Objects subtype generics are bound using the most broad type and not the most specific type

[alaviss]

Example

type
  A = ref object of RootObj
  B = ref object of A
  C = ref object of B

proc foo[T: A](a: T) = echo "got A"
proc foo[T: B](b: T) = echo "got B"

var c = C()
foo(c)

Current Output

got A

Expected Output

got B

Additional Information

• Found during development of CPS

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-06-14
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: a266c549212d1d6e09dbfa01344edbca8b2f6222
active boot switches: -d:release -d:nimUseLinenoise

[alaviss]

Snippet demonstrating a similar issue:

type
  A = ref object of RootObj
  B = ref object of A
  C = ref object of B
  
proc foo[T: A](a: T) = echo "got A"
proc foo(b: B) = echo "got B"

var c = C()
foo(c)

However, in this case a concrete overload is overpowered by a generic overload.

[Araq]

Your second example works according to the spec, the first example should probably produce an ambiguity call.

[metagn]

Works since 2.2