Homebrew GCC 15 not working...

[bredelings]

Hi, while submitting the PR #2041, I attempted to build the formula locally. I get errors like the following:

/home/linuxbrew/.linuxbrew/Cellar/gcc/15.1.0/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;
      |                                ^~~~~~~~~~~~~~~~~~~
/home/linuxbrew/.linuxbrew/Cellar/gcc/15.1.0/include/c++/15/bits/std_mutex.h:208:32: error: cannot convert '<brace-enclosed initializer list>' to 'unsigned int' in initialization
  208 |     __gthread_cond_t _M_cond = __GTHREAD_COND_INIT;
      |                                ^~~~~~~~~~~~~~~~~~~

Someone got errors like this while trying to install in a non-standard prefix:

https://github.com/orgs/Homebrew/discussions/6193

But I am not doing that. Any idea what might be going on?

Weirdly, the PR seems to build just fine on the build bots...