Type comparison when conditional type is target

src/compiler/types.ts

@@ -4737,7 +4737,9 @@ namespace ts {
         /* @internal */
         resolvedInferredTrueType?: Type; // The `trueType` instantiated with the `combinedMapper`, if present
         /* @internal */
-        resolvedDefaultConstraint?: Type;
+        resolvedDefaultConstraintForReading?: Type;
+        /* @internal */
+        resolvedDefaultConstraintForWriting?: Type;
         /* @internal */
         mapper?: TypeMapper;
         /* @internal */

tests/baselines/reference/conditionalTypeAssignabilityWhenDeferred.errors.txt

@@ -0,0 +1,238 @@
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(14,22): error TS2345: Argument of type '"value"' is not assignable to parameter of type 'XX extends XX ? "value" : never'.
+  Type '"value"' is not assignable to type 'never'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(32,20): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(33,21): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(37,24): error TS2345: Argument of type 'T | null' is not assignable to parameter of type 'null extends T | null ? any : never'.
+  Type 'null' is not assignable to type 'null extends T | null ? any : never'.
+    Type 'null' is not assignable to type 'never'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(43,5): error TS2322: Type '{ x: T; y: T; }' is not assignable to type 'T extends T ? { x: T; y: T; } : never'.
+  Type '{ x: T; y: T; }' is not assignable to type 'never'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(50,5): error TS2322: Type 'string' is not assignable to type 'Foo<T>'.
+  Type 'string' is not assignable to type '"a"'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(111,5): error TS2322: Type 'Q' is not assignable to type 'InferBecauseWhyNot<Q>'.
+  Type '(arg: any) => any' is not assignable to type 'InferBecauseWhyNot<Q>'.
+    Type '(arg: any) => any' is not assignable to type 'never'.
+      Type 'Q' is not assignable to type 'never'.
+        Type '(arg: any) => any' is not assignable to type 'never'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(131,1): error TS2322: Type 'true' is not assignable to type 'false'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(132,1): error TS2322: Type 'false' is not assignable to type 'true'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(136,1): error TS2322: Type 'false' is not assignable to type 'true'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(137,1): error TS2322: Type 'true' is not assignable to type 'false'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(147,5): error TS2322: Type 'T extends "a" ? true : false' is not assignable to type 'false'.
+  Type 'boolean' is not assignable to type 'false'.
+    Type 'true' is not assignable to type 'false'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(148,5): error TS2322: Type 'T extends "b" ? true : false' is not assignable to type 'true'.
+  Type 'boolean' is not assignable to type 'true'.
+    Type 'false' is not assignable to type 'true'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(152,5): error TS2322: Type 'false' is not assignable to type 'T extends "a" ? true : false'.
+  Type 'false' is not assignable to type 'never'.
+    Type 'false' is not assignable to type 'true'.
+tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(153,5): error TS2322: Type 'true' is not assignable to type 'T extends "b" ? true : false'.
+  Type 'true' is not assignable to type 'never'.
+    Type 'true' is not assignable to type 'false'.
+
+
+==== tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts (15 errors) ====
+    // #29505
+
+    export type FilterPropsByType<T, TT> = {
+        [K in keyof T]: T[K] extends TT ? K : never
+    }[keyof T];
+
+    function select<
+        T extends string | number,
+        TList extends object,
+        TValueProp extends FilterPropsByType<TList, T>
+    >(property: T, list: TList[], valueProp: TValueProp) {}
+
+    <XX extends string>(x: XX, tipos: { value: XX }[]) => {
+        select(x, tipos, "value");
+                         ~~~~~~~
+!!! error TS2345: Argument of type '"value"' is not assignable to parameter of type 'XX extends XX ? "value" : never'.
+!!! error TS2345:   Type '"value"' is not assignable to type 'never'.
