Introduce detail::always_false<> to make the new static_assert() …

…more readable.

include/pybind11/cast.h

@@ -1372,7 +1372,7 @@ template <typename D>
 template <typename T>
 str_attr_accessor object_api<D>::attr_with_type_hint(const char *key) const {
 #if !defined(__cpp_inline_variables)
-    static_assert(!std::is_same<T, T>::value,
+    static_assert(always_false<T>::value,
                   "C++17 feature __cpp_inline_variables not available: "
                   "https://en.cppreference.com/w/cpp/language/static#Static_data_members");
 #endif

include/pybind11/detail/common.h

@@ -627,6 +627,9 @@ struct instance {
 static_assert(std::is_standard_layout<instance>::value,
               "Internal error: `pybind11::detail::instance` is not standard layout!");
 
+template <typename>
+struct always_false : std::false_type {};
+
 /// from __cpp_future__ import (convenient aliases from C++14/17)
 #if defined(PYBIND11_CPP14)
 using std::conditional_t;