https://godbolt.org/z/EoeGs3E3v

#ifdef WITH_CONCEPT
#  include <concepts>
#  define CONCEPT std::same_as<unsigned>
#else
#  define CONCEPT
#endif

template<unsigned...Ns>
    //requires(sizeof...(Ns) > 0) // workaround
struct Foo {
    static_assert(sizeof...(Ns) > 0);

    static constexpr auto cnt = sizeof...(Ns);
    static constexpr auto cnt0 = (0u + ... + (Ns == 0));

    constexpr Foo() requires(cnt0 == 0) {}

    constexpr explicit(cnt0 == 1)
    Foo(CONCEPT auto...args)
        requires(cnt0 > 0 && cnt0 == sizeof...(args))
    {}
};

template<Foo> void func() {}

int main() {
    // This is okey, but would have same problem if `auto` is replaced with `Foo`
    constexpr auto F = Foo<1>{};

    // Some compilers instantiate Foo<> which has hard error
    func<F>();
}

         No WITH_CONCEPT   WITH_CONCEPT
Clang         ❎                ❎
GCC           ❎                ✅
MSVC          ✅                ✅

What does the standard say about this?

Edit: Partially reduced to Does NTTP construction always happen for CTAD even if value of type of that specialization is given?

Clang output:

<source>:11:19: error: static assertion failed due to requirement 'sizeof...(Ns) > 0'
   11 |     static_assert(sizeof...(Ns) > 0);
      |                   ^~~~~~~~~~~~~~~~~
<source>:18:24: note: in instantiation of template class 'Foo<>' requested here
   18 |     constexpr explicit(cnt0 == 1)
      |                        ^
<source>:31:10: note: while substituting deduced template arguments into function template '<deduction guide for Foo>' [with Ns = (no value), args:auto = <Foo<1>>]
   31 |     func<F>();
      |          ^
<source>:31:5: note: while substituting explicitly-specified template arguments into function template 'func' 
   31 |     func<F>();
      |     ^
<source>:11:33: note: expression evaluates to '0 > 0'
   11 |     static_assert(sizeof...(Ns) > 0);
      |                   ~~~~~~~~~~~~~~^~~
1 error generated.