[PATCH] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]

[Patrick Palka via Gcc-patches]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk and 11/10
once the branch is unfrozen?

        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.
---
 libstdc++-v3/include/bits/ranges_algo.h       |  2 +-
 .../25_algorithms/minmax/constrained.cc       | 23 +++++++++++++++++++
 2 files changed, 24 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..306c495babe 100644
--- a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
@@ -129,6 +129,28 @@ test05()
   VERIFY( result.min == "a"s && result.max == "c"s );
 }
 
+struct A {
+  A() = default;
+  A(const A&) = default;
+  A(A&&) { ++move_count; }
+  A& operator=(const A&) = default;
+  A& operator=(A&&) = default;
+  friend auto operator<=>(const A&, const A&) = default;
+  static inline int move_count = 0;
+};
+
+void
+test06()
+{
+  // PR libstdc++/104858
+  // Verify ranges::minmax doesn't dereference the iterator for the first
+  // element in the range twice.
+  A a;
+  ranges::subrange r = {std::move_iterator(&a), std::move_sentinel(&a + 1)};
+  ranges::minmax(r);
+  VERIFY( A::move_count == 1 );
+}
+
 int
 main()
 {
@@ -137,4 +159,5 @@ main()
   test03();
   test04();
   test05();
+  test06();
 }
-- 
2.36.0.rc2.10.g1ac7422e39