From 791c49df81bb98d1c4c46da7c53545e0a019414d Mon Sep 17 00:00:00 2001
From: Aaron Jomy <aaronjomyjoseph@gmail.com>
Date: Tue, 3 Feb 2026 20:07:40 +0100
Subject: [PATCH] [cmake] add dynamic check for libstdc++fs

Based on https://github.com/root-project/root/commit/957ac3356492765c14098525840bcd56f120da67
Looking at the compiler version is not enough to know if we really need to link against stdc++fs:

"""
Whether linking with libstdc++fs is needed depends on the version of
libstdc++, not on the compiler. For example, it is possible to use an
older library with a newer compiler, or with another compiler that
follows a different versioning scheme (for example Clang).
"""

This adds a similar check in CppInterOp
---
 lib/CppInterOp/CMakeLists.txt | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/CppInterOp/CMakeLists.txt b/lib/CppInterOp/CMakeLists.txt
index 39c1950ca..f131d14bd 100644
--- a/lib/CppInterOp/CMakeLists.txt
+++ b/lib/CppInterOp/CMakeLists.txt
@@ -54,8 +54,17 @@ if(NOT WIN32 AND NOT EMSCRIPTEN)
   list(APPEND link_libs dl)
 endif()
 
-if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.0)
- list(APPEND link_libs stdc++fs)
+# Check if we need to link -lstdc++fs to use <filesystem> (libstdc++ 8 and older).
+include(CheckCXXSourceCompiles)
+set(_filesystem_src "#include <filesystem>
+int main() { std::filesystem::path p = \"path\"; return 0; }")
+check_cxx_source_compiles("${_filesystem_src}" HAVE_NATIVE_FILESYSTEM)
+if(NOT HAVE_NATIVE_FILESYSTEM)
+    set(CMAKE_REQUIRED_LIBRARIES stdc++fs)
+    check_cxx_source_compiles("${_filesystem_src}" NEED_STDCXXFS)
+    if(NEED_STDCXXFS)
+        list(APPEND link_libs stdc++fs)
+    endif()
 endif()
 
 # Get rid of libLLVM-X.so which is appended to the list of static libraries.