The following code doesn't compile due to failed static assertion in GCC,
but does compile in MSVC:
```c++
struct A
{
template <int=0>
constexpr A() noexcept {}
};
struct B : A
{
using A::A;
explicit constexpr B(int) noexcept {}
};
struct C : B
{
using B::B;
template <int=0>
constexpr C() noexcept : a(1) {}
int a = 0;
};
static_assert(C().a == 1);
```
This is not standard compliant, that code should compile. According to the
standard A's constructor is not a default constructor per-se, as in
"special member function" kind of default constructor -- it is a template
for a constructor that takes no arguments, thus A does not have an implicit
default constructor, so nor should B which inherits from it and its
constructor using the using declaration. But GCC seems to think here that
because A is default-constructible (which it is, despite not having "real"
default constructor), that B should get an implicit default constructor.
