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GH-45325: [C++] Improved error message when using clang++ without clang #45326

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GH-45325: [C++] Improved error message when using clang++ without clang #45326

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14 changes: 6 additions & 8 deletions cpp/cmake_modules/SetupCxxFlags.cmake
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,12 @@ include(CheckCXXSourceCompiles)

message(STATUS "System processor: ${CMAKE_SYSTEM_PROCESSOR}")

if(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
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Do we need this check?
In general, we can't mix C/C++ compilers from different vendores, right?
(Can we mix g++ and clang?)

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@WillAyd WillAyd Jan 22, 2025 •
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Yea mixing g++ and clang is the problem, or vice versa. If you just set -DCMAKE_CXX_COMPILER=clang++ then clang++ will be used for cpp files and gcc will be used for c (assuming gcc is the default)

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but wouldn't the following catch that?
if(NOT CMAKE_CXX_COMPILER_ID STREQUAL CMAKE_C_COMPILER_ID)
Why only perform the check if the CMAKE_CXX_COMPILER_ID is clang?

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Ah I see what you are saying. I don't know CMake well enough to know if those labels are always expected to be the same. Happy to make that change.

This originally started by locating the "bug" in SetupCxxFlags.cmake, hence the condition in the first place

if(NOT CMAKE_CXX_COMPILER_ID STREQUAL CMAKE_C_COMPILER_ID)
message(FATAL_ERROR "When using clang++ you must also use clang")
endif()
endif()

if(NOT DEFINED ARROW_CPU_FLAG)
if(CMAKE_SYSTEM_NAME STREQUAL "Emscripten")
set(ARROW_CPU_FLAG "emscripten")
Expand Down Expand Up @@ -430,14 +436,6 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang" OR CMAKE_CXX_COMPILER_ID STRE
"Clang")
# Clang options for all builds

# Using Clang with ccache causes a bunch of spurious warnings that are
# purportedly fixed in the next version of ccache. See the following for details:
#
# http://petereisentraut.blogspot.com/2011/05/ccache-and-clang.html
# http://petereisentraut.blogspot.com/2011/09/ccache-and-clang-part-2.html
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Qunused-arguments")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Qunused-arguments")

# Avoid error when an unknown warning flag is passed
set(CXX_COMMON_FLAGS "${CXX_COMMON_FLAGS} -Wno-unknown-warning-option")
# Add colors when paired with ninja
Expand Down
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