GC_ref on empty string fails with [GCASSERT] incRef: interiorPtr

[iffy]

Calling GC_ref() on an empty string when running with -d:useGcAssert causes a GCASSERT error (which is different than if the string is non-empty).

Example

proc someProc(x:bool):cstring =
  var res:string = ""
  if x:
    res = "yes"
  echo "res.repr: ", res.repr
  GC_ref(res)
  result = res

echo "running someProc(true)"
echo someProc(true)

echo "running someProc(false)"
echo someProc(false)

Current Output

running someProc(true)
res.repr: 0x10d90f058"yes"
yes
running someProc(false)
res.repr: ""
[GCASSERT] incRef: interiorPtr
Traceback (most recent call last)
testthing.nim(13)        testthing
testthing.nim(6)         someProc
gc.nim(198)              nimGCref
gc.nim(116)              incRef

Expected Output

running someProc(true)
res.repr: 0x10d90f058"yes"
yes
running someProc(false)
res.repr: ""

Possible Solution

It seems from the repr output that empty strings are treated specially (and likely don't have a location in memory?) I could change my code to only call GC_ref/unref on non-empty strings, though it would complicate things slightly. If that's the solution, I'm happy to do it.

Additional Information

$ nim -v
Nim Compiler Version 0.19.1 [MacOSX: amd64]
Compiled at 2018-10-19
Copyright (c) 2006-2018 by Andreas Rumpf

git hash: 863d3ef4ed7d18877a42f0060ebe63c34a27cf32
active boot switches: -d:release

[stale]

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[ringabout]

It has been fixed since 0.19.2.