From 85d75d649847c5a5bb476148551a91a37d1e1e29 Mon Sep 17 00:00:00 2001
From: Eduard Valeyev <eduard@valeyev.net>
Date: Mon, 9 Feb 2026 00:47:15 -0500
Subject: [PATCH 1/2] introduced vgkit_check_p0522r0

---
 modules/CheckCXXFeatures.cmake | 54 +++++++++++++++++++++++++++++++++-
 1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/modules/CheckCXXFeatures.cmake b/modules/CheckCXXFeatures.cmake
index 0f58c2f..271b807 100644
--- a/modules/CheckCXXFeatures.cmake
+++ b/modules/CheckCXXFeatures.cmake
@@ -137,4 +137,56 @@ int main() { std::unordered_map<std::string,int> m; m[\"k\"]=1; return 0; }
 
   unset(_clmm_modify_global)
 
-endmacro(vgkit_check_libcxx_linker_mismatch)
\ No newline at end of file
+endmacro(vgkit_check_libcxx_linker_mismatch)
+
+# P0522R0 (relaxed matching of template template arguments) is required by
+# code that passes class templates with defaulted parameters (e.g.
+# small_vector<T, N=...>) to template template parameters expecting fewer
+# args (e.g. template <class> class Container).
+#
+# - GCC has supported this since GCC 7 (default in all C++17 modes)
+# - LLVM Clang enabled it by default in Clang 19; earlier versions need
+#   -frelaxed-template-template-args
+# - AppleClang 17+ works; AppleClang 16 and earlier do not support it at all
+#
+# This macro tests whether P0522R0 works out of the box and, if not, tries
+# adding -frelaxed-template-template-args.
+#
+# Sets VGKIT_P0522R0_COMPILE_FLAG in the caller's scope to the compile
+# option needed (empty string if P0522R0 works natively).  The caller
+# decides how to apply it (e.g. target_compile_options on specific targets).
+# On failure, emits FATAL_ERROR.
+macro(vgkit_check_p0522r0)
+
+  set(VGKIT_P0522R0_COMPILE_FLAG)
+
+  cmake_push_check_state(RESET)
+  check_cxx_source_compiles("
+template <class T, int N = 10> struct SmallVec {};
+template <template <class> class C> void f() { C<int> v; }
+int main() { f<SmallVec>(); return 0; }
+" VGKIT_CXX_HAS_P0522R0)
+  cmake_pop_check_state()
+
+  if (NOT VGKIT_CXX_HAS_P0522R0)
+    cmake_push_check_state(RESET)
+    set(CMAKE_REQUIRED_FLAGS "-frelaxed-template-template-args")
+    check_cxx_source_compiles("
+template <class T, int N = 10> struct SmallVec {};
+template <template <class> class C> void f() { C<int> v; }
+int main() { f<SmallVec>(); return 0; }
+" VGKIT_CXX_HAS_P0522R0_WITH_FLAG)
+    cmake_pop_check_state()
+
+    if (VGKIT_CXX_HAS_P0522R0_WITH_FLAG)
+      set(VGKIT_P0522R0_COMPILE_FLAG "-frelaxed-template-template-args")
+      message(STATUS "P0522R0 requires -frelaxed-template-template-args")
+    else()
+      message(FATAL_ERROR
+        "Compiler does not support P0522R0 (relaxed template template argument matching), "
+        "even with -frelaxed-template-template-args. Use a newer compiler "
+        "(GCC >= 7, Clang >= 19, or AppleClang >= 17).")
+    endif()
+  endif()
+
+endmacro(vgkit_check_p0522r0)
\ No newline at end of file

From 256d9462bb765787f5acb69be154b26d6efba8b6 Mon Sep 17 00:00:00 2001
From: Eduard Valeyev <eduard@valeyev.net>
Date: Mon, 9 Feb 2026 12:24:28 -0500
Subject: [PATCH 2/2] amend vgkit_check_libcxx_linker_mismatch macro in
 d2a1e30a5a2049f70432df660ba678bed7085b05 to produce per-project result so it
 can be run multiple times

---
 modules/CheckCXXFeatures.cmake | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/modules/CheckCXXFeatures.cmake b/modules/CheckCXXFeatures.cmake
index 271b807..00b8eb7 100644
--- a/modules/CheckCXXFeatures.cmake
+++ b/modules/CheckCXXFeatures.cmake
@@ -76,10 +76,10 @@ macro(vgkit_check_libcxx_linker_mismatch)
 #include <unordered_map>
 #include <string>
 int main() { std::unordered_map<std::string,int> m; m[\"k\"]=1; return 0; }
-" VGKIT_LIBCXX_LINKS)
+" VGKIT_LIBCXX_LINKS_${PROJECT_NAME})
     cmake_pop_check_state()
 
-    if (NOT VGKIT_LIBCXX_LINKS)
+    if (NOT VGKIT_LIBCXX_LINKS_${PROJECT_NAME})
       # Derive libc++ lib dir: -print-resource-dir gives <root>/lib/clang/<ver>
       execute_process(
         COMMAND ${CMAKE_CXX_COMPILER} -print-resource-dir
@@ -97,10 +97,10 @@ int main() { std::unordered_map<std::string,int> m; m[\"k\"]=1; return 0; }
 #include <unordered_map>
 #include <string>
 int main() { std::unordered_map<std::string,int> m; m[\"k\"]=1; return 0; }
-" VGKIT_LIBCXX_LINKS_WITH_FLAGS)
+" VGKIT_LIBCXX_LINKS_WITH_FLAGS_${PROJECT_NAME})
         cmake_pop_check_state()
 
-        if (VGKIT_LIBCXX_LINKS_WITH_FLAGS)
+        if (VGKIT_LIBCXX_LINKS_WITH_FLAGS_${PROJECT_NAME})
           if (_clmm_modify_global)
             message(STATUS "libc++ linker mismatch detected; adding -L${_clang_libcxx_dir} to global linker flags")
             string(APPEND CMAKE_EXE_LINKER_FLAGS