g++-14 broken with Xcode 16.3

[pfusik]

brew gist-logs <formula> link OR brew config AND brew doctor output

fox@wiesmac ~ % brew config
HOMEBREW_VERSION: 4.4.29-24-ga3d508f
ORIGIN: https://github.com/Homebrew/brew
HEAD: a3d508f3f90bb302c1efd9b36765ae6f34cb3a0c
Last commit: 13 hours ago
Branch: master
Core tap JSON: 10 Apr 08:13 UTC
Core cask tap JSON: 10 Apr 08:13 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 8
HOMEBREW_SORBET_RUNTIME: set
Homebrew Ruby: 3.3.7 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/3.3.7/bin/ruby
CPU: octa-core 64-bit arm_firestorm_icestorm
Clang: 17.0.0 build 1700
Git: 2.39.5 => /Library/Developer/CommandLineTools/usr/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 15.4-arm64
CLT: 16.3.0.0.1.1742442376
Xcode: 16.3
Rosetta 2: false
fox@wiesmac ~ % brew doctor
Your system is ready to brew.

Verification

• My brew doctor output says Your system is ready to brew. and am still able to reproduce my issue.
• I ran brew update and am still able to reproduce my issue.
• I have resolved all warnings from brew doctor and that did not fix my problem.
• I searched for recent similar issues at https://github.com/Homebrew/homebrew-core/issues?q=is%3Aissue and found no duplicates.

What were you trying to do (and why)?

g++ seems broken by Xcode 16.3 released last week.

What happened (include all command output)?

fox@wiesmac ~ % cat break.cpp
#include

fox@wiesmac ~ % g++-14 -c break.cpp
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/machine/_structs.h:35,
from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/arm/_mcontext.h:36,
from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/machine/_mcontext.h:34,
from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/sys/signal.h:146,
from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/sys/wait.h:109,
from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/_stdlib.h:70,
from /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/stdlib.h:58,
from /opt/homebrew/Cellar/gcc/14.2.0_1/include/c++/14/cstdlib:79,
from break.cpp:1:
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:627:35: error: expected primary-expression before 'unsigned'
627 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:627:26: error: '_Alignof' was not declared in this scope
627 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:633:35: error: expected primary-expression before 'unsigned'
633 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:633:26: error: '_Alignof' was not declared in this scope
633 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:639:35: error: expected primary-expression before 'unsigned'
639 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:639:26: error: '_Alignof' was not declared in this scope
639 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:645:35: error: expected primary-expression before 'unsigned'
645 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include/mach/arm/_structs.h:645:26: error: '_Alignof' was not declared in this scope
645 | } attribute((aligned(_Alignof(unsigned int))));
| ^~~~~~~~

What did you expect to happen?

Compilation succeeds.

Step-by-step reproduction instructions (by running brew commands)

% cat break.cpp
#include <cstdlib>

% g++-14 -c break.cpp

[Bo98]

This has been fixed but we didn't force an update yet. Try brew reinstall gcc.

[pfusik]

It worked, thank you!

[hrishabh314]

brew reinstall gcc didn't work for me. Uninstalling and installing did.