The presence of template specialisations and constexpr functions means that the GP is right here; you cannot decide whether an arbitrary piece of C++ is syntact

The presence of template specialisations and constexpr functions means that the GP is right here; you cannot decide whether an arbitrary piece of C++ is syntactically valid without instantiating templates and/or interpreting constexpr functions. Consider template <int> struct foo { template <int> static int bar(int); }; template <> struct foo<8> { static const int bar = 99; }; constexpr int some_function() { return (int) sizeof(void*); }; Now given the snippet foo<some_function()>::bar<0>(1); then if some_function() returns something other than 8, we use the primary template and foo<N>::bar<0>(1) is a call to a static template member function. But if some_function() does return 8, we use the specialisation and the foo<8>::bar is an int with value 99; so we ask is 99 less than the expression 0>(1) (aka "false", promoted to the int 0). That is, there are two entirely different but valid parses depending on whether we are compiling on a 32- or 64-bit system. Parsing C++ is hard. EDIT: Godbolt example: https://godbolt.org/z/yR3YHW https://godbolt.org/z/yR3YHW