Fix conditional type variance

src/compiler/checker.ts

@@ -10200,8 +10200,11 @@ namespace ts {
                 }
                 else if (source.flags & TypeFlags.Conditional) {
                     if (target.flags & TypeFlags.Conditional) {
-                        if (isTypeIdenticalTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType) &&
-                            isTypeIdenticalTo((<ConditionalType>source).extendsType, (<ConditionalType>target).extendsType)) {
+                        // 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) &&
+                            (isRelatedTo((<ConditionalType>source).checkType, (<ConditionalType>target).checkType) || isRelatedTo((<ConditionalType>target).checkType, (<ConditionalType>source).checkType))) {
                             if (result = isRelatedTo(getTrueTypeFromConditionalType(<ConditionalType>source), getTrueTypeFromConditionalType(<ConditionalType>target), reportErrors)) {
                                 result &= isRelatedTo(getFalseTypeFromConditionalType(<ConditionalType>source), getFalseTypeFromConditionalType(<ConditionalType>target), reportErrors);
                             }

tests/baselines/reference/conditionalTypes2.errors.txt

@@ -0,0 +1,90 @@
+tests/cases/conformance/types/conditional/conditionalTypes2.ts(15,5): error TS2322: Type 'Covariant<A>' is not assignable to type 'Covariant<B>'.
+  Type 'A' is not assignable to type 'B'.
+tests/cases/conformance/types/conditional/conditionalTypes2.ts(19,5): error TS2322: Type 'Contravariant<B>' is not assignable to type 'Contravariant<A>'.
+  Type 'A' is not assignable to type 'B'.
+tests/cases/conformance/types/conditional/conditionalTypes2.ts(24,5): error TS2322: Type 'Invariant<B>' is not assignable to type 'Invariant<A>'.
+  Types of property 'foo' are incompatible.
+    Type 'B extends string ? keyof B : B' is not assignable to type 'A extends string ? keyof A : A'.
+      Type 'keyof B' is not assignable to type 'keyof A'.
+tests/cases/conformance/types/conditional/conditionalTypes2.ts(25,5): error TS2322: Type 'Invariant<A>' is not assignable to type 'Invariant<B>'.
+  Types of property 'foo' are incompatible.
+    Type 'A extends string ? keyof A : A' is not assignable to type 'B extends string ? keyof B : B'.
+      Type 'A' is not assignable to type 'B'.
+
+
+==== tests/cases/conformance/types/conditional/conditionalTypes2.ts (4 errors) ====
+    interface Covariant<T> {
+        foo: T extends string ? T : number;
+    }
+
+    interface Contravariant<T> {
+        foo: T extends string ? keyof T : number;
+    }
+
+    interface Invariant<T> {
+        foo: T extends string ? keyof T : T;
+    }
+
+    function f1<A, B extends A>(a: Covariant<A>, b: Covariant<B>) {
+        a = b;
+        b = a;  // Error
+        ~
+!!! error TS2322: Type 'Covariant<A>' is not assignable to type 'Covariant<B>'.
+!!! error TS2322:   Type 'A' is not assignable to type 'B'.
+    }
+
+    function f2<A, B extends A>(a: Contravariant<A>, b: Contravariant<B>) {
+        a = b;  // Error
+        ~
+!!! error TS2322: Type 'Contravariant<B>' is not assignable to type 'Contravariant<A>'.
+!!! error TS2322:   Type 'A' is not assignable to type 'B'.
+        b = a;
+    }
+
+    function f3<A, B extends A>(a: Invariant<A>, b: Invariant<B>) {
+        a = b;  // Error
+        ~
+!!! error TS2322: Type 'Invariant<B>' is not assignable to type 'Invariant<A>'.
+!!! error TS2322:   Types of property 'foo' are incompatible.
+!!! error TS2322:     Type 'B extends string ? keyof B : B' is not assignable to type 'A extends string ? keyof A : A'.
+!!! error TS2322:       Type 'keyof B' is not assignable to type 'keyof A'.
+        b = a;  // Error
+        ~
+!!! error TS2322: Type 'Invariant<A>' is not assignable to type 'Invariant<B>'.
+!!! error TS2322:   Types of property 'foo' are incompatible.
+!!! error TS2322:     Type 'A extends string ? keyof A : A' is not assignable to type 'B extends string ? keyof B : B'.
+!!! error TS2322:       Type 'A' is not assignable to type 'B'.
+    }
+
+    // Repros from #22860
+
+    class Opt<T> {
+        toVector(): Vector<T> {
+            return <any>undefined;
+        }
+    }
+
+    interface Seq<T> {
+        tail(): Opt<Seq<T>>;
+    }
+
+    class Vector<T> implements Seq<T> {
+        tail(): Opt<Vector<T>> {
+            return <any>undefined;
+        }
+        partition2<U extends T>(predicate:(v:T)=>v is U): [Vector<U>,Vector<Exclude<T, U>>];
+        partition2(predicate:(x:T)=>boolean): [Vector<T>,Vector<T>];
+        partition2<U extends T>(predicate:(v:T)=>boolean): [Vector<U>,Vector<any>] {
+            return <any>undefined;
+        }
+    }
+
+    interface A1<T> {
+        bat: B1<A1<T>>;
+    }
+
+    interface B1<T> extends A1<T> {
+        bat: B1<B1<T>>;
+        boom: T extends any ? true : true
+    }
+

