<algorithm>: fix GH-1932 (#1933)

* <algorithm>: fix GH-1932

... by extracting the constant expression we'd like to short-circuit into a concept.

Fixes #1932.

Author: CaseyCarter

stl/inc/algorithm

@@ -4072,9 +4072,12 @@ namespace ranges {
 
     // VARIABLE ranges::unique_copy
     // clang-format off
+    template <class _It, class _Ty>
+    concept _Is_input_with_value_type = input_iterator<_It> && same_as<iter_value_t<_It>, _Ty>;
+
     template <class _It, class _Out>
     concept _Can_reread_or_store = forward_iterator<_It>
-                                || (input_iterator<_Out> && same_as<iter_value_t<_It>, iter_value_t<_Out>>)
+                                || _Is_input_with_value_type<_Out, iter_value_t<_It>>
                                 || indirectly_copyable_storable<_It, _Out>;
     // clang-format on
     class _Unique_copy_fn : private _Not_quite_object {
@@ -4127,7 +4130,7 @@ namespace ranges {
                 return {_STD move(_First), _STD move(_Result)};
             }
 
-            if constexpr (input_iterator<_Out> && same_as<iter_value_t<_It>, iter_value_t<_Out>>) {
+            if constexpr (_Is_input_with_value_type<_Out, iter_value_t<_It>>) {
                 // Can reread _Result
                 *_Result = *_First;
 

tests/std/tests/P0896R4_ranges_alg_unique_copy/test.cpp

@@ -5,6 +5,7 @@
 #include <cassert>
 #include <concepts>
 #include <ranges>
+#include <sstream>
 #include <utility>
 
 #include <range_algorithm_support.hpp>
@@ -142,8 +143,20 @@ constexpr bool run_tests() {
     return true;
 }
 
+void test_gh1932() {
+    // Defend against regression of GH-1932, in which ranges::unique_copy instantiated
+    // iter_value_t<I> for a non-input iterator I.
+
+    istringstream str("42 42 42");
+    ostringstream result;
+    ranges::unique_copy(istream_iterator<int>{str}, istream_iterator<int>{}, ostream_iterator<int>{result, " "});
+    assert(result.str() == "42 ");
+}
+
 int main() {
     STATIC_ASSERT(run_tests());
     run_tests();
+
+    test_gh1932();
 }
 #endif // TEST_EVERYTHING