CI linux: add clang++-11 C++20 (with LLVM libc++) build

* find-package-test fixed by passing CXX_FLAGS, i.e. -stdlib=libc++
 * std::array usage in compile-test.cc replaced with plain array, because
   <array> header was not included

.github/workflows/linux.yml

@@ -38,6 +38,12 @@ jobs:
             build_type: Debug
             std: 20
             os: ubuntu-20.04
+          - cxx: clang++-11
+            build_type: Debug
+            std: 20
+            cxxflags: -stdlib=libc++
+            os: ubuntu-20.04
+            install: sudo apt install libc++-11-dev libc++abi-11-dev
 
     steps:
     - uses: actions/checkout@v2

test/CMakeLists.txt

@@ -203,6 +203,7 @@ if (FMT_PEDANTIC AND NOT WIN32)
     --build-makeprogram ${CMAKE_MAKE_PROGRAM}
     --build-options
     "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"
+    "-DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}"
     "-DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}"
     "-DFMT_DIR=${PROJECT_BINARY_DIR}"
     "-DPEDANTIC_COMPILE_FLAGS=${PEDANTIC_COMPILE_FLAGS}"

test/compile-test.cc

@@ -323,15 +323,15 @@ TEST(CompileTest, CompileFormatStringLiteral) {
     (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
 template <size_t max_string_length, typename Char = char> struct test_string {
   template <typename T> constexpr bool operator==(const T& rhs) const noexcept {
-    return fmt::basic_string_view<Char>(rhs).compare(buffer.data()) == 0;
+    return fmt::basic_string_view<Char>(rhs).compare(buffer) == 0;
   }
-  std::array<Char, max_string_length> buffer{};
+  Char buffer[max_string_length]{};
 };
 
 template <size_t max_string_length, typename Char = char, typename... Args>
 consteval auto test_format(auto format, const Args&... args) {
   test_string<max_string_length, Char> string{};
-  fmt::format_to(string.buffer.data(), format, args...);
+  fmt::format_to(string.buffer, format, args...);
   return string;
 }