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Invalid C code generation with `iterator` with `when nimvm`: `error: ‘c__r_u3’ undeclared (first use in this function); did you mean ‘c__r_u10’`
Nim Version
Nim Compiler Version 2.2.4 [Linux: amd64]
Compiled at 2025-08-25
Copyright (c) 2006-2025 by Andreas Rumpf
git hash: f7145dd26efeeeb6eeae6fff649db244d81b212d
active boot switches: -d:release
Nim Compiler Version 2.2.5 [Linux: amd64]
Compiled at 2025-08-25
Copyright (c) 2006-2025 by Andreas Rumpf
git hash: c339651ae1d6af6c84f5489f6fe1fdea44ba1fa1
active boot switches: -d:release
Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-08-25
Copyright (c) 2006-2025 by Andreas Rumpf
git hash: b527db9ddd33fd16a8afd8467344fec81a54c84d
active boot switches: -d:release
gcc (Debian 14.3.0-5) 14.3.0
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Description
iterator v(): int =
when nimvm:
yield 0
else:
yield 0
for _ in v():
for c in v():
(; let _: proc() = proc() = discard c)
Current Output
.....................................................................
CC: system/exceptions.nim
CC: std/private/digitsutils.nim
CC: system/dollars.nim
CC: system.nim
CC: r.nim
/tmp/tmp.YWdRaJqN4e/@mr.nim.c: In function ‘colonanonymous___r_u5’:
/tmp/tmp.YWdRaJqN4e/@mr.nim.c:82:7: error: ‘c__r_u3’ undeclared (first use in this function); did you mean ‘c__r_u10’?
82 | (void)c__r_u3;
| ^~~~~~~
| c__r_u10
/tmp/tmp.YWdRaJqN4e/@mr.nim.c:82:7: note: each undeclared identifier is reported only once for each function it appears in
Expected Output
No invalid C code generated
Known Workarounds
No response
Additional Information
No response
Workaround:
import std/sugar
iterator v(): int =
when nimvm:
yield 0
else:
yield 0
for _ in v():
for c in v():
capture c:
(; let _: proc() = proc() = discard c)
Workaround:
import std/sugar
iterator v(): int = when nimvm: yield 0 else: yield 0 for _ in v(): for c in v(): capture c: (; let _: proc() = proc() = discard c)
Well, the general point is that it's presumptively always a Nim bug if pure Nim code generates invalid C. If for whatever reason it's not possible to generate correct C, then the Nim compiler should reject that code in a more intentional way, and it should be defined as invalid Nim code. Conversely, if it is valid Nim code, it should compile correctly.