Remove usage of __builtin_constant_p under IBM XL

include/internal/catch_compiler_capabilities.h

@@ -58,7 +58,20 @@
 #    define CATCH_INTERNAL_START_WARNINGS_SUPPRESSION _Pragma( "clang diagnostic push" )
 #    define CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION  _Pragma( "clang diagnostic pop" )
 
+// As of this writing, IBM XL's implementation of __builtin_constant_p has a bug
+// which results in calls to destructors being emitted for each temporary,
+// without a matching initialization. In practice, this can result in something
+// like `std::string::~string` being called on an uninitialized value.
+//
+// For example, this code will likely segfault under IBM XL:
+// ```
+// REQUIRE(std::string("12") + "34" == "1234")
+// ```
+//
+// Therefore, `CATCH_INTERNAL_IGNORE_BUT_WARN` is not implemented.
+#  if !defined(__ibmxl__)
 #    define CATCH_INTERNAL_IGNORE_BUT_WARN(...) (void)__builtin_constant_p(__VA_ARGS__) /* NOLINT(cppcoreguidelines-pro-type-vararg) */
+#  endif
 
 
 #    define CATCH_INTERNAL_SUPPRESS_GLOBALS_WARNINGS \