Instantiate generic conditional infer source types in the context of the target conditional

src/compiler/checker.ts

@@ -13165,9 +13165,19 @@ namespace ts {
                         // Two conditional types 'T1 extends U1 ? X1 : Y1' and 'T2 extends U2 ? X2 : Y2' are related if
                         // one of T1 and T2 is related to the other, U1 and U2 are identical types, X1 is related to X2,
                         // and Y1 is related to Y2.
-                        if (isTypeIdenticalTo((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType) &&
+                        const sourceParams = (source as ConditionalType).root.inferTypeParameters;
+                        let sourceExtends = (<ConditionalType>source).extendsType;
+                        let mapper: TypeMapper | undefined;
+                        if (sourceParams) {
+                            // If the source has infer type parameters, we instantiate them in the context of the target
+                            const ctx = createInferenceContext(sourceParams, /*signature*/ undefined, InferenceFlags.None, isRelatedTo);
+                            inferTypes(ctx.inferences, (<ConditionalType>target).extendsType, sourceExtends, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
+                            sourceExtends = instantiateType(sourceExtends, ctx.mapper);
+                            mapper = ctx.mapper;
+                        }
+                        if (isTypeIdenticalTo(sourceExtends, (<ConditionalType>target).extendsType) &&
                             (isRelatedTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType) || isRelatedTo((<ConditionalType>target).checkType, (<ConditionalType>source).checkType))) {
-                            if (result = isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), reportErrors)) {
+                            if (result = isRelatedTo(instantiateType(getTrueTypeFromConditionalType(<ConditionalType>source), mapper), getTrueTypeFromConditionalType(<ConditionalType>target), reportErrors)) {
                                 result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), reportErrors);
                             }
                             if (result) {

tests/baselines/reference/identicalGenericConditionalsWithInferRelated.js

@@ -0,0 +1,48 @@
+//// [identicalGenericConditionalsWithInferRelated.ts]
+function f<X>(arg: X) {
+    type Cond1 = X extends [infer A] ? A : never;
+    type Cond2 = X extends [infer A] ? A : never;
+
+    let x: Cond1 = null as any;
+    let y: Cond2 = null as any;
+    x = y; // is err, should be ok
+    y = x; // is err, should be ok
+}
+
+// repro from https://github.com/microsoft/TypeScript/issues/26627
+export type Constructor<T> = new (...args: any[]) => T
+export type MappedResult<T> =
+    T extends Boolean ? boolean :
+    T extends Number ? number :
+    T extends String ? string :
+    T
+
+
+interface X {
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never>
+}
+
+class Y implements X {
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never> {
+        throw new Error()
+    }
+}
+
+
+//// [identicalGenericConditionalsWithInferRelated.js]
+"use strict";
+exports.__esModule = true;
+function f(arg) {
+    var x = null;
+    var y = null;
+    x = y; // is err, should be ok
+    y = x; // is err, should be ok
+}
+var Y = /** @class */ (function () {
+    function Y() {
+    }
+    Y.prototype.decode = function (ctor) {
+        throw new Error();
+    };
+    return Y;
+}());

