=copy and =sink not called in instantiation of forward-declared generic proc

[exelotl]

When a generic proc is invoked before its implementation is defined, =copy and =sink will not be called for any assignments that occur within the proc's implementation.

Example

type
  Foo = object
    n: int

proc `=copy`(dst: var Foo, src: Foo) =
  echo "copying"
  dst.n = src.n

proc `=sink`(dst: var Foo, src: Foo) =
  echo "sinking"
  dst.n = src.n

var a: Foo

proc putValue[T](n: T)

proc main =
  putValue(123)

proc putValue[T](n: T) =
  var b = Foo(n:n)
  a = b
  echo b.n

main()

Current Output

123

Expected Output

copying
123

(or, if echo b.n is commented out):

sinking
123

Additional Information

This appears to be a regression that was introduced between Nim 1.0.0 and 1.2.0 (according to the playground).

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-05-16