default member initialization and noexcept problems

[Quuxplusone]

Bugzilla Link	PR33736
Status	NEW
Importance	P normal
Reported by	Joe Sylve (joe.sylve@gmail.com)
Reported on	2017-07-10 12:57:51 -0700
Last modified on	2020-12-19 04:55:01 -0800
Version	4.0
Hardware	All All
CC	6yearold@gmail.com, d25fe0be@outlook.com, dgregor@apple.com, heavenandhell171@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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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.

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 {};

[Quuxplusone]

Reduced to remove stdlib dependency:

template<typename T> struct B { using type = bool; };

template<typename T> struct S {
  S() noexcept;

  template<typename U, typename B<U>::type = true>
  S(const S<U>&) noexcept;
};

struct A {
    A() noexcept = default;
    A(const A&) noexcept = default;
    S<int> _a {};
};

Looks like we somehow believe that the default member initializer of A::_a is
needed when instantiating B<int>.

[Quuxplusone]

Can't reproduce the problem with Clang 9, 10, 11.