Typechecking can be bypassed for descendant of generics when a typedesc parameter or auto is used

[alaviss]

Example

type
  Either[T, U] = object of RootObj
    t: T
    u: U

  E = object of Either[float, string]

proc test(T: typedesc, e: Either[T, T]) =
  echo "matched ", typeof(e)
  echo e

proc test1[T](e: Either[T, auto]) =
  echo "matched ", typeof(e)
  echo e

proc test2[T](e: Either[T, T]) =
  echo "matched ", typeof(e)
  echo e

proc makeE(): E =
  result.t = -1.0
  result.u = "string"

test(int, makeE())
test1[int](makeE())

when false:
  discard "This doesn't compile as expected"
  test2[int](makeE())

Current Output

matched Either[system.int, system.int]
(t: -4616189618054758400, u: 140610450096208)
matched Either[system.int, system.string]
(t: -4616189618054758400, u: "string")

Expected Output

This should not compile at all

Additional Information

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-09-12
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: 5d1608c9764416e74b36f6a772f16a66d58ddc28
active boot switches: -d:release -d:nimUseLinenoise

[bung87]

changes E = object of Either[float, string] to E = Either[float, string] will be CT error.