[Pitch] Lifting restriction on explicit specialization in function calls

[harlanhaskins]

Hi! Apologies for not following up. I am still keen on getting this in, but agreeing with @Douglas_Gregor’s point in this prior thread:

Unrestricting explicit declarations of function generics

My opinion hasn't shifted since my response in 2016. I still like the restriction that each generic parameter must be mentioned within the function signature, because it (still) pushes us toward better APIs. I still think the explicit specialization syntax is reasonable, but haven't felt much need for it in the intervening years. For me, the only difference since 2016 is that we now support C++ interoperability, so there are C++ function templates that don't follow these rules and can't meanin…

That we should leave in the restriction that you must use the generic parameter within the function signature, e.g.

func perform<X: Something>() {
    X.staticMethod()
}

would be disallowed. That said, if we did want to lift this restriction, we could do so as long as we make sure that the diagnostics are updated to diagnose missing type parameters as missing values rather than as generic constraint mismatches, e.g. perform() diagnosing something like missing type parameter ‘X’ in call to 'perform'with a fixit to insert <<#Type#>>.