Typescript: Type inference for generic functions

[sergey-shandar]

TypeScript Version: 2.0.3

Code

function id<T>(v: T): T { return v; }

function applyToNumber<T>(v: T, toNumber: (x: T) => number): number {
    return toNumber(v);
}

applyToNumber(5, v => v); // ok
applyToNumber(5, id); // ok
applyToNumber("aaa", v => v); // error, as expected.
applyToNumber("aaa", id); // ok, but an error is expected.

Expected behavior:
error

Actual behavior:
no error.

[andy-ms]

Related: #11311

[zpdDG4gta8XKpMCd]

might be dup a new shiny case of #3038

[sergey-shandar]

If it's compatibility problem, it would be really helpful to have a "strictTyping" compilation mode which would prohibit converting the any type to other types without explicit cast.

[zpdDG4gta8XKpMCd]

they say without any it will be an infinite recursion which i don't get because an explicit lambda (v => v in your case) doesn't have this problem, if so.. why would not TS handle all function references as explicit lambdas behind the scenes to avoid this recursion problem

[sergey-shandar]

An ugly workaround

function getId<T>(): (v: T) => T { return v => v; }

applyToNumber(5, getId()); // error, can't infer the 'T' type.
applyToNumber(5, getId<number>()); // ok but 'v => v' looks better.

[mhegazy]

related to https://github.com/Microsoft/TypeScript/issues/5616, and possibly https://github.com/Microsoft/TypeScript/issues/417, https://github.com/Microsoft/TypeScript/issues/5487, https://github.com/Microsoft/TypeScript/issues/9107, https://github.com/Microsoft/TypeScript/issues/9659, https://github.com/Microsoft/TypeScript/issues/9660,
https://github.com/Microsoft/TypeScript/issues/10247, https://github.com/Microsoft/TypeScript/issues/11311, https://github.com/Microsoft/TypeScript/issues/11343, https://github.com/Microsoft/TypeScript/issues/5616

[zpdDG4gta8XKpMCd]

looks like its time to bite that bullet or else you gonna break that all time record of the number of open issues related to the same problemo

[AllainPL]

The sample given:

applyToNumber("aaa", id); // ok, but an error is expected.

in TS 2.6 returns error:

Argument of type '<T>(v: T) => T' is not assignable to parameter of type '(x: string) => number'.
  Type 'string' is not assignable to type 'number'.

So I assume it's fixed.

[mhegazy]

thanks! closing.