tests/baselines/reference/identicalGenericConditionalsWithInferRelated.symbols

@@ -0,0 +1,100 @@
+=== tests/cases/compiler/identicalGenericConditionalsWithInferRelated.ts ===
+function f<X>(arg: X) {
+>f : Symbol(f, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 0))
+>X : Symbol(X, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 11))
+>arg : Symbol(arg, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 14))
+>X : Symbol(X, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 11))
+
+    type Cond1 = X extends [infer A] ? A : never;
+>Cond1 : Symbol(Cond1, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 23))
+>X : Symbol(X, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 11))
+>A : Symbol(A, Decl(identicalGenericConditionalsWithInferRelated.ts, 1, 33))
+>A : Symbol(A, Decl(identicalGenericConditionalsWithInferRelated.ts, 1, 33))
+
+    type Cond2 = X extends [infer A] ? A : never;
+>Cond2 : Symbol(Cond2, Decl(identicalGenericConditionalsWithInferRelated.ts, 1, 49))
+>X : Symbol(X, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 11))
+>A : Symbol(A, Decl(identicalGenericConditionalsWithInferRelated.ts, 2, 33))
+>A : Symbol(A, Decl(identicalGenericConditionalsWithInferRelated.ts, 2, 33))
+
+    let x: Cond1 = null as any;
+>x : Symbol(x, Decl(identicalGenericConditionalsWithInferRelated.ts, 4, 7))
+>Cond1 : Symbol(Cond1, Decl(identicalGenericConditionalsWithInferRelated.ts, 0, 23))
+
+    let y: Cond2 = null as any;
+>y : Symbol(y, Decl(identicalGenericConditionalsWithInferRelated.ts, 5, 7))
+>Cond2 : Symbol(Cond2, Decl(identicalGenericConditionalsWithInferRelated.ts, 1, 49))
+
+    x = y; // is err, should be ok
+>x : Symbol(x, Decl(identicalGenericConditionalsWithInferRelated.ts, 4, 7))
+>y : Symbol(y, Decl(identicalGenericConditionalsWithInferRelated.ts, 5, 7))
+
+    y = x; // is err, should be ok
+>y : Symbol(y, Decl(identicalGenericConditionalsWithInferRelated.ts, 5, 7))
+>x : Symbol(x, Decl(identicalGenericConditionalsWithInferRelated.ts, 4, 7))
+}
+
+// repro from https://github.com/microsoft/TypeScript/issues/26627
+export type Constructor<T> = new (...args: any[]) => T
+>Constructor : Symbol(Constructor, Decl(identicalGenericConditionalsWithInferRelated.ts, 8, 1))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 11, 24))
+>args : Symbol(args, Decl(identicalGenericConditionalsWithInferRelated.ts, 11, 34))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 11, 24))
+
+export type MappedResult<T> =
+>MappedResult : Symbol(MappedResult, Decl(identicalGenericConditionalsWithInferRelated.ts, 11, 54))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 12, 25))
+
+    T extends Boolean ? boolean :
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 12, 25))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    T extends Number ? number :
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 12, 25))
+>Number : Symbol(Number, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    T extends String ? string :
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 12, 25))
+>String : Symbol(String, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    T
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 12, 25))
+
+
+interface X {
+>X : Symbol(X, Decl(identicalGenericConditionalsWithInferRelated.ts, 16, 5))
+
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never>
+>decode : Symbol(X.decode, Decl(identicalGenericConditionalsWithInferRelated.ts, 19, 13))
+>C : Symbol(C, Decl(identicalGenericConditionalsWithInferRelated.ts, 20, 11))
+>Constructor : Symbol(Constructor, Decl(identicalGenericConditionalsWithInferRelated.ts, 8, 1))
+>ctor : Symbol(ctor, Decl(identicalGenericConditionalsWithInferRelated.ts, 20, 39))
+>C : Symbol(C, Decl(identicalGenericConditionalsWithInferRelated.ts, 20, 11))
+>MappedResult : Symbol(MappedResult, Decl(identicalGenericConditionalsWithInferRelated.ts, 11, 54))
+>C : Symbol(C, Decl(identicalGenericConditionalsWithInferRelated.ts, 20, 11))
+>Constructor : Symbol(Constructor, Decl(identicalGenericConditionalsWithInferRelated.ts, 8, 1))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 20, 89))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 20, 89))
+}
+
+class Y implements X {
+>Y : Symbol(Y, Decl(identicalGenericConditionalsWithInferRelated.ts, 21, 1))
+>X : Symbol(X, Decl(identicalGenericConditionalsWithInferRelated.ts, 16, 5))
+
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never> {
+>decode : Symbol(Y.decode, Decl(identicalGenericConditionalsWithInferRelated.ts, 23, 22))
+>C : Symbol(C, Decl(identicalGenericConditionalsWithInferRelated.ts, 24, 11))
+>Constructor : Symbol(Constructor, Decl(identicalGenericConditionalsWithInferRelated.ts, 8, 1))
+>ctor : Symbol(ctor, Decl(identicalGenericConditionalsWithInferRelated.ts, 24, 39))
+>C : Symbol(C, Decl(identicalGenericConditionalsWithInferRelated.ts, 24, 11))
+>MappedResult : Symbol(MappedResult, Decl(identicalGenericConditionalsWithInferRelated.ts, 11, 54))
+>C : Symbol(C, Decl(identicalGenericConditionalsWithInferRelated.ts, 24, 11))
+>Constructor : Symbol(Constructor, Decl(identicalGenericConditionalsWithInferRelated.ts, 8, 1))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 24, 89))
+>T : Symbol(T, Decl(identicalGenericConditionalsWithInferRelated.ts, 24, 89))
+
+        throw new Error()
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+    }
+}
+

