Unrestricting explicit declarations of function generics

[Douglas_Gregor]

@xwu

Your point goes to the crux of the issue here, which @Douglas_Gregor brought up in his reply from many years ago. By not having this feature, we are deliberately requiring that a generic function can only be used if all of its parameters can be specified entirely through a normal parameter or the return type.

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 meaningfully be used from Swift or have more awkward spellings in Swift than in C++.

Doug