Thanks. So I modified a bit:
static constexpr auto result = evaluate();
template <class T> concept C = requires { typename [: X<T>::result :]; };

At 2026-09-17 00:27:53, "Brian Bi" <bbi5291@gmail.com> wrote:

There's never SFINAE for invalid alias declarations.

Brian Bi

On Wed, Sep 16, 2026, 12:20 PM Bingzhi via Std-Discussion <std-discussion@lists.isocpp.org> wrote:

template <class T>
struct X {
  static consteval std::meta::info evaluate()
  { if (std::meta::is_same_type(^^T, ^^int)) return { }; else ... }

  using type = [: evaluate() :];
};
template <class T> concept C = requires { typename X<T>::type; };
static_assert(!C<int>); // error: cannot splice a null reflection


I don't know why it is. Can somebody explain?

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