Add tests for assertion thread safety

tests/ExtraTests/CMakeLists.txt

@@ -553,3 +553,16 @@ set_tests_properties(AmalgamatedFileTest
   PROPERTIES
     PASS_REGULAR_EXPRESSION "All tests passed \\(14 assertions in 3 test cases\\)"
 )
+
+add_executable(ThreadSafetyTests
+  ${TESTS_DIR}/X94-ThreadSafetyTests.cpp
+)
+target_link_libraries(ThreadSafetyTests Catch2_buildall_interface)
+target_compile_definitions(ThreadSafetyTests PUBLIC CATCH_CONFIG_EXPERIMENTAL_THREAD_SAFE_ASSERTIONS)
+add_test(NAME ThreadSafetyTests
+  COMMAND ThreadSafetyTests -r compact
+)
+set_tests_properties(ThreadSafetyTests
+  PROPERTIES
+    PASS_REGULAR_EXPRESSION "assertions: 8001 | 4000 passed | 4001 failed"
+)

tests/ExtraTests/X94-ThreadSafetyTests.cpp

@@ -0,0 +1,44 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+/**\file
+ * Test that assertions and messages are thread-safe.
+ *
+ * This is done by spamming assertions and messages on multiple subthreads.
+ * In manual, this reliably causes segfaults if the test is linked against
+ * a non-thread-safe version of Catch2.
+ *
+ * The CTest test definition should also verify that the final assertion
+ * count is correct.
+ */
+
+#include <catch2/catch_test_macros.hpp>
+
+#include <atomic>
+#include <thread>
+#include <vector>
+
+TEST_CASE( "Failed REQUIRE in the main thread is fine" ) {
+    std::vector<std::thread> threads;
+    for ( size_t t = 0; t < 4; ++t) {
+        threads.emplace_back( [t]() {
+            CAPTURE(t);
+            for (size_t i = 0; i < 1'000; ++i) {
+                CAPTURE(i);
+                CHECK( false );
+                CHECK( true );
+            }
+        } );
+    }
+
+    for (auto& t : threads) {
+        t.join();
+    }
+
+    REQUIRE( false );
+}