Generic function inference fails when using a delegate function parameter that only uses generic types

[patsissons]

The title probably sounds a bit verbose, but this is a bit of a complicated issue to describe. It's best if I paste some code and explain it.

protected Test1<T>(val1: T, sel: (T) => T) {
    return sel(val1);
}

protected Test2<T>(val1: T, sel: (x: T) => T) {
    return sel(val1);
}

protected Test3() {
    let a = this.Test1(123, x => x + 123);
    let b = this.Test2(123, x => x + 123);
}

both Test1 and Test2 perform the exact same function. But in Test3 a will be typeless (any) and b will be strongly typed to number. The only difference is that Test2 uses a delegate parameter instance variable in the declaration (x). Now that I have found a work around this is less of an issue, but others may want to know about this because it's super annoying writing projector functions that don't return strongly typed results.

[RyanCavanaugh]

This declaration:

   protected Test1<T>(val1: T, sel: (T) => T) {

is equivalent to this declaration:

   protected Test1<T>(val1: T, sel: (T_The_Parameter_Name: any) => T) {

The parameter name T has no relation to the generic type parameter T. So we should expect that a will be any because the type parameter T got two inference candidates: number (via val1) and any (via the return type of the function body, where we computed the return expression of the form any + number to be of type any), between which any is chosen because it is more general.

You may be interested in the --noImplicitAny compiler flag, or the discussion in #5187 and #3081.

[patsissons]

Thank you for the explanation, the issue now makes much more sense. I will turn on the flag in my project. I think it definitely makes sense to enable that flag by default to make it an opt out.

I'll close this issue now since it is by design.