[Bug c++/109295] New: deducing brace-enclosed initializer list using import fails to compile

[matthewsatti at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109295

            Bug ID: 109295
           Summary: deducing brace-enclosed initializer list using import
                    <initializer_list> fails to compile
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matthewsatti at gmail dot com
  Target Milestone: ---

Created attachment 54765
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54765&action=edit
pre-processed file using -save-temps

Hello,

I've been experimenting with C++20 modules and came across the following issue.

Given the program:

import <initializer_list>;
int main() {
  auto values = {1, 2, 3};
  return 0;
}

and trying to compile with these commands:

g++ -std=c++20 -Wall -Wextra -fmodules-ts -x c++-system-header initializer_list
g++ -std=c++20 -Wall -Wextra -fmodules-ts main.cxx -o main

produces the compile error:

main.cxx: In function ‘int main()’:
main.cxx:4:31: error: deducing from brace-enclosed initializer list requires
‘#include <initializer_list>’
    1 | import <initializer_list>;
  +++ |+#include <initializer_list>
    2 | 
    3 | int main() {
    4 |   auto values = {1, 2, 3};
      |                         ^

My assumption is that import <initializer_list>; should have worked?

gcc -v output:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.1 20230201 (GCC)

I have also attached the pre-processed output