Improve reduction of similar intersections in type inference (#51405)

* Change criteria for reducing intersections in type inference

* Add regression test

Author: ahejlsberg

src/compiler/checker.ts

@@ -22728,14 +22728,11 @@ namespace ts {
                     }
                     source = getUnionType(sources);
                 }
-                else if (target.flags & TypeFlags.Intersection && some((target as IntersectionType).types,
-                    t => !!getInferenceInfoForType(t) || (isGenericMappedType(t) && !!getInferenceInfoForType(getHomomorphicTypeVariable(t) || neverType)))) {
-                    // We reduce intersection types only when they contain naked type parameters. For example, when
-                    // inferring from 'string[] & { extra: any }' to 'string[] & T' we want to remove string[] and
+                else if (target.flags & TypeFlags.Intersection && !every((target as IntersectionType).types, isNonGenericObjectType)) {
+                    // We reduce intersection types unless they're simple combinations of object types. For example,
+                    // when inferring from 'string[] & { extra: any }' to 'string[] & T' we want to remove string[] and
                     // infer { extra: any } for T. But when inferring to 'string[] & Iterable<T>' we want to keep the
                     // string[] on the source side and infer string for T.
-                    // Likewise, we consider a homomorphic mapped type constrainted to the target type parameter as similar to a "naked type variable"
-                    // in such scenarios.
                     if (!(source.flags & TypeFlags.Union)) {
                         // Infer between identically matching source and target constituents and remove the matching types.
                         const [sources, targets] = inferFromMatchingTypes(source.flags & TypeFlags.Intersection ? (source as IntersectionType).types : [source], (target as IntersectionType).types, isTypeIdenticalTo);

tests/baselines/reference/unionAndIntersectionInference3.js

@@ -86,6 +86,12 @@ declare function foo<T, U>(obj: T & AB<U>): [T, U];
 declare let ab: AB<string>;
 
 let z = foo(ab);  // [AB<string>, string]
+
+// Repro from #51399
+
+declare let a: <T>() => (T extends true ? true : false) & boolean;
+declare let b: <T>() => (T extends true ? true : false) & boolean;
+a = b;
 
 
 //// [unionAndIntersectionInference3.js]
@@ -105,4 +111,5 @@ let x2 = foo2(sa); // unknown
 let y2 = foo2(sx); // { extra: number }
 withRouter(MyComponent);
 let z = foo(ab); // [AB<string>, string]
+a = b;
 export {};

tests/baselines/reference/unionAndIntersectionInference3.symbols

@@ -309,3 +309,19 @@ let z = foo(ab);  // [AB<string>, string]
 >foo : Symbol(foo, Decl(unionAndIntersectionInference3.ts, 80, 33))
 >ab : Symbol(ab, Decl(unionAndIntersectionInference3.ts, 84, 11))
 
+// Repro from #51399
+
+declare let a: <T>() => (T extends true ? true : false) & boolean;
+>a : Symbol(a, Decl(unionAndIntersectionInference3.ts, 90, 11))
+>T : Symbol(T, Decl(unionAndIntersectionInference3.ts, 90, 16))
+>T : Symbol(T, Decl(unionAndIntersectionInference3.ts, 90, 16))
+
+declare let b: <T>() => (T extends true ? true : false) & boolean;
+>b : Symbol(b, Decl(unionAndIntersectionInference3.ts, 91, 11))
+>T : Symbol(T, Decl(unionAndIntersectionInference3.ts, 91, 16))
+>T : Symbol(T, Decl(unionAndIntersectionInference3.ts, 91, 16))
+
+a = b;
+>a : Symbol(a, Decl(unionAndIntersectionInference3.ts, 90, 11))
+>b : Symbol(b, Decl(unionAndIntersectionInference3.ts, 91, 11))
+

tests/baselines/reference/unionAndIntersectionInference3.types

@@ -199,3 +199,22 @@ let z = foo(ab);  // [AB<string>, string]
 >foo : <T, U>(obj: T & AB<U>) => [T, U]
 >ab : AB<string>
 
+// Repro from #51399
+
+declare let a: <T>() => (T extends true ? true : false) & boolean;
+>a : <T>() => (T extends true ? true : false) & boolean
+>true : true
+>true : true
+>false : false
+
+declare let b: <T>() => (T extends true ? true : false) & boolean;
+>b : <T>() => (T extends true ? true : false) & boolean
+>true : true
+>true : true
+>false : false
+
+a = b;
+>a = b : <T>() => (T extends true ? true : false) & boolean
+>a : <T>() => (T extends true ? true : false) & boolean
+>b : <T>() => (T extends true ? true : false) & boolean
+

tests/cases/conformance/types/typeRelationships/typeInference/unionAndIntersectionInference3.ts

@@ -88,3 +88,9 @@ declare function foo<T, U>(obj: T & AB<U>): [T, U];
 declare let ab: AB<string>;
 
 let z = foo(ab);  // [AB<string>, string]
+
+// Repro from #51399
+
+declare let a: <T>() => (T extends true ? true : false) & boolean;
+declare let b: <T>() => (T extends true ? true : false) & boolean;
+a = b;