[VM] Immutable proc argument is incorrectly modified

[konsumlamm]

Example

type
  Wrapper = object
    inner: int

proc assign(r: var Wrapper, a: Wrapper) =
  r = a

proc myEcho(a: Wrapper) =
  var tmp = a
  assign(tmp, Wrapper(inner: 0)) # this shouldn't modify `a`
  echo a

static:
  var result: Wrapper
  assign(result, Wrapper(inner: 1))
  myEcho(result)

Current Output

(inner: 0)

Expected Output

(inner: 1)

Additional Information

The problem seems to be that myEcho directly modifies a, instead of just the tmp copy.

This doesn't occur if

• the code isn't run at compile time
• no wrapper type is used
• normal assignment is used instead of assign

$ nim -v
Nim Compiler Version 1.7.1 [Linux: amd64]
Compiled at 2022-01-27
Copyright (c) 2006-2022 by Andreas Rumpf

git hash: 4b723c0f5350c5bfb9722dace7a40c66a4630a68
active boot switches: -d:release

(This example may seem contrived, but that's because I tried to reduce it as much as possible. I originally encountered this bug while adding debug echos in nim-lang/bigints.)