Passing a function pointer + env to a class constructor fails but works if applied directly

[deech]

When I create a C++ class that emulates a closure with function pointer and environment members and a member function that runs them I get a segfault. If I directly apply the closure in a standalone function it succeeds.

Example

{.emit: """/*TYPESECTION*/
class ClosureClass
{
  void (*c)(void*);
  void* env;
  public:
    ClosureClass(void (*c)(void*), void* env) { c = c; env=env; }
    void run() { (*c)(env); }
};
void apply(void(*c)(void*), void* env) { (*c)(env); }
""".};

type ClosureClass {.importcpp: "ClosureClass".} = object
type Closure = proc () {.nimcall.}

proc makeClosure(): proc () =
  let s = "from closure scope"
  return (proc () = echo s)

proc constructClosureClass(p:Closure,env:pointer):ptr ClosureClass {.importcpp: "new ClosureClass(@)".}
let f = makeClosure()
var c = constructClosureClass(cast[Closure](rawProc f), rawEnv f)

# works fine
proc apply(p:Closure,env:pointer) {.importcpp: "apply(@)".}
apply(cast[Closure](rawProc f), rawEnv f)

# segfaults
proc run(this:ClosureClass) {.importcpp:"#.run()".}
run(c[])

Current Output

from closure scope
Traceback (most recent call last)
/home/deech/Nim/cppclosure.nim(28) cppclosure
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

from closure scope
from closure scope

$ nim -v
Nim Compiler Version 1.3.5 [Linux: amd64]
Compiled at 2020-07-06
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: a754160d654f03584088412f51b153ae6a170adb
active boot switches: -d:release