P0896R4 changes to insert iterators (microsoft#589)

When `defined(__cpp_lib_concepts)`, `(back_|front_|)insert_iterator` are default constructible, and have `ptrdiff_t` as difference type.

Skip libc++ tests broken by this change.

stl/inc/iterator

@@ -25,12 +25,19 @@ class back_insert_iterator { // wrap pushes to back of container as output itera
 public:
     using iterator_category = output_iterator_tag;
     using value_type        = void;
-    using difference_type   = void;
     using pointer           = void;
     using reference         = void;
 
     using container_type = _Container;
 
+#ifdef __cpp_lib_concepts
+    using difference_type = ptrdiff_t;
+
+    constexpr back_insert_iterator() noexcept = default;
+#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
+    using difference_type = void;
+#endif // __cpp_lib_concepts
+
     explicit back_insert_iterator(_Container& _Cont) noexcept /* strengthened */ : container(_STD addressof(_Cont)) {}
 
     back_insert_iterator& operator=(const typename _Container::value_type& _Val) {
@@ -56,7 +63,7 @@ public:
     }
 
 protected:
-    _Container* container; // pointer to container
+    _Container* container = nullptr;
 };
 
 // FUNCTION TEMPLATE back_inserter
@@ -72,12 +79,19 @@ class front_insert_iterator { // wrap pushes to front of container as output ite
 public:
     using iterator_category = output_iterator_tag;
     using value_type        = void;
-    using difference_type   = void;
     using pointer           = void;
     using reference         = void;
 
     using container_type = _Container;
 
+#ifdef __cpp_lib_concepts
+    using difference_type = ptrdiff_t;
+
+    constexpr front_insert_iterator() noexcept = default;
+#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
+    using difference_type = void;
+#endif // __cpp_lib_concepts
+
     explicit front_insert_iterator(_Container& _Cont) : container(_STD addressof(_Cont)) {}
 
     front_insert_iterator& operator=(const typename _Container::value_type& _Val) { // push value into container
@@ -103,7 +117,7 @@ public:
     }
 
 protected:
-    _Container* container; // pointer to container
+    _Container* container = nullptr;
 };
 
 // FUNCTION TEMPLATE front_inserter
@@ -119,12 +133,19 @@ class insert_iterator { // wrap inserts into container as output iterator
 public:
     using iterator_category = output_iterator_tag;
     using value_type        = void;
-    using difference_type   = void;
     using pointer           = void;
     using reference         = void;
 
     using container_type = _Container;
 
+#ifdef __cpp_lib_concepts
+    using difference_type = ptrdiff_t;
+
+    insert_iterator() = default;
+#else // ^^^ __cpp_lib_concepts / !__cpp_lib_concepts vvv
+    using difference_type = void;
+#endif // __cpp_lib_concepts
+
     insert_iterator(_Container& _Cont, typename _Container::iterator _Where)
         : container(_STD addressof(_Cont)), iter(_Where) {}
 
@@ -155,8 +176,8 @@ public:
     }
 
 protected:
-    _Container* container; // pointer to container
-    typename _Container::iterator iter; // iterator into container
+    _Container* container = nullptr;
+    typename _Container::iterator iter{};
 };
 
 // FUNCTION TEMPLATE inserter

tests/libcxx/skipped_tests.txt

@@ -784,6 +784,11 @@ language.support\cmp\cmp.weakord\weakord.pass.cpp
 # error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
 containers\sequences\array\array.creation\to_array.pass.cpp
 
+# Tests that need to learn that insert iterators have non-void difference type in C++20
+iterators\predef.iterators\insert.iterators\back.insert.iterator\types.pass.cpp
+iterators\predef.iterators\insert.iterators\front.insert.iterator\types.pass.cpp
+iterators\predef.iterators\insert.iterators\insert.iterator\types.pass.cpp
+
 
 # *** LIKELY STL BUGS ***
 # Not yet analyzed, likely STL bugs. Assertions and other runtime failures.

tests/std/tests/P0896R4_ranges_iterator_machinery/test.cpp

@@ -5,8 +5,10 @@
 #include <compare>
 #include <concepts>
 #include <iterator>
+#include <list>
 #include <type_traits>
 #include <utility>
+#include <vector>
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
@@ -2770,6 +2772,29 @@ namespace iter_ops {
     }
 } // namespace iter_ops
 
+namespace insert_iterators {
+    template <class Container>
+    constexpr bool test() {
+        using std::back_insert_iterator, std::front_insert_iterator, std::insert_iterator;
+        using std::default_initializable, std::is_nothrow_default_constructible_v, std::iter_difference_t,
+            std::ptrdiff_t, std::same_as;
+
+        STATIC_ASSERT(default_initializable<back_insert_iterator<Container>>);
+        STATIC_ASSERT(is_nothrow_default_constructible_v<back_insert_iterator<Container>>);
+        STATIC_ASSERT(default_initializable<front_insert_iterator<Container>>);
+        STATIC_ASSERT(is_nothrow_default_constructible_v<front_insert_iterator<Container>>);
+        STATIC_ASSERT(default_initializable<insert_iterator<Container>>);
+        STATIC_ASSERT(same_as<iter_difference_t<back_insert_iterator<Container>>, ptrdiff_t>);
+        STATIC_ASSERT(same_as<iter_difference_t<front_insert_iterator<Container>>, ptrdiff_t>);
+        STATIC_ASSERT(same_as<iter_difference_t<insert_iterator<Container>>, ptrdiff_t>);
+
+        return true;
+    }
+
+    STATIC_ASSERT(test<std::list<double>>());
+    STATIC_ASSERT(test<std::vector<int>>());
+} // namespace insert_iterators
+
 int main() {
     iterator_cust_swap_test::test();
     iter_ops::test();