std::is_same_v needs a compile config selector

[jl]

Describe the bug
Some compilers will appear to support C++17 but not support std::is_same_v properly, however, unlike other C++17 features with have a compile flag such as CATCH_CONFIG_NO_CPP17_BYTE there isn't an analogous one for std::is_same_v.

Expected behavior
User of catch should be able to #define CATCH_CONFIG_NO_CPP17_IS_SAME_V in a broken compiler and have catch work.

Reproduction steps
Compile a project with clang 8.0.0, observe the following error:

catch.hpp:686:10: error: no template named 'is_same_v' in namespace 'std'

cmake flags used:

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS NO)

Platform information:

• OS: 64-bit linux
• Compiler+version: clang 8.0.0
• Catch version: v2.9.2
• Cmake version: 3.12

** Proposed patch (to single header)

diff --git a/catch.hpp b/catch.hpp
--- a/catch.hpp
+++ b/catch.hpp
@@ -311,6 +311,14 @@ namespace Catch {
 #  endif // __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)
 #endif // __has_include

+////////////////////////////////////////////////////////////////////////////////
+// Check if is_same_v is available and usable
+#if defined(__has_include)
+#  if __has_include(<type_traits>) && defined(CATCH_CPP17_OR_GREATER)
+#    define CATCH_INTERNAL_CONFIG_CPP17_IS_SAME_V
+#  endif // __has_include(<type_traits>) && defined(CATCH_CPP17_OR_GREATER)
+#endif // __has_include
+
 ////////////////////////////////////////////////////////////////////////////////
 // Check if variant is available and usable
 #if defined(__has_include)
@@ -369,6 +377,10 @@ namespace Catch {
 #  define CATCH_CONFIG_CPP17_BYTE
 #endif

+#if defined(CATCH_INTERNAL_CONFIG_CPP17_IS_SAME_V) && !defined(CATCH_CONFIG_NO_CPP17_IS_SAME_V) && !defined(CATCH_CONFIG_CPP17_IS_SAME_V)                               
+#  define CATCH_CONFIG_CPP17_IS_SAME_V
+#endif
+
 #if defined(CATCH_CONFIG_EXPERIMENTAL_REDIRECT)
 #  define CATCH_INTERNAL_CONFIG_NEW_CAPTURE
 #endif
@@ -677,7 +689,7 @@ inline auto operator "" _catch_sr( char const* rawChars, std::size_t size ) noex                                                                     

 namespace Catch{

-#ifdef CATCH_CPP17_OR_GREATER
+#if defined(CATCH_CONFIG_CPP17_IS_SAME_V)
        template <typename...>
        inline constexpr auto is_unique = std::true_type{};

[horenmar]

Since the trait that uses it is no longer in use, I just removed the use completely.