Possible gcc 15 modules regression w/ TU-local entity in constexpr function

[Marcel Laverdet <marcel@xxxxxxxxxxxx>]

I ran into the following error while attempting to compile the following
simple case in gcc 15.1.0. gcc 14 and clang 20 compile it without any
problems. Is this a regression or is this behavior expected?


root[18:56:11] [/workspace] $ cat a.h
constexpr int flag = 0;
constexpr bool check() {
        return flag == 0;
}

root[18:57:41] [/workspace] $ cat a.cc
module;
#include "a.h"
export module M;
export using ::check;

root[18:57:54] [/workspace] $ gcc-14 -fmodules-ts -c a.cc
# it compiles fine

root[18:57:57] [/workspace] $ gcc-15 -fmodules-ts -c a.cc
In file included from a.cc:2:
a.h:2:16: error: 'constexpr bool check()' exposes TU-local entity 'flag'
    2 | constexpr bool check() {
      |                ^~~~~
a.h:1:15: note: 'flag' declared with internal linkage
    1 | constexpr int flag = 0;
      |               ^~~~

root[18:58:00] [/workspace] $ gcc-15 --version
gcc-15 (Debian 15.1.0-1) 15.1.0
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