Specially handle array<T, 0> for non-default-constructible T (microsoft#2296)

Author: CaseyCarter

stl/inc/array

@@ -615,6 +615,8 @@ template <class _First, class... _Rest>
 array(_First, _Rest...) -> array<typename _Enforce_same<_First, _Rest...>::type, 1 + sizeof...(_Rest)>;
 #endif // _HAS_CXX17
 
+struct _Empty_array_element {};
+
 template <class _Ty>
 class array<_Ty, 0> {
 public:
@@ -728,47 +730,47 @@ public:
         _STL_REPORT_ERROR("array subscript out of range");
 #endif // _CONTAINER_DEBUG_LEVEL > 0
 
-        return _Elems[0];
+        return *data();
     }
 
     _NODISCARD const_reference operator[](size_type) const noexcept /* strengthened */ {
 #if _CONTAINER_DEBUG_LEVEL > 0
         _STL_REPORT_ERROR("array subscript out of range");
 #endif // _CONTAINER_DEBUG_LEVEL > 0
 
-        return _Elems[0];
+        return *data();
     }
 
     _NODISCARD reference front() noexcept /* strengthened */ {
 #if _CONTAINER_DEBUG_LEVEL > 0
         _STL_REPORT_ERROR("array<T, 0>::front() invalid");
 #endif // _CONTAINER_DEBUG_LEVEL > 0
 
-        return _Elems[0];
+        return *data();
     }
 
     _NODISCARD const_reference front() const noexcept /* strengthened */ {
 #if _CONTAINER_DEBUG_LEVEL > 0
         _STL_REPORT_ERROR("array<T, 0>::front() invalid");
 #endif // _CONTAINER_DEBUG_LEVEL > 0
 
-        return _Elems[0];
+        return *data();
     }
 
     _NODISCARD reference back() noexcept /* strengthened */ {
 #if _CONTAINER_DEBUG_LEVEL > 0
         _STL_REPORT_ERROR("array<T, 0>::back() invalid");
 #endif // _CONTAINER_DEBUG_LEVEL > 0
 
-        return _Elems[0];
+        return *data();
     }
 
     _NODISCARD const_reference back() const noexcept /* strengthened */ {
 #if _CONTAINER_DEBUG_LEVEL > 0
         _STL_REPORT_ERROR("array<T, 0>::back() invalid");
 #endif // _CONTAINER_DEBUG_LEVEL > 0
 
-        return _Elems[0];
+        return *data();
     }
 
     _NODISCARD _CONSTEXPR17 _Ty* data() noexcept {
@@ -783,7 +785,9 @@ public:
         _Xout_of_range("invalid array<T, 0> subscript");
     }
 
-    _Ty _Elems[1];
+    conditional_t<disjunction_v<is_default_constructible<_Ty>, _Is_implicitly_default_constructible<_Ty>>, _Ty,
+        _Empty_array_element>
+        _Elems[1];
 };
 
 template <class _Ty, size_t _Size, enable_if_t<_Size == 0 || _Is_swappable<_Ty>::value, int> = 0>

tests/libcxx/expected_results.txt

@@ -69,6 +69,9 @@ std/language.support/support.limits/support.limits.general/concepts.version.pass
 # Bogus test believes that optional<non_constexpr_destructor> cannot be a literal type
 std/utilities/optional/optional.object/optional.object.dtor/dtor.pass.cpp:0 FAIL
 
+# Bogus test believes that copyability of array<T, 0> must be the same as array<T, 1>
+std/containers/sequences/array/array.cons/implicit_copy.pass.cpp FAIL
+
 
 # *** INTERACTIONS WITH CONTEST / C1XX THAT UPSTREAM LIKELY WON'T FIX ***
 # Tracked by VSO-593630 "<filesystem> Enable libcxx filesystem tests"
@@ -343,13 +346,6 @@ std/input.output/file.streams/fstreams/filebuf.virtuals/underflow.pass.cpp FAIL
 std/containers/sequences/array/array.fill/fill.fail.cpp FAIL
 std/containers/sequences/array/array.swap/swap.fail.cpp FAIL
 
-# GH-942 <array>: std::array<T,0> doesn't compile - when type is not default constructible
-std/containers/sequences/array/array.cons/implicit_copy.pass.cpp FAIL
-std/containers/sequences/array/array.cons/initialization.pass.cpp FAIL
-std/containers/sequences/array/array.data/data_const.pass.cpp FAIL
-std/containers/sequences/array/array.data/data.pass.cpp FAIL
-std/containers/sequences/array/iterators.pass.cpp FAIL
-
 # GH-1006 <algorithm>: debug checks for predicates are observable
 std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp FAIL
 std/algorithms/alg.sorting/alg.min.max/minmax_init_list_comp.pass.cpp FAIL

tests/libcxx/skipped_tests.txt

@@ -69,6 +69,9 @@ language.support\support.limits\support.limits.general\concepts.version.pass.cpp
 # Bogus test believes that optional<non_constexpr_destructor> cannot be a literal type
 utilities\optional\optional.object\optional.object.dtor\dtor.pass.cpp
 
+# Bogus test believes that copyability of array<T, 0> must be the same as array<T, 1>
+containers\sequences\array\array.cons\implicit_copy.pass.cpp
+
 
 # *** INTERACTIONS WITH CONTEST / C1XX THAT UPSTREAM LIKELY WON'T FIX ***
 # Tracked by VSO-593630 "<filesystem> Enable libcxx filesystem tests"
@@ -343,13 +346,6 @@ input.output\file.streams\fstreams\filebuf.virtuals\underflow.pass.cpp
 containers\sequences\array\array.fill\fill.fail.cpp
 containers\sequences\array\array.swap\swap.fail.cpp
 
-# GH-942 <array>: std::array<T,0> doesn't compile - when type is not default constructible
-containers\sequences\array\array.cons\implicit_copy.pass.cpp
-containers\sequences\array\array.cons\initialization.pass.cpp
-containers\sequences\array\array.data\data_const.pass.cpp
-containers\sequences\array\array.data\data.pass.cpp
-containers\sequences\array\iterators.pass.cpp
-
 # GH-1006 <algorithm>: debug checks for predicates are observable
 algorithms\alg.sorting\alg.merge\inplace_merge_comp.pass.cpp
 algorithms\alg.sorting\alg.min.max\minmax_init_list_comp.pass.cpp