C generation loses the signed underlying type of a scoped enum

[71iq]

Version

Compiler: 7.0, commit 43e10b90fc83ed2a436fb79b241be8dbf3dc6846
Platform: Linux x86_64 (LP64)
Reference/compiler for generated C: GCC 13.3.0
Preset: linux-gcc-debug, with EDG_CPFE_USE_ASAN=OFF

Relevant configuration:

#define BACK_END_IS_C_GEN_BE 1
#define C_GEN_BE_GENERATES_ANSI_C 1
#define GNU_EXTENSIONS_ALLOWED 1
#define TARG_CHAR_BIT 8
#define TARG_SIZEOF_INT 4
#define TARG_SIZEOF_LONG 8
#define TARG_SIZEOF_POINTER 8

Code to reproduce

#include <cassert>

enum class ClassE { ZERO = 0 };

int main()
{
    ClassE e = static_cast<ClassE>(-1);
    long widened = static_cast<long>(e);
    assert(widened == -1);
}

Compiler options:

--c++20 --no_exceptions --gnu_version=130300 --gen_c_file_name enum.cpp.int.c enum.cpp.ii

Behavior

A scoped enum without an explicit enum-base has the fixed underlying type int. Converting ClassE(-1) to long should therefore yield -1.

The reference program compiled with GCC exits with status 0. The translated program aborts on widened == -1; widened is instead 4294967295.

The backend emits a C enum:

enum ClassE { _ZN6ClassE4ZEROE };
enum ClassE e = (enum ClassE)(-1);
long widened = (long)e;

GCC gives this generated C enum an unsigned representation. The frontend itself correctly identifies the original C++ underlying type: this check passes with the same GNU options:

static_assert(__is_same(__underlying_type(ClassE), int));

The problem appears to be preserving signedness during C generation.

References: C++20 [dcl.enum]/5 and /8: https://timsong-cpp.github.io/cppwp/n4861/dcl.enum#5.

The attachment contains the full --dump_configuration output and self-contained reproduction inputs.

Source, generated C, results, and full configuration

Reporting acknowledgements

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• I have checked open issues and existing discussions before filing this report.
• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
• I have reduced the reproducing source code to the best of my ability.
• I have selected at least one standard configuration or uploaded a non-standard configuration dump.
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