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

[Joe Sylve via cfe-dev]

Please note this on the bug report

On Tue, Jul 11, 2017, 13:46 Dimitry Andric <dimitry at andric.com> wrote:

> The shortest reduction I could come up with is this:
>
> template <bool> struct a;
> template <class> struct B {};
> template <class b, bool = B<b>::f> struct g;
> template <class c, class, int = g<c>::f> struct i;
> template <class> class j {
> public:
>   j();
>   template <class d> j(j<d>, typename a<i<d *, int *>::f>::e = int());
> };
> struct k {
>   k() noexcept = default;
>   k(k &) = default;
>   j<int> h{};
> };
>
> All versions of g++ that I tried compile this without error, while clang
> says:
>
> testcase.cpp:11:3: error: default member initializer for 'h' needed within
> definition of enclosing class 'k' outside of member functions
>   k() noexcept = default;
>   ^
> testcase.cpp:3:27: note: in instantiation of template class 'B<int *>'
> requested here
> template <class b, bool = B<b>::f> struct g;
>                           ^
> testcase.cpp:4:33: note: in instantiation of default argument for 'g<int
> *>' required here
> template <class c, class, int = g<c>::f> struct i;
>                                 ^~~~
> testcase.cpp:8:41: note: in instantiation of default argument for 'i<int
> *, int *>' required here
>   template <class d> j(j<d>, typename a<i<d *, int *>::f>::e = int());
>                                         ^~~~~~~~~~~~~
> testcase.cpp:5:24: note: while substituting deduced template arguments
> into function template 'j' [with d = int]
> template <class> class j {
>                        ^
> testcase.cpp:13:10: note: default member initializer declared here
>   j<int> h{};
>          ^
> 1 error generated.
>
> -Dimitry
>
> > On 11 Jul 2017, at 16:54, Joe Sylve <joe.sylve at gmail.com> wrote:
> >
> > 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 {};
> >> _______________________________________________
> >> cfe-dev mailing list
> >> cfe-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
>
>
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