[Bug c/114657] New: Invalid type conversion from some _BitInt bit-fields

["juuso.alasuutari at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114657

            Bug ID: 114657
           Summary: Invalid type conversion from some _BitInt bit-fields
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juuso.alasuutari at gmail dot com
  Target Milestone: ---

Created attachment 57911
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57911&action=edit
Preprocessed test program source

Summary
-------

GCC's type conversion from some bit-precise integer-typed bit-fields
does not match the description in the C23 standard.


Expected vs. observed behavior
------------------------------

Based on footnote on page 47 of ISO/IEC 9899/2024 6.3.1.1 (n3320.pdf),
the converted type of a _BitInt bit-field should match the original
bit-precise integer:

  "E.g. unsigned _BitInt(7): 2 is a bit-field that can hold the
   values 0, 1, 2, 3, and converts to unsigned _BitInt(7)."

My expectation would then be to see the following output, which does
in fact happen when compiling with clang-19:

  $ ./test
  unsigned _BitInt(7)

This is what GCC does:

  $ ./test
  unsigned _BitInt(2)


Output of gcc-14 -v
-------------------

Using built-in specs.
COLLECT_GCC=gcc-14
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/14/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 14-20240330-1'
--with-bugurl=file:///usr/share/doc/gcc-14/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2,rust --prefix=/usr
--with-gcc-major-version-only --program-suffix=-14
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify
--enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/reproducible-path/gcc-14-14-20240330/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/reproducible-path/gcc-14-14-20240330/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver
--enable-checking=yes,extra,rtl --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.0.1 20240330 (experimental) [master r14-9728-g6fc84f680d0]
(Debian 14-20240330-1) 


Compilation command
-------------------

gcc-14 -std=gnu23 -Wall -Wextra -Wpedantic -save-temps -o test test.c