gcc@15.1.0 / gcc@13.3.0 on x86_64-linux-gnu: a stale fixincludes pthread.h in the payload shadows glibc 2.44, so <memory>/<mutex> do not compile

[Sunrisepeak]

Summary

With --toolchain gcc@15.1.0 (and gcc@13.3.0) on x86_64-linux-gnu, any C++ translation unit that reaches libstdc++'s threading primitives fails to compile — including a two-line program that only includes <memory>. The default gcc@16.1.0 is not affected.

include/c++/15.1.0/ext/concurrence.h:257:32: error: cannot convert '<brace-enclosed initializer list>' to 'unsigned int' in initialization
  257 |     __gthread_cond_t _M_cond = __GTHREAD_COND_INIT;
include/c++/15.1.0/bits/std_mutex.h:208:32: error: cannot convert '<brace-enclosed initializer list>' to 'unsigned int' in initialization

Reproduction

mcpp 2026.9.21.3, xlings 2026.9.16.1, glibc payload xim-x-glibc 2.44.

mkdir -p probe/src && cd probe
printf '[package]\nname = "probe"\nversion = "0.1.0"\n' > mcpp.toml
printf '#include <memory>\nint main() { return *std::make_unique<int>(0); }\n' > src/main.cpp
mcpp build --toolchain gcc@15.1.0      # fails as above
mcpp build --toolchain gcc@16.1.0      # builds

Cause

The gcc 15.1.0 and 13.3.0 payloads ship a fixincludes copy of glibc's pthread.h, taken from the machine the toolchain was built on:

lib/gcc/x86_64-linux-gnu/15.1.0/include-fixed/pthread.h
  "It has been auto-edited by fixincludes from:
   /home/xlings/.xlings_data/subos/linux/usr/include/pthread.h"   (glibc, Copyright 2002-2024)

mcpp adds the realised glibc with -idirafter <xim-x-glibc/2.44>/include, so gcc's include-fixed/ is searched first and this stale pthread.h wins over glibc 2.44's. Its initializer is for the older pthread_cond_t layout:

header	PTHREAD_COND_INITIALIZER
gcc 15.1.0 include-fixed/pthread.h	{ { {0}, {0}, {0, 0}, {0, 0}, 0, 0, {0, 0} } }
glibc 2.44 include/pthread.h	{ { {0}, {0}, {0, 0}, 0, 0, {0, 0}, 0, 0 } }

glibc 2.44's struct __pthread_cond_s is __wseq, __g1_start, __g1_orig_size, __wrefs, __g_signals[2], ..., so the stale initializer puts a brace list where an unsigned int is — the error above.

Evidence that this file alone is the cause:

• include-fixed/ contents per payload: 13.3.0 → pthread.h README, 15.1.0 → pthread.h README, 16.1.0 → README only (and 16.1.0 works).
• With mcpp's own flags plus -isystem <dir> containing a pthread.h that just #includes glibc 2.44's, gcc 15.1.0 compiles the same sources cleanly (verified on yaml-cpp 0.8.0/0.9.0, cli11, re2).
• -fsyntax-only on #include <mutex> with exactly mcpp's -idirafter flags: gcc 13.3.0 exit 1, gcc 15.1.0 exit 1.
• The x86_64-linux-musl target of gcc 15.1.0 (xim-x-musl-gcc) is unaffected.

Suggested fix

Repackage gcc 13.3.0 / 15.1.0 the way 16.1.0 is packaged — without the fixincludes pthread.h (or with include-fixed/ emptied of headers generated from the build host's glibc). A fixed header describes the build machine's C library, and mcpp always supplies its own glibc, so it can only ever disagree with it.

Found while validating compat.yaml-cpp in mcpplibs/mcpp-index#464 on gcc 15.