Make warnings in assertions fire for GCC/Clang again

The old code caused warnings to fire under MSVC, and Clang <3.8.
I could not find a GCC version where it worked, but I assume that it
did at some point.

This new code causes all of MSVC, GCC, Clang, in current versions,
to emit signed/unsigned comparison warning in test like this:

```cpp
TEST_CASE() {
    int32_t i = -1;
    uint32_t j = 1;
    REQUIRE(i != j);
}
```

Where previously only MSVC would emit the warning.

Fixes #1880

src/catch2/internal/catch_compiler_capabilities.hpp

@@ -56,6 +56,9 @@
 
 #    define CATCH_INTERNAL_SUPPRESS_UNUSED_VARIABLE_WARNINGS \
          _Pragma( "GCC diagnostic ignored \"-Wunused-variable\"" )
+
+#    define CATCH_INTERNAL_IGNORE_BUT_WARN(...) (void)__builtin_constant_p(__VA_ARGS__)
+
 #endif
 
 #if defined(__clang__)
@@ -63,6 +66,9 @@
 #    define CATCH_INTERNAL_START_WARNINGS_SUPPRESSION _Pragma( "clang diagnostic push" )
 #    define CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION  _Pragma( "clang diagnostic pop" )
 
+#    define CATCH_INTERNAL_IGNORE_BUT_WARN(...) (void)__builtin_constant_p(__VA_ARGS__)
+
+
 #    define CATCH_INTERNAL_SUPPRESS_GLOBALS_WARNINGS \
          _Pragma( "clang diagnostic ignored \"-Wexit-time-destructors\"" ) \
          _Pragma( "clang diagnostic ignored \"-Wglobal-constructors\"")
@@ -142,6 +148,8 @@
 #  define CATCH_INTERNAL_START_WARNINGS_SUPPRESSION __pragma( warning(push) )
 #  define CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION  __pragma( warning(pop) )
 
+#  define CATCH_INTERNAL_IGNORE_BUT_WARN(...) (void)sizeof(__VA_ARGS__)
+
 #  if _MSC_VER >= 1900 // Visual Studio 2015 or newer
 #    define CATCH_INTERNAL_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS
 #  endif
@@ -340,6 +348,11 @@
 #   define CATCH_INTERNAL_SUPPRESS_ZERO_VARIADIC_WARNINGS
 #endif
 
+// The goal of this macro is to avoid evaluation of the arguments, but
+// still have the compiler warn on problems inside...
+#if !defined(CATCH_INTERNAL_IGNORE_BUT_WARN)
+#   define CATCH_INTERNAL_IGNORE_BUT_WARN(...)
+#endif
 
 #if defined(__APPLE__) && defined(__apple_build_version__) && (__clang_major__ < 10)
 #   undef CATCH_INTERNAL_SUPPRESS_UNUSED_TEMPLATE_WARNINGS

src/catch2/internal/catch_test_macro_impl.hpp

@@ -46,6 +46,8 @@
 ///////////////////////////////////////////////////////////////////////////////
 #define INTERNAL_CATCH_TEST( macroName, resultDisposition, ... ) \
     do { \
+        /* The expression should not be evaluated, but warnings should hopefully be checked */ \
+        CATCH_INTERNAL_IGNORE_BUT_WARN(__VA_ARGS__); \
         Catch::AssertionHandler catchAssertionHandler( macroName##_catch_sr, CATCH_INTERNAL_LINEINFO, CATCH_INTERNAL_STRINGIFY(__VA_ARGS__), resultDisposition ); \
         INTERNAL_CATCH_TRY { \
             CATCH_INTERNAL_START_WARNINGS_SUPPRESSION \
@@ -54,8 +56,7 @@
             CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION \
         } INTERNAL_CATCH_CATCH( catchAssertionHandler ) \
         INTERNAL_CATCH_REACT( catchAssertionHandler ) \
-    } while( (void)0, (false) && static_cast<bool>( !!(__VA_ARGS__) ) ) // the expression here is never evaluated at runtime but it forces the compiler to give it a look
+    } while( false )
-    // The double negation silences MSVC's C4800 warning, the static_cast forces short-circuit evaluation if the type has overloaded &&.
 
 ///////////////////////////////////////////////////////////////////////////////
 #define INTERNAL_CATCH_IF( macroName, resultDisposition, ... ) \

tests/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp

@@ -300,7 +300,7 @@ TEST_CASE("analyse", "[approvals][benchmark]") {
     CHECK(analysis.outliers.low_severe == 0);
     CHECK(analysis.outliers.high_mild == 0);
     CHECK(analysis.outliers.high_severe == 0);
-    CHECK(analysis.outliers.samples_seen == samples.size());
+    CHECK(analysis.outliers.samples_seen == static_cast<int>(samples.size()));
 
     CHECK(analysis.outlier_variance < 0.5);
     CHECK(analysis.outlier_variance > 0);

tests/SelfTest/UsageTests/Compilation.tests.cpp

@@ -202,10 +202,19 @@ namespace { namespace CompilationTests {
         inline static void synchronizing_callback( void * ) { }
     }
 
+#if defined (_MSC_VER)
+#pragma warning(push)
+// The function pointer comparison below triggers warning because of
+// calling conventions
+#pragma warning(disable:4244)
+#endif
     TEST_CASE("#925: comparing function pointer to function address failed to compile", "[!nonportable]" ) {
         TestClass test;
         REQUIRE(utility::synchronizing_callback != test.testMethod_uponComplete_arg);
     }
+#if defined (_MSC_VER)
+#pragma warning(pop)
+#endif
 
     TEST_CASE( "#1027: Bitfields can be captured" ) {
         struct Y {