[llvm-bugs] [Bug 40124] New: [regression] std::array<int, 0>::begin() should be constexpr but isn't

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40124

            Bug ID: 40124
           Summary: [regression] std::array<int, 0>::begin() should be
                    constexpr but isn't
           Product: libc++
           Version: 7.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tonyelewis at hotmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Compiling the following with `-fsyntax-only -std=c++17 -stdlib=libc++` :


~~~
#include <array>

template <typename Range>
inline constexpr bool f(const Range &prmRange) {
        prmRange.begin();
        return true;
}

void some_function() {
        static_assert( f( ::std::array<int, 1>{ 0 } ) );
        static_assert( f( ::std::array<int, 0>{   } ) );
}
~~~


I get:


~~~
a.cpp:11:17: error: static_assert expression is not an integral constant
expression
        static_assert( f( ::std::array<int, 0>{   } ) );
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cpp:5:11: note: non-constexpr function 'begin' cannot be used in a constant
expression
        prmRange.begin();
                 ^
a.cpp:11:17: note: in call to 'f(::std::array<int, 0>{})'
        static_assert( f( ::std::array<int, 0>{   } ) );
                       ^
/home/lewis/source/llvm/bin/../include/c++/v1/array:273:20: note: declared here
    const_iterator begin() const _NOEXCEPT {return const_iterator(data());}
                   ^
1 error generated.
~~~


It looks like the zero-size specialisation of `::std::array` has some
constexprs missing. From what I can see in "26.3.7.1 Class template array
overview [array.overview]" and "26.3.7.8 Zero sized arrays [array.zero]" of the
C++17 draft, a zero-sized array should have constexpr `begin()` (etc).

This appears to be a regression: on Godbolt, I'm seeing this fail to compile in
this way under 7.0 and trunk, but compile cleanly under 6.0.

Thanks very much for all work on libc++.

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