[Concepts] Consider concept declarations for name resolution

[konsumlamm]

Motivation

Currently, the following example fails to compile (see nim-lang/Nim#17420):

type
  Comparable = concept
    proc compare(a, b: Self): int # so that there are no conflicts with `cmp`

proc test[T: Comparable](x, y: T) =
  echo compare(x, y) # Error: undeclared identifier: 'compare'

The reason is that the declarations in a concept body are not considered for name resolution. However, this would be very useful for defining concepts and functions that operate on types implementing the concept, without having such a type in scope.

Solution

Concepts define a dummy proc that is considered for name resolution. It wouldn't need to return anything useful, since it can only be called when the concept is implemented for the type it's called with anyway. This could either be done by generating a proc that just returns the default value of the return type (although this wouldn't work when you can define types without a default value, see #290 and #252) or by using compiler magic.

[Araq]

Why not instead let the concepts inject a set of mixin declarations inside the procs that use the concept?

[konsumlamm]

Why not instead let the concepts inject a set of mixin declarations inside the procs that use the concept?

If that works, sure, I've just never used mixin. I don't have any strong preference for the implementation, let's do whatever works well.

EDIT: Looking at the documentation for mixin, that seems to be exactly the mechanism we want here.

[konsumlamm]

@Araq is there anything blocking this?

[Araq]

Feel free to give it a try, the core team has currently no capacity to work on it.