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

[Joe Sylve via cfe-dev]

It is, but I'm not sure that libc++ is the cause or just points out the
bug. If you look at the stack overflow link, you'll see an example of
different code that triggers a similar error without the use of any library
stuff.

On Tue, Jul 11, 2017, 03:45 Dimitry Andric <dimitry at andric.com> wrote:

> FYI, this looks like an issue in libc++.  If I compile your sample with
> the latest and greatest clang, but point it to the g++ 7.1.1 libstdc++
> headers, it works just fine.
>
> -Dimitry
>
> On 10 Jul 2017, at 20:55, Joe Sylve via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
>
> 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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