+    };
+
+    // #29662
+
+    declare function onlyNullablePlease<T extends null extends T ? any : never>(
+        value: T
+    ): void;
+
+    declare function onlyNullablePlease2<
+        T extends [null] extends [T] ? any : never
+    >(value: T): void;
+
+    declare var z: string | null;
+    onlyNullablePlease(z); // works as expected
+    onlyNullablePlease2(z); // works as expected
+
+    declare var y: string;
+    onlyNullablePlease(y); // error as expected
+                       ~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+    onlyNullablePlease2(y); // error as expected
+                        ~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+
+    <T>(t: T) => {
+        var x: T | null = Math.random() > 0.5 ? null : t;
+        onlyNullablePlease(x); // should work
+                           ~
+!!! error TS2345: Argument of type 'T | null' is not assignable to parameter of type 'null extends T | null ? any : never'.
+!!! error TS2345:   Type 'null' is not assignable to type 'null extends T | null ? any : never'.
+!!! error TS2345:     Type 'null' is not assignable to type 'never'.
+        onlyNullablePlease2(x); // should work
+    };
+
+    <T>(t1: { x: T; y: T }, t2: T extends T ? { x: T; y: T } : never) => {
+        t1 = t2; // OK
+        t2 = t1; // should fail
+        ~~
+!!! error TS2322: Type '{ x: T; y: T; }' is not assignable to type 'T extends T ? { x: T; y: T; } : never'.
+!!! error TS2322:   Type '{ x: T; y: T; }' is not assignable to type 'never'.
+    };
+
+    type Foo<T> = T extends true ? string : "a";
+
+    <T>(x: Foo<T>, s: string) => {
+        x = "a"; // Currently an error, should be ok
+        x = s; // Error
+        ~
+!!! error TS2322: Type 'string' is not assignable to type 'Foo<T>'.
+!!! error TS2322:   Type 'string' is not assignable to type '"a"'.
+    };
+
+    // #26933
+
+    type Distributive<T> = T extends { a: number } ? { a: number } : { b: number };
+    <T>() => {
+        const o = { a: 1, b: 2 };
+        const x: [T] extends [string]
+            ? { y: number }
+            : { a: number; b: number } = undefined!;
+        // Simple case: OK
+        const o1: [T] extends [number] ? { a: number } : { b: number } = o;
+        // Simple case where source happens to be a conditional type: also OK
+        const x1: [T] extends [number]
+            ? ([T] extends [string] ? { y: number } : { a: number })
+            : ([T] extends [string] ? { y: number } : { b: number }) = x;
+        // Infer type parameters: no good
+        const o2: [T] extends [[infer U]] ? U : { b: number } = o;
+
+        // The next 4 are arguable - if you choose to ignore the `never` distribution case,
+        // then they're all good. The `never` case _is_ a bit of an outlier - we say distributive types
+        // look approximately like the sum of their branches, but the `never` case bucks that.
+        // There's an argument for the result of dumping `never` into a distributive conditional
+        // being not `never`, but instead the intersection of the branches - a much more precise bound
+        // on that "impossible" input.
+
+        // Distributive where T might instantiate to never: no good
+        const o3: Distributive<T> = o;
+        // Distributive where T & string might instantiate to never: also no good
+        const o4: Distributive<T & string> = o;
+        // Distributive where {a: T} cannot instantiate to never: OK
+        const o5: Distributive<{ a: T }> = o;
+        // Distributive where check type is a conditional which returns a non-never type upon instantiation with `never` but can still return never otherwise: no good
+        const o6: Distributive<[T] extends [never] ? { a: number } : never> = o;
+    };
+
+    type Wrapped<T> = { ___secret: T };
+    type Unwrap<T> = T extends Wrapped<infer U> ? U : T;
+
+    declare function set<T, K extends keyof T>(
+        obj: T,
+        key: K,
+        value: Unwrap<T[K]>
+    ): Unwrap<T[K]>;
+
+    class Foo2 {
+        prop!: Wrapped<string>;
+
+        method() {
+            set(this, "prop", "hi"); // <-- type error
+        }
+    }
+
+    set(new Foo2(), "prop", "hi"); // <-- typechecks
+
+    type InferBecauseWhyNot<T> = [T] extends [(p: infer P1) => any]
+        ? P1 | T
+        : never;
+
+    <Q extends (arg: any) => any>(x: Q): InferBecauseWhyNot<Q> => {
+        return x;
+        ~~~~~~~~~
+!!! error TS2322: Type 'Q' is not assignable to type 'InferBecauseWhyNot<Q>'.
