Overloading resolution - Specializing generic proc like $

[mratsim]

Related to #5644, #1991 and commit f8c921d.

Overloading resolution works as expected when the proc definition and the proc call are in the same file.
However there are conflicts when proc is called from another module.
It might be as "intended" from the manual

Rule #3: Generic match: f is a generic type and a matches, for instance a is int and f is a generic (constrained) parameter type (like in [T] or [T: int|char].

but this prevents specializing proc like $ for typeclass and maybe concepts:

Test case in 2 files

# Filename: test1.nim

type
  Matrix*[M, N: static[int], T: SomeReal] = object
    data: ref array[N * M, T]
  Matrix32*[M, N: static[int]] = Matrix[M, N, float32]
  Matrix64*[M, N: static[int]] = Matrix[M, N, float64]

proc zeros*(M,N: static[int], T: typedesc[float64]): Matrix[M,N,T] =
  new result.data
  for i in 0 .. < (M * N):
    result.data[i] = 0

proc `$`*[M, N: static[int]](m: Matrix32[M, N] or Matrix64[M, N]): string =
  result = "Matrix[" & $M & ", " & $N & "]"

File2

import test1

proc baz*[T](a: T) =
  echo a

let a = zeros(2,2, float64)

baz(a)

Error

ambiguous call; both system.$(x: T) and test1.$(m: Matrix32[$.M, $.N] or Matrix64[$.M, $.N]) match for: (Matrix[2, 2, system.float64])

[Clyybber]

Works on devel. (replace SomeReal with SomeFloat in the testcase)

[Araq]

This needs a test case.