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Covariance breaks when checking for undefinded in a type #55161
Description
Bug Report
I'm using the solid-js npm package which defines a type called Setter:
type Setter<T> = (undefined extends T ? () => undefined : {}) & (<U extends T>(value: (prev: T) => U) => U) & (<U extends T>(value: Exclude<U, Function>) => U) & (<U extends T>(value: Exclude<U, Function> | ((prev: T) => U)) => U);I don't fully understand the reasoning behind it, but it's giving me some problems:
type Setter<T> = (undefined extends T ? () => undefined : {}) & (<U extends T>(value: (prev: T) => U) => U) & (<U extends T>(value: Exclude<U, Function>) => U) & (<U extends T>(value: Exclude<U, Function> | ((prev: T) => U)) => U);
class A { a = 1 }
class B extends A { b = 2 }
type Test<T> = { 0: Setter<T> };
declare const a: Test<A>;
const b: Test<B> = a; // ERROR!I created an object with the property 0 which contains a Setter and it appears to not be covariant.
Inspecting the error it looks like that the problem is originated from this part undefined extends T ? () => undefined : {}, infact if I remove it this works, moreover the problem would still occur if that was the only part of the type:
type Setter<T> = undefined extends T ? () => undefined : {};
class A { a = 1 }
class B extends A { b = 2 }
type Test<T> = { 0: Setter<T> };
declare const a: Test<A>;
const b: Test<B> = a; // ERROR!I think it is an error because the types are the same:
type TA = Test<A>;
// ^? type TA = { 0: {}; }
type TB = Test<B>;
// ^? type TB = { 0: {}; }Additionally if Test were to be defined as [ Setter<T> ] the error would not occur:
type Setter<T> = undefined extends T ? () => undefined : {};
class A { a = 1 }
class B extends A { b = 2 }
type Test<T> = [ Setter<T> ];
declare const a: Test<A>;
const b: Test<B> = a; // OK?!?!🔎 Search Terms
- undefined extends T
- covariance
- covariant
🕗 Version & Regression Information
This changed between versions 3.3.3 and 3.5.1. (The only playground version in which it doesn't occur is 3.3.3)
⏯ Playground Link
💻 Code
type Setter<T> = (undefined extends T ? () => undefined : {}) & (<U extends T>(value: (prev: T) => U) => U) & (<U extends T>(value: Exclude<U, Function>) => U) & (<U extends T>(value: Exclude<U, Function> | ((prev: T) => U)) => U);
// type Setter<T> = (<U extends T>(value: (prev: T) => U) => U) & (<U extends T>(value: Exclude<U, Function>) => U) & (<U extends T>(value: Exclude<U, Function> | ((prev: T) => U)) => U);
// type Setter<T> = undefined extends T ? () => undefined : {};
class A { a = 1 }
class B extends A { b = 2 }
type Test<T> = { 0: Setter<T> };
// type Test<T> = [ Setter<T> ];
declare const a: Test<A>;
const b: Test<B> = a; // ERROR!
type TA = Test<A>;
// ^?
type TB = Test<B>;
// ^?🙁 Actual behavior
Can't assign b with the value of a.
It is wrong at the very least because it is not consistent
🙂 Expected behavior
Can assign b with the value of a
Activity
RyanCavanaugh commentedon Jul 27, 2023
I believe the bug here is that we're not marking
U extends Tas being an unmeasurable variance computation onT.cc Anders Hejlsberg (@ahejlsberg)
weswigham commentedon Jul 27, 2023
#43887 I thought I already had a PR out to fix this once before. :P
saltman424 commentedon Dec 14, 2023
Wesley Wigham (@weswigham) since #43887 was never merged, will this issue be addressed by #54866?