Is it possible to implement always_false in the C++ standard library?

Question

There are cases where one uses an always_false helper to e.g. cause unconditional static_assert failure if instantiation of some template is attempted:

Answer 1

In C++20, with lambda, you might do something like:

template <class... T> struct always_false : std::false_type {};

// To have true, but for a type that user code can't reuse as lambda types are unique.
template <> struct always_false<decltype([](){})> : std::true_type{};

Answer 2

just use option -fdelayed-template-parsing

Answer 3

To paraphrase Jarod's idea, It could be something like

template <class... T> struct always_false : std::false_type {};

template <> struct always_false</* implementation defined */> : std::true_type{};

Where /* implementation defined */ can be filled by std::_ReservedIdentifer. User code can't access it, since the identifier is reserved to the library, but there exists a specialization that is true. That should avoid questions about the ODR and lambdas in specializations.