Use C++17 fallthrough tag instead of disabling warning

Signed-off-by: Ross Younger <crazyscot@gmail.com>

include/rapidjson/internal/regex.h

@@ -23,7 +23,6 @@
 RAPIDJSON_DIAG_PUSH
 RAPIDJSON_DIAG_OFF(padded)
 RAPIDJSON_DIAG_OFF(switch-enum)
-RAPIDJSON_DIAG_OFF(implicit-fallthrough)
 #elif defined(_MSC_VER)
 RAPIDJSON_DIAG_PUSH
 RAPIDJSON_DIAG_OFF(4512) // assignment operator could not be generated
@@ -32,9 +31,6 @@ RAPIDJSON_DIAG_OFF(4512) // assignment operator could not be generated
 #ifdef __GNUC__
 RAPIDJSON_DIAG_PUSH
 RAPIDJSON_DIAG_OFF(effc++)
-#if __GNUC__ >= 7
-RAPIDJSON_DIAG_OFF(implicit-fallthrough)
-#endif
 #endif
 
 #ifndef RAPIDJSON_REGEX_VERBOSE
@@ -291,6 +287,7 @@ private:
                     if (!CharacterEscape(ds, &codepoint))
                         return; // Unsupported escape character
                     // fall through to default
+                    RAPIDJSON_DELIBERATE_FALLTHROUGH;
 
                 default: // Pattern character
                     PushOperand(operandStack, codepoint);
@@ -520,6 +517,7 @@ private:
                 else if (!CharacterEscape(ds, &codepoint))
                     return false;
                 // fall through to default
+                RAPIDJSON_DELIBERATE_FALLTHROUGH;
 
             default:
                 switch (step) {
@@ -529,6 +527,7 @@ private:
                         break;
                     }
                     // fall through to step 0 for other characters
+                    RAPIDJSON_DELIBERATE_FALLTHROUGH;
 
                 case 0:
                     {

include/rapidjson/rapidjson.h

@@ -591,6 +591,19 @@ RAPIDJSON_NAMESPACE_END
 #endif
 #endif // RAPIDJSON_HAS_CXX11_RANGE_FOR
 
+///////////////////////////////////////////////////////////////////////////////
+// C++17 features
+
+#if defined(__has_cpp_attribute)
+# if __has_cpp_attribute(fallthrough)
+#  define RAPIDJSON_DELIBERATE_FALLTHROUGH [[fallthrough]]
+# else
+#  define RAPIDJSON_DELIBERATE_FALLTHROUGH
+# endif
+#else
+# define RAPIDJSON_DELIBERATE_FALLTHROUGH
+#endif
+
 //!@endcond
 
 //! Assertion (in non-throwing contexts).