libstdc++: Fix std::array<T, 0>::data() to be a constant expression [PR108258]

When I refactored the __array_traits helper I broke this.

libstdc++-v3/ChangeLog:

	PR libstdc++/108258
	* include/std/array (__array_traits<T, 0>::operator T*()): Add
	constexpr.
	* testsuite/23_containers/array/element_access/constexpr_c++17.cc: Check
	std::array<T, 0>::data().

2 files changed, 17 insertions, 4 deletions

diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index e26390e..c50a201 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -69,7 +69,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
        // Conversion to a pointer produces a null pointer.
        __attribute__((__always_inline__))
-       operator _Tp*() const noexcept { return nullptr; }
+       constexpr operator _Tp*() const noexcept { return nullptr; }
      };
 
      using _Is_swappable = true_type;
diff --git a/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_c++17.cc b/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_c++17.cc
index b6878fd..b92aa5c 100644
--- a/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_c++17.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_c++17.cc
@@ -34,21 +34,34 @@ constexpr std::size_t test01()
   auto v2  = a.at(2);
   auto v3  = a.front();
   auto v4  = a.back();
-  return v1 + v2 + v3 + v4;
+  auto v5 = *a.data();
+  return v1 + v2 + v3 + v4 + v5;
 }
 
 static_assert( test01() == (55 + 66 + 0 + 2) );
 
 constexpr std::size_t test02()
 {
-  // array
+  // const array
   typedef std::array<std::size_t, 6> array_type;
   const array_type a = { { 0, 55, 66, 99, 4115, 2 } };
   auto v1  = a[1];
   auto v2  = a.at(2);
   auto v3  = a.front();
   auto v4  = a.back();
-  return v1 + v2 + v3 + v4;
+  auto v5 = *a.data();
+  return v1 + v2 + v3 + v4 + v5;
 }
 
 static_assert( test02() == (55 + 66 + 0 + 2) );
+
+constexpr bool test_zero()
+{
+  // zero-sized array (PR libstdc++/108258)
+  std::array<int, 0> a{};
+  auto v4 = a.data();
+  // The standard says this is unspecified, it's null for our implementation:
+  return a.data() == nullptr;
+}
+
+static_assert( test_zero() );