Avoid truncation warnings for contiguous iterators with unusual difference types (#4898)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/xutility

@@ -4659,7 +4659,7 @@ _OutCtgIt _Copy_memmove(_CtgIt _First, _CtgIt _Last, _OutCtgIt _Dest) {
     if constexpr (is_pointer_v<_OutCtgIt>) {
         return reinterpret_cast<_OutCtgIt>(_Dest_ch + _Count);
     } else {
-        return _Dest + (_LastPtr - _FirstPtr);
+        return _Dest + static_cast<_Iter_diff_t<_OutCtgIt>>(_LastPtr - _FirstPtr);
     }
 }
 
@@ -4984,7 +4984,7 @@ _CtgIt2 _Copy_backward_memmove(_CtgIt1 _First, _CtgIt1 _Last, _CtgIt2 _Dest) {
     if constexpr (is_pointer_v<_CtgIt2>) {
         return static_cast<_CtgIt2>(_Result);
     } else {
-        return _Dest - (_LastPtr - _FirstPtr);
+        return _Dest - static_cast<_Iter_diff_t<_CtgIt2>>(_LastPtr - _FirstPtr);
     }
 }
 

tests/std/test.lst

@@ -230,6 +230,7 @@ tests\GH_003119_error_category_ctor
 tests\GH_003246_cmath_narrowing
 tests\GH_003570_allocate_at_least
 tests\GH_003617_vectorized_meow_element
+tests\GH_003663_cast_contiguous_iterator_difference_type
 tests\GH_003676_format_large_hh_mm_ss_values
 tests\GH_003735_char_traits_signatures
 tests\GH_003840_tellg_when_reading_lf_file_in_text_mode

tests/std/tests/GH_003663_cast_contiguous_iterator_difference_type/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_20_matrix.lst

tests/std/tests/GH_003663_cast_contiguous_iterator_difference_type/test.compile.pass.cpp

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <algorithm>
+#include <iterator>
+
+class ContiguousIterator {
+public:
+    using value_type        = int;
+    using difference_type   = int;
+    using element_type      = int;
+    using iterator_category = std::contiguous_iterator_tag;
+    int* operator->() const;
+    int& operator*() const;
+    int& operator[](int) const;
+    ContiguousIterator& operator++();
+    ContiguousIterator operator++(int);
+    ContiguousIterator& operator--();
+    ContiguousIterator operator--(int);
+    ContiguousIterator& operator+=(int);
+    ContiguousIterator& operator-=(int);
+    friend auto operator<=>(ContiguousIterator, ContiguousIterator) = default;
+    friend int operator-(ContiguousIterator, ContiguousIterator);
+    friend ContiguousIterator operator+(ContiguousIterator, int);
+    friend ContiguousIterator operator-(ContiguousIterator, int);
+    friend ContiguousIterator operator+(int, ContiguousIterator);
+};
+
+static_assert(std::contiguous_iterator<ContiguousIterator>);
+
+// GH-3663 <algorithm>/<iterator>: contiguous iterators with non-ptrdiff_t difference types
+void test() {
+    int* p = nullptr;
+    std::copy(p, p, ContiguousIterator{});
+    std::sort(ContiguousIterator{}, ContiguousIterator{});
+}