SE-0341: Opaque Parameter Declarations

[allevato (Tony Allevato)]

@AlexanderM

Would it be possible to reference the type variables of sequence1 and sequence2 in a where clause? E.g. something like:

func eagerConcatenate(
    _ sequence1: some Sequence, _ sequence2: some Sequence
) -> [T] where type(of: sequence1).Element == type(of: sequence2).Element

It was mentioned in the pitch thread that this wouldn't be possible.

I agree with the rationale there; if there's a critical need to refer to the member types of the generic parameter type, then the generic parameter type should just be given an explicit name. This proposal aims to reduce boilerplate for a certain class of common uses of generics, but I think it would be bad to start adding other ways to extract things like member types from an unnamed generic parameter. Something like the syntax above would effectively create two parallel generics syntaxes in the language, instead of one just being sugar for the more complex one.