Use strict subtype relation in getCommonSupertype

internal/checker/inference.go

@@ -1462,11 +1462,7 @@ func (c *Checker) getCommonSupertype(types []*Type) *Type {
 	if c.literalTypesWithSameBaseType(primaryTypes) {
 		supertype = c.getUnionType(primaryTypes)
 	} else {
-		for _, t := range primaryTypes {
-			if supertype == nil || c.isTypeSubtypeOf(supertype, t) {
-				supertype = t
-			}
-		}
+		supertype = c.getSingleCommonSupertype(primaryTypes)
 	}
 	// Add any nullable types that occurred in the candidates back to the result.
 	if core.Same(primaryTypes, types) {
@@ -1475,6 +1471,27 @@ func (c *Checker) getCommonSupertype(types []*Type) *Type {
 	return c.getNullableType(supertype, c.getCombinedTypeFlags(types)&TypeFlagsNullable)
 }
 
+func (c *Checker) getSingleCommonSupertype(types []*Type) *Type {
+	// First, find the leftmost type for which no type to the right is a strict supertype, and if that
+	// type is a strict supertype of all other candidates, return it. Otherwise, return the leftmost type
+	// for which no type to the right is a (regular) supertype.
+	candidate := c.findLeftmostType(types, (*Checker).isTypeStrictSubtypeOf)
+	if core.Every(types, func(t *Type) bool { return t == candidate || c.isTypeStrictSubtypeOf(t, candidate) }) {
+		return candidate
+	}
+	return c.findLeftmostType(types, (*Checker).isTypeSubtypeOf)
+}
+
+func (c *Checker) findLeftmostType(types []*Type, f func(c *Checker, s *Type, t *Type) bool) *Type {
+	var candidate *Type
+	for _, t := range types {
+		if candidate == nil || f(c, candidate, t) {
+			candidate = t
+		}
+	}
+	return candidate
+}
+
 // Return the leftmost type for which no type to the right is a subtype.
 func (c *Checker) getCommonSubtype(types []*Type) *Type {
 	var subtype *Type

testdata/baselines/reference/compiler/strictBestCommonSupertype.symbols

@@ -0,0 +1,64 @@
+//// [tests/cases/compiler/strictBestCommonSupertype.ts] ////
+
+=== strictBestCommonSupertype.ts ===
+// https://github.com/microsoft/typescript-go/issues/1222
+
+class Store<T = object> {
+>Store : Symbol(Store, Decl(strictBestCommonSupertype.ts, 0, 0))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 2, 12))
+
+    select<K>(mapFn: (state: T) => K) {};
+>select : Symbol(select, Decl(strictBestCommonSupertype.ts, 2, 25))
+>K : Symbol(K, Decl(strictBestCommonSupertype.ts, 3, 11))
+>mapFn : Symbol(mapFn, Decl(strictBestCommonSupertype.ts, 3, 14))
+>state : Symbol(state, Decl(strictBestCommonSupertype.ts, 3, 22))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 2, 12))
+>K : Symbol(K, Decl(strictBestCommonSupertype.ts, 3, 11))
+}
+
+const store: Store = inject(Store);
+>store : Symbol(store, Decl(strictBestCommonSupertype.ts, 6, 5))
+>Store : Symbol(Store, Decl(strictBestCommonSupertype.ts, 0, 0))
+>inject : Symbol(inject, Decl(strictBestCommonSupertype.ts, 6, 35))
+>Store : Symbol(Store, Decl(strictBestCommonSupertype.ts, 0, 0))
+
+function inject<T>(token: ProviderToken<T>): T {
+>inject : Symbol(inject, Decl(strictBestCommonSupertype.ts, 6, 35))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 8, 16))
+>token : Symbol(token, Decl(strictBestCommonSupertype.ts, 8, 19))
+>ProviderToken : Symbol(ProviderToken, Decl(strictBestCommonSupertype.ts, 14, 1))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 8, 16))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 8, 16))
+
+    return {} as T;
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 8, 16))
+}
+
+interface Type<T> extends Function {
+>Type : Symbol(Type, Decl(strictBestCommonSupertype.ts, 10, 1))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 12, 15))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    new (...args: any[]): T;
+>args : Symbol(args, Decl(strictBestCommonSupertype.ts, 13, 9))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 12, 15))
+}
+
+type ProviderToken<T> = Type<T> | AbstractType<T>;
+>ProviderToken : Symbol(ProviderToken, Decl(strictBestCommonSupertype.ts, 14, 1))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 16, 19))
+>Type : Symbol(Type, Decl(strictBestCommonSupertype.ts, 10, 1))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 16, 19))
+>AbstractType : Symbol(AbstractType, Decl(strictBestCommonSupertype.ts, 16, 50))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 16, 19))
+
+interface AbstractType<T> extends Function {
+>AbstractType : Symbol(AbstractType, Decl(strictBestCommonSupertype.ts, 16, 50))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 18, 23))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    prototype: T;
+>prototype : Symbol(prototype, Decl(strictBestCommonSupertype.ts, 18, 44))
+>T : Symbol(T, Decl(strictBestCommonSupertype.ts, 18, 23))
+}
+

testdata/baselines/reference/compiler/strictBestCommonSupertype.types

@@ -0,0 +1,42 @@
+//// [tests/cases/compiler/strictBestCommonSupertype.ts] ////
+
+=== strictBestCommonSupertype.ts ===
+// https://github.com/microsoft/typescript-go/issues/1222
+
+class Store<T = object> {
+>Store : Store<T>
+
+    select<K>(mapFn: (state: T) => K) {};
+>select : <K>(mapFn: (state: T) => K) => void
+>mapFn : (state: T) => K
+>state : T
+}
+
+const store: Store = inject(Store);
+>store : Store<object>
+>inject(Store) : Store<any>
+>inject : <T>(token: ProviderToken<T>) => T
+>Store : typeof Store
+
+function inject<T>(token: ProviderToken<T>): T {
+>inject : <T>(token: ProviderToken<T>) => T
+>token : ProviderToken<T>
+
+    return {} as T;
+>{} as T : T
+>{} : {}
+}
+
+interface Type<T> extends Function {
+    new (...args: any[]): T;
+>args : any[]
+}
+
+type ProviderToken<T> = Type<T> | AbstractType<T>;
+>ProviderToken : ProviderToken<T>
+
+interface AbstractType<T> extends Function {
+    prototype: T;
+>prototype : T
+}
+

testdata/tests/cases/compiler/strictBestCommonSupertype.ts

@@ -0,0 +1,24 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/typescript-go/issues/1222
+
+class Store<T = object> {
+    select<K>(mapFn: (state: T) => K) {};
+}
+
+const store: Store = inject(Store);
+
+function inject<T>(token: ProviderToken<T>): T {
+    return {} as T;
+}
+
+interface Type<T> extends Function {
+    new (...args: any[]): T;
+}
+
+type ProviderToken<T> = Type<T> | AbstractType<T>;
+
+interface AbstractType<T> extends Function {
+    prototype: T;
+}