+!!! error TS2322:   Type '(arg: any) => any' is not assignable to type 'InferBecauseWhyNot<Q>'.
+!!! error TS2322:     Type '(arg: any) => any' is not assignable to type 'never'.
+!!! error TS2322:       Type 'Q' is not assignable to type 'never'.
+!!! error TS2322:         Type '(arg: any) => any' is not assignable to type 'never'.
+    };
+
+    type InferBecauseWhyNotDistributive<T> = T extends (p: infer P1) => any
+        ? P1 | T
+        : never;
+
+    <Q extends (arg: any) => any>(
+        x: Q
+    ): InferBecauseWhyNotDistributive<Q> => {
+        return x; // should fail
+    };
+
+    let t: true;
+    let f: false;
+
+    let a: "a" extends "a" ? true : false = undefined!;
+    let b: "a" extends "b" ? true : false = undefined!;
+
+    t = a;
+    f = a; // !!! error TS2322: Type 'true' is not assignable to type 'false'.
+    ~
+!!! error TS2322: Type 'true' is not assignable to type 'false'.
+    t = b; // !!! error TS2322: Type 'false' is not assignable to type 'true'.
+    ~
+!!! error TS2322: Type 'false' is not assignable to type 'true'.
+    f = b;
+
+    a = true;
+    a = false; // !!! error TS2322: Type 'false' is not assignable to type 'true'.
+    ~
+!!! error TS2322: Type 'false' is not assignable to type 'true'.
+    b = true; // !!! error TS2322: Type 'true' is not assignable to type 'false'.
+    ~
+!!! error TS2322: Type 'true' is not assignable to type 'false'.
+    b = false;
+
+    // #23132
+
+    <T extends "a">() => {
+        let a: T extends "a" ? true : false = undefined!;
+        let b: T extends "b" ? true : false = undefined!;
+
+        t = a;
+        f = a; // !!! error TS2322: Type 'T extends "a" ? true : false' is not assignable to type 'false'.
+        ~
+!!! error TS2322: Type 'T extends "a" ? true : false' is not assignable to type 'false'.
+!!! error TS2322:   Type 'boolean' is not assignable to type 'false'.
+!!! error TS2322:     Type 'true' is not assignable to type 'false'.
+        t = b; // !!! error TS2322: Type 'T extends "b" ? true : false' is not assignable to type 'true'.
+        ~
+!!! error TS2322: Type 'T extends "b" ? true : false' is not assignable to type 'true'.
+!!! error TS2322:   Type 'boolean' is not assignable to type 'true'.
+!!! error TS2322:     Type 'false' is not assignable to type 'true'.
+        f = b;
+
+        a = true;
+        a = false; // !!! error TS2322: Type 'false' is not assignable to type 'T extends "a" ? true : false'.
+        ~
+!!! error TS2322: Type 'false' is not assignable to type 'T extends "a" ? true : false'.
+!!! error TS2322:   Type 'false' is not assignable to type 'never'.
+!!! error TS2322:     Type 'false' is not assignable to type 'true'.
+        b = true; // !!! error TS2322: Type 'true' is not assignable to type 'T extends "b" ? true : false'.
+        ~
+!!! error TS2322: Type 'true' is not assignable to type 'T extends "b" ? true : false'.
+!!! error TS2322:   Type 'true' is not assignable to type 'never'.
+!!! error TS2322:     Type 'true' is not assignable to type 'false'.
+        b = false;
+    };
+