[Bug libstdc++/82716] New: struct/class vs. tuple_element/tuple_size

[marc.mutz at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82716

            Bug ID: 82716
           Summary: struct/class vs. tuple_element/tuple_size
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marc.mutz at kdab dot com
  Target Milestone: ---

I'm trying to support the tuple protocol for Qt's QPair:

namespace std {
    // these have to be 'class', because MSVC warns about struct/class
mismatch, and
    // in the std, they are classes:
    template <typename T1, typename T2>
    class tuple_size<QT_PREPEND_NAMESPACE(QPair)<T1,T2>> : public
std::integral_constant<size_t, 2> {};
    template <typename T1, typename T2>
    class tuple_element<0, QT_PREPEND_NAMESPACE(QPair)<T1, T2>> { public: using
type = T1; };
    template <typename T1, typename T2>
    class tuple_element<1, QT_PREPEND_NAMESPACE(QPair)<T1, T2>> { public: using
type = T2; };
}

I get these warnings from clang:

68 : <source>:68:5: error: 'tuple_size' defined as a class template here but
previously declared as a struct template [-Werror,-Wmismatched-tags]
    class tuple_size<~~~> : public std::integral_constant<std::size_t, 2> {};
    ^
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/utility:88:5:
note: did you mean class here?
    struct tuple_size;
    ^
71 : <source>:71:5: error: 'tuple_element' defined as a class template here but
previously declared as a struct template [-Werror,-Wmismatched-tags]
    class tuple_element<I, ~~~~>
    ^
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/utility:112:5:
note: did you mean class here?
    struct tuple_element;
    ^
2 errors generated.
Compiler exited with result code 1

Here's a stripped-down example: https://godbolt.org/g/sUZxAQ
Works with -stdlib=libc++: https://godbolt.org/g/bW4u1u

The standard says that tuple_element and tuple_size are struct and continues to
use it as class (http://eel.is/c++draft/tuple.helper), and so does libstdc++,
which means that there's no way to write a tuple_size specialisation that will
avoid the warning.

Expected: libstdc++ recognizes that it may be used with compilers that warn
about struct/class mismatch, picks one of the two, and sticks to it, so one can
at least use #ifdefs to pick which one to use.