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Avoid errors on newly introduced warnings in Clang 21 #15768

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ktf merged 1 commit into from
Sep 4, 2026
+37 −2
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Avoid errors on newly introduced warnings in Clang 21 #15768

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39 changes: 37 additions & 2 deletions dependencies/O2CompileFlags.cmake
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Original file line number Diff line number Diff line change
Expand Up @@ -66,8 +66,43 @@ else()
message(STATUS "Building without compiler warnings enabled.")
endif()

string(JOIN " " CMAKE_C_WARNINGS "-Wno-unknown-warning-option" "-Wno-vla-cxx-extension" ${O2_C_ENABLED_WARNINGS} ${O2_C_ENABLED_WARNINGS_NO_ERROR})
string(JOIN " " CMAKE_CXX_WARNINGS "-Wno-unknown-warning-option" "-Wno-vla-cxx-extension" ${O2_CXX_ENABLED_WARNINGS} ${O2_CXX_ENABLED_WARNINGS_NO_ERROR})
# Diagnostics that newer compilers added and that fire on existing, deliberate
# code. Warn, never fail.
#
# DELIBERATELY OUTSIDE the if(O2_ENABLE_WARNINGS) block above. That block is OFF
# unless ALIBUILD_O2_WARNINGS is set in the environment, so none of its
# -Wno-error= pairs are emitted in the builds that actually matter here: the PR
# checkers, where alidist o2.sh appends a bare -Werror for ALIBUILD_O2_TESTS.
# That combination is why -Wnonnull was fatal on Apple clang 21 even though
# `nonnull` is listed in O2_COMMON_WARNINGS.
#
# Appended after the externally supplied CXXFLAGS (see the
# CMAKE_CXX_FLAGS_<CONFIG> assignment below), so these win over that -Werror.
#
# Clang only, and that is not incidental. -Wno-unknown-warning-option makes an
# unknown name harmless on clang, including clang 16 (Xcode 16.2, still on part
# of the macOS fleet), which knows none of these. GCC gives no such guarantee:
# it swallows an unknown -Wno-foo silently but rejects -Wno-error=foo with a
# hard "no option '-Wfoo'" error, so emitting these unconditionally broke every
# GCC build. Three of the four are clang-only spellings anyway.
#
# Introduced 2026-09-04, when the macOS builders moved to Apple clang 21
# (Xcode 26.6) and every O2 PR check on them began failing on a different new
# diagnostic each time the previous one was fixed. Fixing the source is still
# preferable where the code is actually wrong -- three such fixes went in that
# day -- but some of these fire on intentional constructs: FlatObject relocates
# flat objects bitwise BY DESIGN, and its types delete their copy constructors
# precisely so nobody copies them the C++ way, which is what
# -Wnontrivial-memcall objects to.
if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
set(O2_NEW_COMPILER_WARNINGS_NO_ERROR "nontrivial-memcall;deprecated-literal-operator;final-dtor-non-final-class;nonnull")
o2_build_warning_flags(PREFIX "-Wno-error="
OUTPUTVARNAME O2_NEW_COMPILER_NO_ERROR_FLAGS
WARNINGS ${O2_NEW_COMPILER_WARNINGS_NO_ERROR})
endif()

string(JOIN " " CMAKE_C_WARNINGS "-Wno-unknown-warning-option" "-Wno-vla-cxx-extension" ${O2_C_ENABLED_WARNINGS} ${O2_C_ENABLED_WARNINGS_NO_ERROR} ${O2_NEW_COMPILER_NO_ERROR_FLAGS})
string(JOIN " " CMAKE_CXX_WARNINGS "-Wno-unknown-warning-option" "-Wno-vla-cxx-extension" ${O2_CXX_ENABLED_WARNINGS} ${O2_CXX_ENABLED_WARNINGS_NO_ERROR} ${O2_NEW_COMPILER_NO_ERROR_FLAGS})

string(REGEX MATCH "-O[0-9]+" CMAKE_FLAGS_OPT_VALUE "${CMAKE_CXX_FLAGS}")
if(NOT CMAKE_FLAGS_OPT_VALUE OR CMAKE_FLAGS_OPT_VALUE STREQUAL "-O0" OR CMAKE_FLAGS_OPT_VALUE STREQUAL "-O1")
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