol(x, Decl(unionAndIntersectionInference1.ts, 86, 26))
+>T : Symbol(T, Decl(unionAndIntersectionInference1.ts, 86, 20))
+>U : Symbol(U, Decl(unionAndIntersectionInference1.ts, 86, 22))
+>T : Symbol(T, Decl(unionAndIntersectionInference1.ts, 86, 20))
+>U : Symbol(U, Decl(unionAndIntersectionInference1.ts, 86, 22))
+
+let x1: string = f1('a');
+>x1 : Symbol(x1, Decl(unionAndIntersectionInference1.ts, 88, 3))
+>f1 : Symbol(f1, Decl(unionAndIntersectionInference1.ts, 83, 1))
+
+let x2: string = f2('a');
+>x2 : Symbol(x2, Decl(unionAndIntersectionInference1.ts, 89, 3))
+>f2 : Symbol(f2, Decl(unionAndIntersectionInference1.ts, 85, 43))
+

tests/baselines/reference/unionAndIntersectionInference1.types

@@ -179,3 +179,56 @@ pigify(mbp).walks; // Ok, mbp is treated as Man
 >mbp : Man & Bear
 >walks : boolean
 
+// Repros from #29815
+
+interface ITest {
+  name: 'test'
+>name : "test"
+}
+
+const createTestAsync = (): Promise<ITest> => Promise.resolve().then(() => ({ name: 'test' }))
+>createTestAsync : () => Promise<ITest>
+>(): Promise<ITest> => Promise.resolve().then(() => ({ name: 'test' })) : () => Promise<ITest>
+>Promise.resolve().then(() => ({ name: 'test' })) : Promise<ITest | { name: "test"; }>
+>Promise.resolve().then : <TResult1 = void, TResult2 = never>(onfulfilled?: (value: void) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>Promise.resolve() : Promise<void>
+>Promise.resolve : { <T>(value: T | PromiseLike<T>): Promise<T>; (): Promise<void>; }
+>Promise : PromiseConstructor
+>resolve : { <T>(value: T | PromiseLike<T>): Promise<T>; (): Promise<void>; }
+>then : <TResult1 = void, TResult2 = never>(onfulfilled?: (value: void) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>() => ({ name: 'test' }) : () => { name: "test"; }
+>({ name: 'test' }) : { name: "test"; }
+>{ name: 'test' } : { name: "test"; }
+>name : "test"
+>'test' : "test"
+
+const createTest = (): ITest => {
+>createTest : () => ITest
+>(): ITest => {  return { name: 'test' }} : () => ITest
+
+  return { name: 'test' }
+>{ name: 'test' } : { name: "test"; }
+>name : "test"
+>'test' : "test"
+}
+
+declare function f1<T, U>(x: T | U): T | U;
+>f1 : <T, U>(x: T | U) => T | U
+>x : T | U
+
+declare function f2<T, U>(x: T & U): T & U;
+>f2 : <T, U>(x: T & U) => T & U
+>x : T & U
+
+let x1: string = f1('a');
+>x1 : string
+>f1('a') : "a"
+>f1 : <T, U>(x: T | U) => T | U
+>'a' : "a"
+
+let x2: string = f2('a');
+>x2 : string
+>f2('a') : "a"
+>f2 : <T, U>(x: T & U) => T & U
+>'a' : "a"
+

tests/cases/compiler/jqueryInference.ts

@@ -10,4 +10,4 @@ declare function shouldBeIdentity<T, U>(p: DoNothingAlias<T, U>): MyPromise<T, U
 
 declare const p1: MyPromise<boolean, any>;
 var p2 = shouldBeIdentity(p1);
-var p2: MyPromise<boolean, {}>;
+var p2: MyPromise<boolean, any>;

tests/cases/conformance/types/typeRelationships/typeInference/unionAndIntersectionInference1.ts

@@ -1,3 +1,5 @@
+// @target: es2015
+
 // Repro from #2264
 
 interface Y { 'i am a very certain type': Y }
@@ -70,3 +72,21 @@ declare var mbp: Man & Bear;
 
 pigify(mbp).oinks; // OK, mbp is treated as Pig
 pigify(mbp).walks; // Ok, mbp is treated as Man
+
+// Repros from #29815
+
+interface ITest {
+  name: 'test'
+}
+
+const createTestAsync = (): Promise<ITest> => Promise.resolve().then(() => ({ name: 'test' }))
+
+const createTest = (): ITest => {
+  return { name: 'test' }
+}
+
+declare function f1<T, U>(x: T | U): T | U;
+declare function f2<T, U>(x: T & U): T & U;
+
+let x1: string = f1('a');
+let x2: string = f2('a');