[clang] Class gets an implicit default constructor when it has a user-provided constructor

[carljohnsonnewhampshire-hue]

The following code doesn't compile due to failed static assertion in Clang, but does compile in MSVC:

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 clang 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.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (carljohnsonnewhampshire-hue)

The following code doesn't compile due to failed static assertion in Clang, 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 clang 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.
</details>