Can use param.T in return type only with type classes

[gabbhack]

With type class

type Test[T] = object

proc run(self: Test): self.T = discard

discard run(Test[int]())

https://play.nim-lang.org/#ix=44m4

Current output

As expected

With generic

type Test[T] = object

proc run[T](self: Test[T]): self.T = discard

discard run(Test[int]())

https://play.nim-lang.org/#ix=44m5

Current Output

/usercode/in.nim(5, 12) template/generic instantiation of `run` from here
/usercode/in.nim(3, 29) Error: cannot evaluate at compile time: self

With auto

type Test[T] = object

proc run(self: Test[auto]): self.T = discard

discard run(Test[int]())

https://play.nim-lang.org/#ix=44m6

Current Output

/usercode/in.nim(3, 33) Error: undeclared field: 'T'

$ nim -v
Nim Compiler Version 1.6.6 [Windows: amd64]
Compiled at 2022-05-05
Copyright (c) 2006-2021 by Andreas Rumpf

active boot switches: -d:release

[hamidb80]

workaround:

type Test[T] = object

proc run[T](self: Test[T]): T = discard

discard run(Test[int]())

[metagn]

Doing

type Test[T] = object

proc run[T](self: Test[T]): typeof(self).T = discard

discard run(Test[int]())

gives

fatal.nim(53)            sysFatal
Error: unhandled exception: field 'intVal' is not accessible for type 'TFullReg' using 'kind = rkNode' [FieldDefect]