[gcc r12-8178] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]

[gcc r12-8178] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]

[Patrick Palka]

https://gcc.gnu.org/g:7e76cef873342a66408c126abceaf7dbddd3f477

commit r12-8178-g7e76cef873342a66408c126abceaf7dbddd3f477
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Apr 15 14:41:14 2022 -0400

    libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]
    
            PR libstdc++/104858
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ranges_algo.h (__minmax_fn): Avoid dereferencing
            __first twice at the start.
            * testsuite/25_algorithms/minmax/constrained.cc (test06): New test.

Diff:
---
 libstdc++-v3/include/bits/ranges_algo.h            |  2 +-
 .../testsuite/25_algorithms/minmax/constrained.cc  | 30 ++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 62dc605080a..3d30fb1428c 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -3084,7 +3084,7 @@ namespace ranges
 	auto __last = ranges::end(__r);
 	__glibcxx_assert(__first != __last);
 	auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
-	minmax_result<range_value_t<_Range>> __result = {*__first, *__first};
+	minmax_result<range_value_t<_Range>> __result = {*__first, __result.min};
 	if (++__first == __last)
 	  return __result;
 	else
diff --git a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
index 90882afb6d0..59864043a0a 100644
--- a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
@@ -20,6 +20,7 @@
 
 #include <algorithm>
 #include <string>
+#include <utility>
 #include <vector>
 #include <testsuite_hooks.h>
 #include <testsuite_iterators.h>
@@ -129,6 +130,34 @@ test05()
   VERIFY( result.min == "a"s && result.max == "c"s );
 }
 
+struct A {
+  A() = delete;
+  A(int i) : i(i) { }
+  A(const A&) = default;
+  A(A&& other) : A(std::as_const(other)) { ++move_count; }
+  A& operator=(const A&) = default;
+  A& operator=(A&& other) {
+    ++move_count;
+    return *this = std::as_const(other);
+  };
+  friend auto operator<=>(const A&, const A&) = default;
+  static inline int move_count = 0;
+  int i;
+};
+
+void
+test06()
+{
+  // PR libstdc++/104858
+  // Verify ranges::minmax doesn't dereference the iterator for the first
+  // element in the range twice.
+  A a(42);
+  ranges::subrange r = {std::move_iterator(&a), std::move_sentinel(&a + 1)};
+  auto result = ranges::minmax(r);
+  VERIFY( A::move_count == 1 );
+  VERIFY( result.min.i == 42 && result.max.i == 42 );
+}
+
 int
 main()
 {
@@ -137,4 +166,5 @@ main()
   test03();
   test04();
   test05();
+  test06();
 }