GCC breaks on Xcode / CLT updates sometimes

[scivision]

Output of brew config

HOMEBREW_VERSION: 5.1.2
ORIGIN: https://github.com/Homebrew/brew
HEAD: aa3933a4271380c7f9b1d63f63e3b141b15d6185
Last commit: 13 hours ago
Branch: stable
Core tap JSON: 30 Mar 16:14 UTC
Core cask tap JSON: 30 Mar 16:14 UTC
HOMEBREW_PREFIX: /opt/homebrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /var/run/com.apple.launchd.Poi2PEPOMf/org.xquartz:0
HOMEBREW_DOWNLOAD_CONCURRENCY: 16
HOMEBREW_EDITOR: nano
HOMEBREW_FORBID_PACKAGES_FROM_PATHS: set
HOMEBREW_MAKE_JOBS: 8
Homebrew Ruby: 4.0.2 => /opt/homebrew/Library/Homebrew/vendor/portable-ruby/4.0.2/bin/ruby
CPU: octa-core 64-bit arm_blizzard_avalanche
Clang: 21.0.0 build 2100
Git: 2.53.0 => /opt/homebrew/bin/git
Curl: 8.7.1 => /usr/bin/curl
macOS: 26.4-arm64
CLT: 26.4.0.0.1774242506
Xcode: 26.4
Metal Toolchain: N/A
Rosetta 2: false

Output of brew doctor

brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Some installed casks are deprecated or disabled.
You should find replacements for the following casks:
  imagej

Warning: Some installed formulae are deprecated or disabled.
You should find replacements for the following formulae:
  icu4c@77

Description of issue

tl;dr: when a macOS / Xcode / CLT update breaks GCC, should I file a Homebrew issue, or just wait a few weeks for the "gcc" bottle to be rebuilt, setting SDKROOT= in the meantime?

---

Every several months when Xcode / Command Line Tools updates something internally such as headers (it doesn't happen on every version change of Xcode / CLT) GCC stops being able to compile C++ code due to conflicts with the headers in the standard library - sometimes even just <iostream>.
I am almost always able to workaround this by specifying environment variable SDKROOT to point to the previous SDK. This inability to compile with GCC has just happened again on upgrading to macOS 26.4, Xcode 26.4, CLT 26.4.

The current break occurs for this main.cpp (which header breaks varies with Xcode / CLT release):

#include <mach-o/loader.h>

int main(){ return 0; }

% g++-15 main.cpp

n file included from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:78,
                 from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/arm/thread_status.h:40,
                 from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/machine/thread_status.h:35,
                 from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach-o/loader.h:52,
                 from main.c:1:
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:299:1: error: expected constructor, destructor, or type conversion before '(' token
  299 | xnu_static_assert_struct_size(mach_msg_type_descriptor_t, 12);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:308:1: error: expected constructor, destructor, or type conversion before '(' token
  308 | xnu_static_assert_struct_size_kernel_user(mach_msg_port_descriptor_t, 16, 12);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

<snip several screens of errors>

Here's the verbose output

% g++-15 main.c -v

Using built-in specs.
COLLECT_GCC=g++-15
COLLECT_LTO_WRAPPER=/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/aarch64-apple-darwin25/15/lto-wrapper
Target: aarch64-apple-darwin25
Configured with: ../configure --prefix=/opt/homebrew/opt/gcc --libdir=/opt/homebrew/opt/gcc/lib/gcc/current --disable-nls --enable-checking=release --with-gcc-major-version-only --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-15 --with-gmp=/opt/homebrew/opt/gmp --with-mpfr=/opt/homebrew/opt/mpfr --with-mpc=/opt/homebrew/opt/libmpc --with-isl=/opt/homebrew/opt/isl --with-zstd=/opt/homebrew/opt/zstd --with-pkgversion='Homebrew GCC 15.2.0_1' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues --with-system-zlib --build=aarch64-apple-darwin25 --with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.2.0 (Homebrew GCC 15.2.0_1) 
COLLECT_GCC_OPTIONS='-v' '-mmacosx-version-min=26.0.0' '-asm_macosx_version_min=26.0' '-nodefaultexport' '-mcpu=apple-m1' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a-'
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/aarch64-apple-darwin25/15/cc1plus -quiet -v -iprefix /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/ -D__DYNAMIC__ main.c -fPIC -quiet -dumpdir a- -dumpbase main.c -dumpbase-ext .c -mmacosx-version-min=26.0.0 -mcpu=apple-m1 -mlittle-endian -mabi=lp64 -version -o /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//cc4btLue.s
GNU C++17 (Homebrew GCC 15.2.0_1) version 15.2.0 (aarch64-apple-darwin25)
	compiled by GNU C version 15.2.0, GMP version 6.3.0, MPFR version 4.2.2, MPC version 1.3.1, isl version isl-0.27-GMP

warning: MPC header version 1.3.1 differs from library version 1.4.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../aarch64-apple-darwin25/include"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/aarch64-apple-darwin25"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/backward"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/local/include"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include-fixed"
ignoring nonexistent directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../aarch64-apple-darwin25/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/aarch64-apple-darwin25
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/backward
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include-fixed
 /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/System/Library/Frameworks
End of search list.
Compiler executable checksum: a37d1cd6599b6720a977e39427c2228b

Here's what the successful workaround with SDKROOT outputs:

% SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/ g++-15 main.c -v
Using built-in specs.
COLLECT_GCC=g++-15
COLLECT_LTO_WRAPPER=/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/aarch64-apple-darwin25/15/lto-wrapper
Target: aarch64-apple-darwin25
Configured with: ../configure --prefix=/opt/homebrew/opt/gcc --libdir=/opt/homebrew/opt/gcc/lib/gcc/current --disable-nls --enable-checking=release --with-gcc-major-version-only --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-15 --with-gmp=/opt/homebrew/opt/gmp --with-mpfr=/opt/homebrew/opt/mpfr --with-mpc=/opt/homebrew/opt/libmpc --with-isl=/opt/homebrew/opt/isl --with-zstd=/opt/homebrew/opt/zstd --with-pkgversion='Homebrew GCC 15.2.0_1' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues --with-system-zlib --build=aarch64-apple-darwin25 --with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.2.0 (Homebrew GCC 15.2.0_1) 
COLLECT_GCC_OPTIONS='-v' '--sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/' '-mmacosx-version-min=26.0.0' '-asm_macosx_version_min=26.0' '-nodefaultexport' '-mcpu=apple-m1' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a-'
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/aarch64-apple-darwin25/15/cc1plus -quiet -v -iprefix /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/ -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/ -D__DYNAMIC__ main.c -fPIC -quiet -dumpdir a- -dumpbase main.c -dumpbase-ext .c -mmacosx-version-min=26.0.0 -mcpu=apple-m1 -mlittle-endian -mabi=lp64 -version -o /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//ccjDASpg.s
GNU C++17 (Homebrew GCC 15.2.0_1) version 15.2.0 (aarch64-apple-darwin25)
	compiled by GNU C version 15.2.0, GMP version 6.3.0, MPFR version 4.2.2, MPC version 1.3.1, isl version isl-0.27-GMP

warning: MPC header version 1.3.1 differs from library version 1.4.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../aarch64-apple-darwin25/include"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/aarch64-apple-darwin25"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/backward"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/local/include"
ignoring duplicate directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include-fixed"
ignoring nonexistent directory "/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/../../../../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../aarch64-apple-darwin25/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk//Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/aarch64-apple-darwin25
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../../../../include/c++/15/backward
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/include-fixed
 /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk//System/Library/Frameworks
End of search list.
Compiler executable checksum: a37d1cd6599b6720a977e39427c2228b
COLLECT_GCC_OPTIONS='-v' '--sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/' '-mmacosx-version-min=26.0.0'  '-nodefaultexport' '-mcpu=apple-m1' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a-'
 as -arch arm64 -v -mmacosx-version-min=26.0 -o /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//cccYaX9g.o /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//ccjDASpg.s
Apple clang version 21.0.0 (clang-2100.0.123.102)
Target: arm64-apple-darwin25.4.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1as -triple arm64-apple-macosx26.0.0 -target-sdk-version=15.4 -filetype obj -main-file-name ccjDASpg.s -target-cpu apple-m1 -target-feature +v8.5a -target-feature +aes -target-feature +altnzcv -target-feature +ccdp -target-feature +ccpp -target-feature +complxnum -target-feature +crc -target-feature +dotprod -target-feature +flagm -target-feature +fp-armv8 -target-feature +fp16fml -target-feature +fptoint -target-feature +fullfp16 -target-feature +jsconv -target-feature +lse -target-feature +neon -target-feature +pauth -target-feature +perfmon -target-feature +predres -target-feature +ras -target-feature +rcpc -target-feature +rdm -target-feature +sb -target-feature +sha2 -target-feature +sha3 -target-feature +specrestrict -target-feature +ssbs -fdebug-compilation-dir=/Users/micha/code/macos-xcode-diagnose -dwarf-debug-producer "Apple clang version 21.0.0 (clang-2100.0.123.102)" -dwarf-version=5 -mrelocation-model pic -o /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//cccYaX9g.o /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//ccjDASpg.s
COMPILER_PATH=/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/aarch64-apple-darwin25/15/:/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/
LIBRARY_PATH=/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/:/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/:/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../../
COLLECT_GCC_OPTIONS='-v' '--sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk/' '-mmacosx-version-min=26.0.0'  '-nodefaultexport' '-mcpu=apple-m1' '-mlittle-endian' '-mabi=lp64' '-dumpdir' 'a.'
 /opt/homebrew/Cellar/gcc/15.2.0_1/bin/../libexec/gcc/aarch64-apple-darwin25/15/collect2 -demangle -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk// -dynamic -arch arm64 -macos_version_min 26.0.0 -o a.out -L/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15 -L/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc -L/opt/homebrew/Cellar/gcc/15.2.0_1/bin/../lib/gcc/current/gcc/aarch64-apple-darwin25/15/../../.. -lemutls_w -lheapt_w /var/folders/dh/k7c2gppn12v3w0plnf_xz94c0000gn/T//cccYaX9g.o -lstdc++ -lgcc -lSystem -no_compact_unwind -rpath @loader_path -rpath /opt/homebrew/Cellar/gcc/15.2.0_1/lib/gcc/current/gcc/aarch64-apple-darwin25/15 -rpath /opt/homebrew/Cellar/gcc/15.2.0_1/lib/gcc/current/gcc -rpath /opt/homebrew/Cellar/gcc/15.2.0_1/lib/gcc/current

[svillemot]

Same issue here. The compilation of Dynare with Homebrew fails since upgrading XCode to 26.4: https://git.dynare.org/Dynare/dynare/-/jobs/99661

[Bo98]

This appears to be a macOS SDK bug where that header is using _Static_assert under C++ when it should use static_assert.

Clang isn't affected because it accepts _Static_assert under C++ as a Clang-specific extension: https://clang.llvm.org/docs/LanguageExtensions.html#c-keywords-supported-in-all-language-modes.

cc @fxcoudert - do we need to get GCC to support this extension?

[Bo98]

I've reported this bug to Apple: FB22501039