Mark code past FAIL and SKIP as unreachable

This helps the compiler see that the execution does not continue,
which in turn means that it is not an error if the user does not
return value from the function after FAIL/SKIP.

Closes #2941

src/catch2/internal/catch_unreachable.hpp

@@ -0,0 +1,52 @@
+
+//              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
+#ifndef CATCH_UNREACHABLE_HPP_INCLUDED
+#define CATCH_UNREACHABLE_HPP_INCLUDED
+
+/**\file
+ * Polyfill `std::unreachable`
+ *
+ * We need something like `std::unreachable` to tell the compiler that
+ * some macros, e.g. `FAIL` or `SKIP`, do not continue execution in normal
+ * manner, and should handle it as such, e.g. not warn if there is no return
+ * from non-void function after a `FAIL` or `SKIP`.
+ */
+
+#include <exception>
+
+#if defined( __cpp_lib_unreachable ) && __cpp_lib_unreachable > 202202L
+#    include <utility>
+namespace Catch {
+    namespace Detail {
+        using Unreachable = std::unreachable;
+    }
+} // namespace Catch
+
+#else // vv If we do not have std::unreachable, we implement something similar
+
+namespace Catch {
+    namespace Detail {
+
+        [[noreturn]]
+        inline void Unreachable() noexcept {
+#    if defined( NDEBUG )
+#        if defined( _MSC_VER ) && !defined( __clang__ )
+            __assume( false );
+#        elif defined( __GNUC__ )
+            __builtin_unreachable();
+#        endif
+#    endif // ^^ NDEBUG
+            std::terminate();
+        }
+
+    } // namespace Detail
+} // end namespace Catch
+
+#endif
+
+#endif // CATCH_UNREACHABLE_HPP_INCLUDED