Concepts and cross-module generic binding madness

[alaviss]

Example

handles.nim:

type
  FD* = distinct cint

type
  AnyFD* = concept fd
    close(fd)

proc close*(fd: FD) =
  discard

type
  Handle*[T: AnyFD] = object
    fd: T

proc close*[T: AnyFD](h: var Handle[T]) =
  close h.fd

proc initHandle*[T: AnyFD](fd: T): Handle[T] =
  Handle[T](fd: fd)

files.nim:

import handles

type
  File* = ref object
    handle: Handle[FD]

proc close*[T: File](f: T) =
  f.handle.close()

proc newFile*(fd: FD): File =
  File(handle: initHandle(FD -1))

test.nim:

import files
from handles import FD
#import handles <- do this and it works

let wr = newFile(FD -1)
close wr

Current Output

test.nim(6, 7) template/generic instantiation of `close` from here
files.nim(8, 11) Error: type mismatch: got <Handle[handles.FD]>
but expected one of:
proc close(f: File)
  first type mismatch at position: 1
  required type for f: File
  but expression 'f.handle' is of type: Handle[handles.FD]
proc close(fd: FD)
  first type mismatch at position: 1
  required type for fd: FD
  but expression 'f.handle' is of type: Handle[handles.FD]
proc close[T: AnyFD](h: var Handle[T])
  first type mismatch at position: 1
  required type for h: var Handle[close.T]
  but expression 'f.handle' is of type: Handle[handles.FD]
test.nim(6, 7) template/generic instantiation of `close` from here
handles.nim(6, 10) AnyFD: type mismatch: got <Alias>
but expected one of:
proc close(f: File)
  first type mismatch at position: 1
  required type for f: File
  but expression 'fd' is of type: Alias
proc close[T: File](f: T)
  first type mismatch at position: 1
  required type for f: T: File
  but expression 'fd' is of type: Alias

expression: close(fd)
proc close[T: File](f: T)
  first type mismatch at position: 1
  required type for f: T: File
  but expression 'f.handle' is of type: Handle[handles.FD]

expression: close(f.handle)

Additional Information

• This issue is reduced from alaviss/nim-sys#anyfd-concept

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-01-15
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: 7b632f9ccbd4d15fa9fe4ca45b6fea22b259c81d
active boot switches: -d:release -d:nimUseLinenoise

[mratsim]

I would be surprised if it's not another instance of Generic sandwiches #11225. (tag #8677). I.e. not due to concept but by how generics are implemented and how generic (early) symbol resolution works.