GCC 15 from Homebrew fails to compile C++ code on Ubuntu 25.04 (glibc 2.41) - __GTHREAD_COND_INIT error

[yuki2006]

brew config output

HOMEBREW_VERSION: 4.5.2
ORIGIN: https://github.com/Homebrew/brew
HEAD: xxx
Last commit: xxx
Core tap JSON: xxx
Core tap origin: https://github.com/Homebrew/homebrew-core
Core tap HEAD: xxx
Core tap branch: master
HOMEBREW_PREFIX: /home/linuxbrew/.linuxbrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 2
Homebrew Ruby: 3.3.6
CPU: Generic x86_64
Clang: N/A
Git: 2.48.1
Curl: 8.12.1
Kernel: Linux 6.14.x x86_64
OS: Ubuntu 25.04 (plucky)
Host glibc: 2.41
/usr/bin/gcc: 14.2.0
glibc: N/A

brew doctor output

Your system is ready to brew.

What you tried to do

Compile a simple C++ program using Homebrew's GCC 15 on Ubuntu 25.04

What happened (include command output)

$ /home/linuxbrew/.linuxbrew/bin/g++-15 -std=c++23 main.cpp

In file included from /home/linuxbrew/.linuxbrew/Cellar/gcc/15.2.0/include/c++/15/x86_64-pc-linux-gnu/bits/gthr-default.h:35,
...
/home/linuxbrew/.linuxbrew/Cellar/gcc/15.2.0/include/c++/15/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;
      |                                ^~~~~~~~~~~~~~~~~~~

What you expected to happen

The program should compile successfully.

Environment

• OS: Ubuntu 25.04 (Plucky Puffin)
• glibc: 2.41
• Homebrew GCC: 15.2.0 (installed via brew install gcc)
• System GCC: 15.0.1 (from apt)

Root Cause

Homebrew's GCC 15 was built against a different version of glibc. The __GTHREAD_COND_INIT macro (which expands to PTHREAD_COND_INITIALIZER) in Homebrew's GCC headers is incompatible with glibc 2.41's pthread_cond_t structure.

Minimal Reproduction

docker run --rm ubuntu:25.04 bash -c '
  apt-get update -qq && apt-get install -y -qq curl git build-essential

  # Install Homebrew
  NONINTERACTIVE=1 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
  eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"

  # Install GCC from Homebrew
  brew install gcc

  # Create test file
  echo "#include <bits/stdc++.h>
int main() { return 0; }" > main.cpp

  # This fails
  /home/linuxbrew/.linuxbrew/bin/g++-15 -std=c++23 main.cpp
'