[cfe-dev] Problem with default member initialization and noexcept default constructor

[Joe Sylve via cfe-dev]

I'm seeing an error that is similar to the question raised at
https://stackoverflow.com/questions/43819314.  That question seems to not
have any consensus as to what the problem may be.

I would appreciate if someone could shed some light on my issue and let me
know if this is a known compiler bug or if clang is working as intended.

The following code produces a compile-time error in clang++ 4.0.0 and
later, but works fine in gcc as well as previous versions of clang.

#include <memory>

struct A {
    A() noexcept = default;
    A(const A&) noexcept = default;

    std::shared_ptr<int> _a {};
};

int main() {
    return 0;
}

It's worth noting that if I remove the copy constructor, this compiles
fine.  If I remove the noexcept qualification from the default constructor
it also compiles fine.

The error I'm getting is as follows:

<source>:4:5: error: default member initializer for '_a' needed within
definition of enclosing class 'A' outside of member functions
    A() noexcept = default;
    ^
/opt/compiler-explorer/clang-4.0.0/bin/../include/c++/v1/type_traits:1306:29:
note: in instantiation of template class 'std::__1::is_class<int *>'
requested here
template <class _Tp, bool = is_class<_Tp>::value>
                            ^
/opt/compiler-explorer/clang-4.0.0/bin/../include/c++/v1/type_traits:1311:71:
note: in instantiation of default argument for '__libcpp_abstract<int *>'
required here
template <class _Tp> struct _LIBCPP_TEMPLATE_VIS is_abstract : public
__libcpp_abstract<_Tp> {};

^~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/clang-4.0.0/bin/../include/c++/v1/type_traits:1384:39:
note: in instantiation of template class 'std::__1::is_abstract<int *>'
requested here
                                     !is_abstract<_T2>::value> {};
                                      ^
/opt/compiler-explorer/clang-4.0.0/bin/../include/c++/v1/memory:3918:39:
note: in instantiation of template class 'std::__1::is_convertible<int *,
int *>' requested here
                   typename enable_if<is_convertible<_Yp*,
element_type*>::value, __nat>::type = __nat())
                                      ^
/opt/compiler-explorer/clang-4.0.0/bin/../include/c++/v1/memory:3883:28:
note: while substituting deduced template arguments into function template
'shared_ptr' [with _Yp = int]
class _LIBCPP_TEMPLATE_VIS shared_ptr
                           ^
<source>:7:26: note: default member initializer declared here
    std::shared_ptr<int> _a {};
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