Computed types violate isa transitivity

[yglukhov]

Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-12-06
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: c3a20fa
active boot switches: -d:release

Description

Playing with computed types I wanted to express smth like the following (pseudocode):

type Foo[T] = object of Foo[getParentType(T)]

However the resulting types seem to have broken isa relation. The following code shows a runtime problem, where A is B, B is C, but A is not C, which violates transitivity common sense.

import macros

type
  JSObj {.inheritable.} = object
  JSExternObjBase {.inheritable.} = object
  Document = object

macro parent(t: typedesc): untyped =
  let n = t.getType()[1]
  result = case $n
           of "JSObj": ident "JSExternObjBase"
           of "Document": ident "JSExternObjDocument"
           else: assert(false); nil
  echo "parent(", n, ") = ", repr result

type
  JSExternObj[T] = parent(T)
  JSExternObjDocument = object of JSExternObj[JSObj]

assert JSExternObj[Document] is JSExternObjDocument
assert JSExternObjDocument is JSExternObj[JSObj]
assert JSExternObj[Document] is JSExternObj[JSObj] # FAILS HERE

Current Output

Error: unhandled exception: .../test.nim(22, 1) `JSExternObj[Document] is JSExternObj[JSObj]`  [AssertionDefect]

Expected Output

No errors

Known Workarounds

No response

Additional Information

No response