fix cppcorecheck warnings (#703)

* Added c++17 test configurations for clang5.0 and clang6.0

* Fixed CppCoreCheck warnings in GSL and tests

- Added CMakeSettings.json for VS Open Folder configuration
	- So we can easily run CppCoreCheck in VS
- Fixed CppCorecheck warnings where it made sense
- Suppressed the rest
	- Some suppression does not work due to compiler/tool bugs,
          so replaced by #pragma disable
	- CppCoreCheck has noise, suppressed those with comments
	- Catch produces many warnings, blanket-supressed them all
- Had to fix clang formatting to keep attributes in place
	- clang-format does not support attributes, so I am using
        - "CommentPragmas: '^ NO-FORMAT:'" to skip formatiting on them
- Removed GSL_NOEXCEPT macro, removed incorred noexcepts

* Ignore unknown attributes

* ignore unknown attributes in noexception mode tests

* fixed C26472 in at()

* created GSL_SUPPRESS macro to allow all compilers to parse suppression attributes

* try to fix gcc compilation problems with attributes

* ignore gsl::suppress for gcc

* move suppression to function level on return statements

clang5.0 and up does not allow attributes on return statemets in constexpr functions

* move suppression to function level on return statements

* use GSL_SUPPRESS in algorithm_tests

* Addressed PR comments

1 files changed, 20 insertions, 5 deletions

diff --git a/include/gsl/gsl_byte b/include/gsl/gsl_byte
index 6b770fd..bc18886 100644
--- a/include/gsl/gsl_byte
+++ b/include/gsl/gsl_byte
@@ -17,14 +17,28 @@
 #ifndef GSL_BYTE_H
 #define GSL_BYTE_H
 
+//
+// make suppress attributes work for some compilers
+// Hopefully temporary until suppresion standardization occurs
+//
+#if defined(_MSC_VER)
+#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
+#else
+#if defined(__clang__)
+#define GSL_SUPPRESS(x) [[gsl::suppress("x")]]
+#else
+#define GSL_SUPPRESS(x)
+#endif // __clang__
+#endif // _MSC_VER
+
 #include <type_traits>
 
 #ifdef _MSC_VER
 
 #pragma warning(push)
 
-// don't warn about function style casts in byte related operators
-#pragma warning(disable : 26493)
+// Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
+#pragma warning(disable : 26493) // don't use c-style casts // TODO: MSVC suppression in templates does not always work
 
 #ifndef GSL_USE_STD_BYTE
 // this tests if we are under MSVC and the standard lib has std::byte and it is enabled
@@ -57,12 +71,12 @@
 
 #define GSL_USE_STD_BYTE 0
 
-#endif //defined(__cplusplus) && (__cplusplus >= 201703L)
+#endif //defined(__cplusplus) && (__cplusplus >= 201703L) &&
        //   (defined(__cpp_lib_byte) && (__cpp_lib_byte >= 201603)  ||
        //    defined(_LIBCPP_VERSION) && (_LIBCPP_VERSION >= 5000))
 #endif // GSL_USE_STD_BYTE
 
-#endif           // _MSC_VER
+#endif // _MSC_VER
 
 // Use __may_alias__ attribute on gcc and clang
 #if defined __clang__ || (__GNUC__ > 5)
@@ -75,7 +89,6 @@ namespace gsl
 {
 #if GSL_USE_STD_BYTE
 
-
 using std::byte;
 using std::to_integer;
 
@@ -160,6 +173,8 @@ constexpr byte to_byte_impl(T t) noexcept
     return static_cast<byte>(t);
 }
 template <>
+// NOTE: need suppression since c++14 does not allow "return {t}"
+// GSL_SUPPRESS(type.4) // NO-FORMAT: attribute // TODO: suppression does not work
 constexpr byte to_byte_impl<true, unsigned char>(unsigned char t) noexcept
 {
     return byte(t);