GNU mode accepts pointer-to-unsigned-char reinterpret_cast with a warning instead of an error

[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

static_assert(sizeof(void*) > sizeof(unsigned char),
              "This reproducer requires pointers wider than unsigned char");

unsigned char narrow(void *p)
{
    return reinterpret_cast<unsigned char>(p);
}

Compiler options:

--c++20 --gnu_version=130300 --strict_gnu narrow.cpp

Behavior

On this target, pointers are 64 bits and unsigned char is 8 bits. The destination cannot hold all pointer values, so the cast is ill-formed.

GCC rejects it without -fpermissive and exits with status 1:

error: cast from 'void*' to 'unsigned char' loses precision [-fpermissive]

GNU emulation instead generates C and exits with status 0, issuing:

warning: conversion from pointer to smaller integer

Strict ISO mode (--c++20 --strict, without GNU emulation) correctly rejects it and exits with status 2.

This report concerns GNU compatibility and diagnostic severity. The warning is a diagnostic; is accepting the cast as an extension intentional?

Reference: C++20 [expr.reinterpret.cast]/1 and /4: https://timsong-cpp.github.io/cppwp/n4861/expr.reinterpret.cast#4.

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

Source, generated C, results, and full configuration

Reporting acknowledgements

I acknowledge that:

• 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.
• I understand code shared via this form is NOT confidential.
• I have permission to share the reproducing source code and grant permission for the project to use the source code (or derivatives of the source code) to create tests.
• I agree that any tests created using the reproducing source code (or deriatives of it) will be licensed under project license unless I included a LICENSE.txt file in my .zip or .tar.gz upload. If I included a LICENSE.txt file, any tests created using the reproducing source code will be governed by the terms of the license I provided instead.