Fix StringFixedSize infinite template recursion

On MSC, `sizeof...(Args) != 1 ||` does not short-circuit properly, so
`std::tuple_element_t<0, std::tuple<Args...>>` is treated as an
out-of-bounds access.

This felt like a good use-case for concepts, but the fix could also have
been done while keeping std::enable_if.

tools/quake3/common/stringfixedsize.h

@@ -24,6 +24,7 @@
 #include "stream/textstream.h"
 #include "string/string.h"
 #include "inout.h"
+#include <type_traits>
 
 
 /// \brief A TextOutputStream which writes to a null terminated fixed length char array.
@@ -37,10 +38,11 @@ public:
 	StringFixedSize() {
 		clear();
 	}
-	template<typename Arg, typename ... Args, typename = std::enable_if_t<sizeof...(Args) != 0 || //prevent override of copy constructor
-	             !std::is_same_v<StringFixedSize, std::decay_t<Arg>>>>
-	explicit StringFixedSize( Arg&& arg, Args&& ... args ){
-		operator()( std::forward<Arg>( arg ), std::forward<Args>( args ) ... );
+	template<typename ... Args>
+		// prevent override of copy constructor
+		requires ( !std::is_same_v<std::tuple<std::decay_t<Args>...>, std::tuple<StringFixedSize>> )
+	explicit StringFixedSize(Args&& ... args) {
+		operator()( std::forward<Args>( args ) ... );
 	}
 	std::size_t write( const char* buffer, std::size_t length ) override {
 		if( m_length + length < SIZE ){