Codegen error when using methods with var parameters

[alistairkeys]

A codegen error can be triggered by creating a subclass and calling a method with 'var' parameters.

The following snippet produces an error when it gets to obj3. If you remove 'var' from the method parameters then it compiles.

Example

type
  BaseClass = ref object of RootObj
  SubClass = ref object of BaseClass
  
method foo(x: var BaseClass) {.base.} = echo "base" 
method foo(x: var SubClass) = echo "sub"
  
var obj1 = BaseClass()
obj1.foo()

var obj2: BaseClass = SubClass()
obj2.foo()

var obj3 = SubClass()
obj3.foo()

https://play.nim-lang.org/#ix=3s68

I'm not sure whether I'd expect this to work or to give a 'nice' error but a codegen error doesn't seem right.

Current Output

Without 'var' params:
base
sub
sub

With 'var' params (output taken from the Nim playground snippet mentioned earlier):

Hint: used config file '/playground/nim/config/nim.cfg' [Conf]
Hint: used config file '/playground/nim/config/config.nims' [Conf]
....
Hint: gcc -c  -w -fmax-errors=3   -I/playground/nim/lib -I/usercode -o /usercode/nimcache/stdlib_io.nim.c.o /usercode/nimcache/stdlib_io.nim.c [Exec]
Hint: gcc -c  -w -fmax-errors=3   -I/playground/nim/lib -I/usercode -o /usercode/nimcache/stdlib_system.nim.c.o /usercode/nimcache/stdlib_system.nim.c [Exec]
Hint: gcc -c  -w -fmax-errors=3   -I/playground/nim/lib -I/usercode -o /usercode/nimcache/@min.nim.c.o /usercode/nimcache/@min.nim.c [Exec]
/usercode/nimcache/@min.nim.c: In function 'NimMainModule':
/usercode/nimcache/@min.nim.c:537:39: error: expected ';' before '->' token
  T4_ = &&obj3__RMWab9bcqUlZuOCGx7H85cA->Sup;
                                       ^~
/usercode/nimcache/@min.nim.c:537:2: error: label 'obj3__RMWab9bcqUlZuOCGx7H85cA' used but not defined
  T4_ = &&obj3__RMWab9bcqUlZuOCGx7H85cA->Sup;
  ^~~
Error: execution of an external program failed: 'gcc -c  -w -fmax-errors=3   -I/playground/nim/lib -I/usercode -o /usercode/nimcache/@min.nim.c.o /usercode/nimcache/@min.nim.c'

If only obj1 and obj2 are in the example, it compiles and gives output. It seems related to creating a subclass specifically and calling the method with var parameters.

Version

Windows 10 (Version 20H2, build 19042.1052)

Nim Compiler Version 1.4.8 [Windows: amd64]
Compiled at 2021-05-25
Copyright (c) 2006-2021 by Andreas Rumpf

active boot switches: -d:release

I tried yesterday's nightly (1.5.1) and that gave an error too.

[jackvandrunen]

I just tracked this down because I was testing JS codegen for a library I wrote that works fine with C, and uses methods with var parameters (but only calls them when casting to the base type, which as you noted works). This fails silently on the JS backend. As in, it will simply not call any method, and not print anything in your example.

I'm not sure whether I'd expect this to work or to give a 'nice' error but a codegen error doesn't seem right.

Is there a reason you'd expect it not to work? I'm going to be needing to redesign some things if so...

Edit: ok, ok, I see now that there are like 3 or 4 open issues related to this problem, and it's not supposed to work.

[alistairkeys]

When I originally mentioned this error on Discord, someone replied saying they wouldn't expect this to work ("No I don't think you should be able to have var methods as that implies you can change where the var points") and it made me unsure. However, if it can be made to work then I won't say no! :)

[Araq]

This code should not compile as there is no subtype relation between var ref's.