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

[Michał Górny via llvm-commits]

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

Append `-nostd*++` flags only if we have checked that a trivial C++ program cannot link without them.  This is meant as a workaround for #90332.  While the flags could still incorrectly be passed to the C compiler, this should at least no longer happen when we are using a C++ compiler with the C++ runtime provided, which is the case Gentoo was hitting.

Fixes #90332

>From 6fdffd08ef687bf86213caf1900ef810d15eaeab Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20G=C3=B3rny?= <mgorny at gentoo.org>
Date: Mon, 4 Aug 2025 11:06:13 +0200
Subject: [PATCH] [runtimes] Append `-nostd*++` flags only if necessary
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Append `-nostd*++` flags only if we have checked that a trivial C++
program cannot link without them.  This is meant as a workaround
for #90332.  While the flags could still incorrectly be passed
to the C compiler, this should at least no longer happen when we are
using a C++ compiler with the C++ runtime provided, which is the case
Gentoo was hitting.

Fixes #90332

Signed-off-by: Michał Górny <mgorny at gentoo.org>
---
 runtimes/CMakeLists.txt | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index bfb4341b172cc..27658832e93e1 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -160,13 +160,25 @@ 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++")
+
+# TODO: this is incorrect, as these variables are passed to the C compiler.
+# Until a proper solution is found, append them only if necessary to avoid
+# some of the resulting breakage.
+# See https://github.com/llvm/llvm-project/issues/90332
+check_cxx_source_compiles("int main() {return 0;}" CXX_LINKS_WITHOUT_NOSTD)
+if (NOT CXX_LINKS_WITHOUT_NOSTD)
+  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()
+  check_cxx_source_compiles("int main() {return 0;}" CXX_LINKS_WITH_NOSTD)
+  if (NOT CXX_LINKS_WITH_NOSTD)
+    message(FATAL_ERROR "C++ compiler fails to link C++ programs, check CMake logs")
+  endif()
 endif()
 
 # Avoid checking whether the compiler is working.