[llvm] [runtimes] Append `-nostd*++` flags only if necessary (PR #151930)

[Michał Górny via llvm-commits]

https://github.com/mgorny updated https://github.com/llvm/llvm-project/pull/151930

>From 0e83408518d38c346d28f11456b4aa29dd0bba72 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20G=C3=B3rny?= <mgorny at gentoo.org>
Date: Thu, 7 Aug 2025 14:54:09 +0200
Subject: [PATCH] [runtimes] Append `-nostd*++` flags only for Clang
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Append `-nostdlib++` and `-nostdinc++` flags to `CMAKE_REQUIRED_FLAGS`
only if we are actually building with Clang.  These flags are also
passed to the C compiler, which is not allowed in GCC.  Since CMake
implicitly performs some tests using the C compiler, this can lead
to incorrect check results.  This should be safe, since FWIU we only
need them when bootstrapping Clang.

Even though we know that Clang supports these flags, we still need
to explicitly check if they work, as in some scenarios adding
`-nostdlib++` actually breaks the build.  See PR #108357 for examples
of that.

Fixes #90332

Signed-off-by: Michał Górny <mgorny at gentoo.org>
---
 runtimes/CMakeLists.txt | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index bfb4341b172cc..e3f75073bf0a7 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -160,13 +160,26 @@ endif()
 # Check for -nostdlib++ first; if there's no C++ standard library yet,
 # all check_cxx_compiler_flag commands will fail until we add -nostdlib++
 # (or -nodefaultlibs).
-llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
-endif()
-check_cxx_compiler_flag(-nostdinc++ CXX_SUPPORTS_NOSTDINCXX_FLAG)
-if (CXX_SUPPORTS_NOSTDINCXX_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
+#
+# CMAKE_REQUIRED_FLAGS is used both for C and C++ compilation. This breaks
+# compiling with GCC (and we should not need to force -nostd* for GCC anyway),
+# so apply it only with Clang. However, since there are cases when appending
+# the flag actually breaks the build, still perform the checks rather than
+# appending it unconditionally.
+#
+# TODO: find a better solution. See the discussion on:
+# https://github.com/llvm/llvm-project/issues/90332
+# https://github.com/llvm/llvm-project/pull/108357
+
+if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+  llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+  if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+  endif()
+  check_cxx_compiler_flag(-nostdinc++ CXX_SUPPORTS_NOSTDINCXX_FLAG)
+  if (CXX_SUPPORTS_NOSTDINCXX_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
+  endif()
 endif()
 
 # Avoid checking whether the compiler is working.