Another attempt at fixing old compilers with partial C++20 support

Author: horenmar

src/catch2/internal/catch_compiler_capabilities.hpp

@@ -36,7 +36,10 @@
 #endif
 
 // Matchers are only constexpr-able in C++20
-#if defined( CATCH_CPP20_OR_GREATER )
+#if defined( CATCH_CPP20_OR_GREATER ) && \
+    defined( __cpp_constexpr_dynamic_alloc ) && \
+    __cpp_constexpr_dynamic_alloc >= 201907L
+#    define CATCH_INTERNAL_CONSTEXPR_MATCHERS_ENABLED
 #    define CATCH_CPP20_CONSTEXPR constexpr
 #else
 #    define CATCH_CPP20_CONSTEXPR

tests/SelfTest/UsageTests/MatchersConstexpr.tests.cpp

@@ -9,7 +9,7 @@
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/matchers/catch_matchers_templated.hpp>
 
-#if defined(CATCH_CPP20_OR_GREATER)
+#if defined( CATCH_INTERNAL_CONSTEXPR_MATCHERS_ENABLED )
 
 namespace {
     struct MatchAllMatcher final : public Catch::Matchers::MatcherGenericBase {