```
Module validation failed: Invalid bitcast
%4 = bitcast i32* %3 to %"(Spectator::Arguments(T, NT) | Nil)", !dbg !16
(Exception)
from /crystal/src/int.cr:541:9 in 'finish'
from /crystal/src/compiler/crystal/codegen/codegen.cr:71:7 in 'codegen'
from /crystal/src/compiler/crystal/compiler.cr:173:16 in 'compile'
from /crystal/src/compiler/crystal/command/spec.cr:98:14 in 'spec'
from /crystal/src/hash.cr:901:11 in '__crystal_main'
from /crystal/src/crystal/main.cr:115:5 in 'main'
from src/env/__libc_start_main.c:94:2 in 'libc_start_main_stage2'
Error: you've found a bug in the Crystal compiler. ...
```
I haven't been able to reproduce this in a simplified form.
Passing the `constraint` argument in NullStub#and_return to ValueStub.new triggers the bug.
Removing the `constraint` argument fixes the issue.
