Unexpected type argument inference

[ltblue]

Bug Report

🔎 Search Terms

generic type argument inference

🕗 Version & Regression Information

4.4.4

⏯ Playground Link

Playground link with relevant code

💻 Code

type Type<T extends 1|2> = 
  T extends 1 ? number : 
  T extends 2 ? string :
  never;

type Foo<T extends 1|2> = (x: {a: T, b: Type<T>}) => void;

function foo1(x: {a: 1, b: number} | {a: 2, b: string}) {}
function foo2(x: {a: 2, b: string} | {a: 1, b: number}) {}

function bar<T extends 1|2> (a: T, b: Foo<T>) {}

bar(1, foo1); // ok
bar(2, foo1); // error: Argument of type '2' is not assignable to parameter of type '1'.(2345)

bar(1, foo2); // error: Argument of type '1' is not assignable to parameter of type '2'.(2345)
bar(2, foo2); // ok

🙁 Actual behavior

In bar, foo1 is always inferred to be Foo<1> and foo2 is always inferred to be Foo<2>

🙂 Expected behavior

It is supposed to be no error there because foo1 and foo2 have the same type, which can satisfy all the four calls.

[andrewbranch]

Duplicate of #45603, already fixed in nightly

[ltblue]

Duplicate of #45603, already fixed in nightly

Thanks. But I found it doesn't work if I use function overload instead of union type in nightly. Here is the code.

type Type<T extends 1|2> = 
  T extends 1 ? number : 
  T extends 2 ? string :
  never;

type Foo<T extends 1|2> = (x: {a: T, b: Type<T>}) => void;

function foo1(x: {a: 1, b: number}): void;
function foo1(x: {a: 2, b: string}): void;
function foo1(x: any) {}

function bar<T extends 1|2> (a: T, b: Foo<T>) {}

bar(1, foo1); // error: Argument of type '1' is not assignable to parameter of type '2'.(2345)
bar(2, foo1); // ok

Playground v4.5.0-dev.20211101

[andrewbranch]

That's expected; inference doesn't pick overloads; it just uses the last one.