Clash when using trivially copy/move-assignable union in constexpr - c++

Question

Considering the following code
struct S
{
constexpr S() = default;
constexpr S(S const &) = default;
constexpr S(S &) = default;
constexpr S(S &&) = default;
#if 1
S & operator = (S const &) = default;
S & operator = (S &) = default;
S & operator = (S &&) = default;
#else
constexpr S & operator = (S const &) = default;
constexpr S & operator = (S &) = default;
constexpr S & operator = (S &&) = default;
#endif
~S() = default;
};
struct U
{
union
{
S s;
};
constexpr U() : s{} { ; }
};
inline constexpr bool test()
{
U v;
U w;
v = w;
return true;
}
static_assert(test());
I found out that there is contradiction.
I want to use union (or euqally union-like class) in constexpr.
Version #if 1 gives an error (error: constexpr function never produces a constant expression):
main.cpp:31:23: error: constexpr function never produces a constant expression [-Winvalid-constexpr]
inline constexpr bool test()
^
main.cpp:35:7: note: non-constexpr function 'operator=' cannot be used in a constant expression
v = w;
^
main.cpp:19:8: note: declared here
struct U
^
main.cpp:39:15: error: static_assert expression is not an integral constant expression
static_assert(test());
^~~~~~
main.cpp:35:7: note: non-constexpr function 'operator=' cannot be used in a constant expression
v = w;
^
main.cpp:39:15: note: in call to 'test()'
static_assert(test());
^
main.cpp:19:8: note: declared here
struct U
^
2 errors generated.
But version #if 0 also give an error (error: defaulted definition of copy assignment operator is not constexpr):
main.cpp:12:5: error: defaulted definition of copy assignment operator is not constexpr
constexpr S & operator = (S const &) = default;
^
main.cpp:13:5: error: defaulted definition of copy assignment operator is not constexpr
constexpr S & operator = (S &) = default;
^
main.cpp:14:5: error: defaulted definition of move assignment operator is not constexpr
constexpr S & operator = (S &&) = default;
^
main.cpp:35:7: error: object of type 'U' cannot be assigned because its copy assignment operator is implicitly deleted
v = w;
^
main.cpp:24:11: note: copy assignment operator of 'U' is implicitly deleted because field 's' has no copy assignment operator
S s;
^
4 errors generated.
LIVE EXAMPLE
All above is for version of clang 3.7.0, but newer version clang 3.8.0 (trunk 253951) nothing says about constexpr S & operator = (S const &) = default; (it allows to compile such a code), but consequences are the same.
For trivial copy/move-constructors, but user-provided assignment operators analogous code (construction) compiles fine.
I think the problem (bug) is in user-declared special functions, but it should be legal at all.
Also please look at the last and last but one clauses of this code. Quite well-designed (at my mind) variant becomes useless in constexpr, due to difference in interpretation by compiler of next definitions:
struct S {};
and:
struct S
{
constexpr S() = default;
constexpr S(S const &) = default;
constexpr S(S &) = default;
constexpr S(S &&) = default;
S & operator = (S const &) = default;
S & operator = (S &) = default;
S & operator = (S &&) = default;
~S() = default;
};
But both is equivalent at my mind (the second recognized by compiler as not POD, but who cares?).
Why defaulted assignment operator of empty struct is not constexpr?
UPDATE:
Removing the declaration of destructor makes problem to disappear. So even user-declared (not user-provided) destructor make implicitly defined defaulted assignment operators to be not marked as constexpr and to be non-trivial. It definitely bug because no one source telling about this.