Compiler error when proc refers to type with unused generic params.

[etan-status]

When a type is defined to be generic on an object type, but is not actually using that type in its definition, as in Foo[T] = object (T is not used in the body of Foo[T]), there are compiler errors when trying to access the generic type in a proc. It seems that the only a single implementation is generated for the different T in that case, leading to C compilation errors when multiple different implementations are requested in a single compilation unit.

Example

type
  Foo[T] = object

  Bar[F: Foo] = object
    c: proc(t: F.T)

proc cb[F](v: Bar[F]) =
  v.c(default(F.T))

type
  X = object
    x: uint32
  Y = object
    x: uint32
proc cbX(v: X) = discard
proc cbY(v: Y) = discard

let
  x = Bar[Foo[X]](c: cbX)
  y = Bar[Foo[Y]](c: cbY)
x.cb()
y.cb()

Current Output

error: passing 'tyObject_Y__U7uoXYJsXAmQ0DnkqRcTeA' (aka 'struct tyObject_Y__U7uoXYJsXAmQ0DnkqRcTeA') to parameter of incompatible type 'tyObject_X__nhXs9ay3Yu0ODg9bF6LB8cRA' (aka 'struct tyObject_X__nhXs9ay3Yu0ODg9bF6LB8cRA')
        nimZeroMem((void*)(&T1_), sizeof(tyObject_Y__U7uoXYJsXAmQ0DnkqRcTeA));  v.c.ClE_0? v.c.ClP_0(T1_, v.c.ClE_0):((TM__cz1FP2xNs9aBzwtJ0lyd4Jg_8)(v.c.ClP_0))(T1_);
                                                                                                     ^~~
1 error generated.

Expected Output

0
0

Possible Solution

• As a workaround, use T in the definition of Foo, e.g., Foo[T] = (T,) instead of Foo[T] = object.
• Second workaround is to only ever use a single instantiation in a compilation unit, e.g., put x and y into separate programs. The problem only occurs when there are multiple instantiations of different types.

Additional Information

% nim -v
Nim Compiler Version 1.2.16 [MacOSX: amd64]
Compiled at 2022-02-14
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: c6a9f27b3e36bae9aacec1bd6c37893fb98fd33f
active boot switches: -d:release