microsoft / microsoft/TypeScript

Adding an overload breaks inference of compositional functions

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Description

Tried fixing this, but ran into another case where an overload breaks type inference:

declare function reverse(a: ReadonlyArray<number>): ReadonlyArray<number>;

declare function incrementEvery(list: ReadonlyArray<number>): ReadonlyArray<number>;
declare function incrementEvery(x: ReadonlySet<number>): ReadonlySet<number>;

declare function compose<T>(
    second: (x: ReadonlyArray<number>) => T,
    first: (x0: ReadonlyArray<number>) => ReadonlyArray<number>,
): (x0: ReadonlyArray<number>) => T;

const works = compose<ReadonlyArray<number>>(incrementEvery, reverse);
const broke: (nums: ReadonlyArray<number>) => ReadonlyArray<number> = compose(incrementEvery, reverse);
src/a.ts(12,79): error TS2345: Argument of type '{ (list: ReadonlyArray<number>): ReadonlyArray<number>; (x: ReadonlySet<number>): ReadonlySet<num...' is not assignable to parameter of type '(x: ReadonlyArray<number>) => ReadonlySet<number>'.
  Type 'ReadonlyArray<number>' is not assignable to type 'ReadonlySet<number>'.
    Property 'has' is missing in type 'ReadonlyArray<number>'.

Unlike the above issue, this was broken in all the way back in 2.0.

Originally posted by @andy-ms in https://github.com/microsoft/TypeScript/issues/23352#issuecomment-380844184

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  3. Fork the repository and make your change on a branch.
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Research direction

Start with the minimal reproduction in src/a.ts, comparing the inferred compose call with the explicit generic invocation. Investigate why the overloaded incrementEvery is inferred as the wrong function type; done means the unannotated compose call is accepted with the stated function type without regressing the overload cases.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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