tests/baselines/reference/conditionalTypes2.js

@@ -0,0 +1,128 @@
+//// [conditionalTypes2.ts]
+interface Covariant<T> {
+    foo: T extends string ? T : number;
+}
+
+interface Contravariant<T> {
+    foo: T extends string ? keyof T : number;
+}
+
+interface Invariant<T> {
+    foo: T extends string ? keyof T : T;
+}
+
+function f1<A, B extends A>(a: Covariant<A>, b: Covariant<B>) {
+    a = b;
+    b = a;  // Error
+}
+
+function f2<A, B extends A>(a: Contravariant<A>, b: Contravariant<B>) {
+    a = b;  // Error
+    b = a;
+}
+
+function f3<A, B extends A>(a: Invariant<A>, b: Invariant<B>) {
+    a = b;  // Error
+    b = a;  // Error
+}
+
+// Repros from #22860
+
+class Opt<T> {
+    toVector(): Vector<T> {
+        return <any>undefined;
+    }
+}
+
+interface Seq<T> {
+    tail(): Opt<Seq<T>>;
+}
+
+class Vector<T> implements Seq<T> {
+    tail(): Opt<Vector<T>> {
+        return <any>undefined;
+    }
+    partition2<U extends T>(predicate:(v:T)=>v is U): [Vector<U>,Vector<Exclude<T, U>>];
+    partition2(predicate:(x:T)=>boolean): [Vector<T>,Vector<T>];
+    partition2<U extends T>(predicate:(v:T)=>boolean): [Vector<U>,Vector<any>] {
+        return <any>undefined;
+    }
+}
+
+interface A1<T> {
+    bat: B1<A1<T>>;
+}
+
+interface B1<T> extends A1<T> {
+    bat: B1<B1<T>>;
+    boom: T extends any ? true : true
+}
+
+
+//// [conditionalTypes2.js]
+"use strict";
+function f1(a, b) {
+    a = b;
+    b = a; // Error
+}
+function f2(a, b) {
+    a = b; // Error
+    b = a;
+}
+function f3(a, b) {
+    a = b; // Error
+    b = a; // Error
+}
+// Repros from #22860
+var Opt = /** @class */ (function () {
+    function Opt() {
+    }
+    Opt.prototype.toVector = function () {
+        return undefined;
+    };
+    return Opt;
+}());
+var Vector = /** @class */ (function () {
+    function Vector() {
+    }
+    Vector.prototype.tail = function () {
+        return undefined;
+    };
+    Vector.prototype.partition2 = function (predicate) {
+        return undefined;
+    };
+    return Vector;
+}());
+
+
+//// [conditionalTypes2.d.ts]
+interface Covariant<T> {
+    foo: T extends string ? T : number;
+}
+interface Contravariant<T> {
+    foo: T extends string ? keyof T : number;
+}
+interface Invariant<T> {
+    foo: T extends string ? keyof T : T;
+}
+declare function f1<A, B extends A>(a: Covariant<A>, b: Covariant<B>): void;
+declare function f2<A, B extends A>(a: Contravariant<A>, b: Contravariant<B>): void;
+declare function f3<A, B extends A>(a: Invariant<A>, b: Invariant<B>): void;
+declare class Opt<T> {
+    toVector(): Vector<T>;
+}
+interface Seq<T> {
+    tail(): Opt<Seq<T>>;
+}
+declare class Vector<T> implements Seq<T> {
+    tail(): Opt<Vector<T>>;
+    partition2<U extends T>(predicate: (v: T) => v is U): [Vector<U>, Vector<Exclude<T, U>>];
+    partition2(predicate: (x: T) => boolean): [Vector<T>, Vector<T>];
+}
+interface A1<T> {
+    bat: B1<A1<T>>;
+}
+interface B1<T> extends A1<T> {
+    bat: B1<B1<T>>;
+    boom: T extends any ? true : true;
+}