[libcxx-commits] [PATCH] D65161: [libc++] Implement LWG 2510

[Louis Dionne via Phabricator via libcxx-commits]

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================
Comment at: libcxx/include/tuple:959
 #ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+tuple() -> tuple<>;
+
----------------
@Quuxplusone Any idea why this is needed? Otherwise, I get:


```
<snip>/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.cnstr/implicit_deduction_guides.pass.cpp:125:16: error: no viable constructor or deduction guide for deduction of template arguments of 'tuple'
    std::tuple t1{};
               ^
<snip>/libcxx/include/tuple:643:5: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    tuple()
    ^
<snip>/libcxx/include/tuple:650:5: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    tuple()
    ^
<snip>/libcxx/include/tuple:680:5: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
    ^
<snip>/libcxx/include/tuple:698:14: note: candidate template ignored: substitution failure [with _Tp = <>, _Dummy = true]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
    explicit tuple(const _Tp& ... __t) _NOEXCEPT_((__all<is_nothrow_copy_constructible<_Tp>::value...>::value))
             ^
<snip>/libcxx/include/tuple:762:9: note: candidate template ignored: substitution failure [with _Tp = <>, _Up = <>]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
        tuple(_Up&&... __u)
        ^
<snip>/libcxx/include/tuple:795:9: note: candidate template ignored: substitution failure [with _Tp = <>, _Up = <>]: cannot reference member of primary template because deduced class template specialization 'tuple<>' is an explicit specialization
        tuple(_Up&&... __u)
        ^
<snip>/libcxx/include/__tuple:159:52: note: candidate function template not viable: requires 1 argument, but 0 were provided
template <class ..._Tp> class _LIBCPP_TEMPLATE_VIS tuple;
                                                   ^
<snip>/libcxx/include/tuple:653:5: note: candidate function template not viable: requires 1 argument, but 0 were provided
    tuple(tuple const&) = default;
    ^
<snip>/libcxx/include/tuple:654:5: note: candidate function template not viable: requires 1 argument, but 0 were provided
    tuple(tuple&&) = default;
    ^
<snip>/libcxx/include/tuple:664:5: note: candidate function template not viable: requires 2 arguments, but 0 were provided
    tuple(_AllocArgT, _Alloc const& __a)
    ^
<snip>/libcxx/include/tuple:716:7: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
      tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
      ^
<snip>/libcxx/include/tuple:736:7: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
      tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
      ^
<snip>/libcxx/include/tuple:822:9: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
        tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
        ^
<snip>/libcxx/include/tuple:842:9: note: candidate function template not viable: requires at least 2 arguments, but 0 were provided
        tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
        ^
<snip>/libcxx/include/tuple:852:9: note: candidate function template not viable: requires single argument '__t', but no arguments were provided
        tuple(_Tuple&& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, _Tuple>::value))
        ^
<snip>/libcxx/include/tuple:857:9: note: candidate function template not viable: requires single argument '__t', but no arguments were provided
        tuple(const _Tuple& __t) _NOEXCEPT_((is_nothrow_constructible<_BaseT, const _Tuple&>

[...]
```


Repository:
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  https://reviews.llvm.org/D65161/new/

https://reviews.llvm.org/D65161