tests/baselines/reference/identicalGenericConditionalsWithInferRelated.types

@@ -0,0 +1,65 @@
+=== tests/cases/compiler/identicalGenericConditionalsWithInferRelated.ts ===
+function f<X>(arg: X) {
+>f : <X>(arg: X) => void
+>arg : X
+
+    type Cond1 = X extends [infer A] ? A : never;
+>Cond1 : X extends [infer A] ? A : never
+
+    type Cond2 = X extends [infer A] ? A : never;
+>Cond2 : X extends [infer A] ? A : never
+
+    let x: Cond1 = null as any;
+>x : X extends [infer A] ? A : never
+>null as any : any
+>null : null
+
+    let y: Cond2 = null as any;
+>y : X extends [infer A] ? A : never
+>null as any : any
+>null : null
+
+    x = y; // is err, should be ok
+>x = y : X extends [infer A] ? A : never
+>x : X extends [infer A] ? A : never
+>y : X extends [infer A] ? A : never
+
+    y = x; // is err, should be ok
+>y = x : X extends [infer A] ? A : never
+>y : X extends [infer A] ? A : never
+>x : X extends [infer A] ? A : never
+}
+
+// repro from https://github.com/microsoft/TypeScript/issues/26627
+export type Constructor<T> = new (...args: any[]) => T
+>Constructor : Constructor<T>
+>args : any[]
+
+export type MappedResult<T> =
+>MappedResult : MappedResult<T>
+
+    T extends Boolean ? boolean :
+    T extends Number ? number :
+    T extends String ? string :
+    T
+
+
+interface X {
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never>
+>decode : <C extends Constructor<any>>(ctor: C) => MappedResult<C extends Constructor<infer T> ? T : never>
+>ctor : C
+}
+
+class Y implements X {
+>Y : Y
+
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never> {
+>decode : <C extends Constructor<any>>(ctor: C) => MappedResult<C extends Constructor<infer T> ? T : never>
+>ctor : C
+
+        throw new Error()
+>new Error() : Error
+>Error : ErrorConstructor
+    }
+}
+

tests/cases/compiler/identicalGenericConditionalsWithInferRelated.ts

@@ -0,0 +1,28 @@
+function f<X>(arg: X) {
+    type Cond1 = X extends [infer A] ? A : never;
+    type Cond2 = X extends [infer A] ? A : never;
+
+    let x: Cond1 = null as any;
+    let y: Cond2 = null as any;
+    x = y; // is err, should be ok
+    y = x; // is err, should be ok
+}
+
+// repro from https://github.com/microsoft/TypeScript/issues/26627
+export type Constructor<T> = new (...args: any[]) => T
+export type MappedResult<T> =
+    T extends Boolean ? boolean :
+    T extends Number ? number :
+    T extends String ? string :
+    T
+
+
+interface X {
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never>
+}
+
+class Y implements X {
+    decode<C extends Constructor<any>>(ctor: C): MappedResult<C extends Constructor<infer T> ? T : never> {
+        throw new Error()
+    }
+}