libstdc++: Fix constraint recursion in std::expected's operator== [PR119714]

This std::expected friend operator== is prone to constraint recursion
after CWG 2369 for the same reason as basic_const_iterator's comparison
operators were before the r15-7757-g4342c50ca84ae5 workaround.  This
patch works around the constraint recursion here in a similar manner,
by making the function parameter of type std::expected dependent in a
trivial way.

	PR libstdc++/119714
	PR libstdc++/112490

libstdc++-v3/ChangeLog:

	* include/std/expected (expected::operator==): Replace
	non-dependent std::expected function parameter with a dependent
	one of type expected<_Vp, _Er> where _Vp matches _Tp.
	* testsuite/20_util/expected/119714.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

diff --git a/libstdc++-v3/include/std/expected b/libstdc++-v3/include/std/expected
index 5dc1dfbe5b8a954826d2779a9cbc51c953b5e5f0..60f1565f15b9272de7402a0fb70c270cf740d44d 100644 (file)
--- a/libstdc++-v3/include/std/expected
+++ b/libstdc++-v3/include/std/expected
@@ -1169,13 +1169,13 @@ namespace __expected
            return !__y.has_value() && bool(__x.error() == __y.error());
        }
 
-      template<typename _Up>
+      template<typename _Up, same_as<_Tp> _Vp>
        requires (!__expected::__is_expected<_Up>)
          && requires (const _Tp& __t, const _Up& __u) {
            { __t == __u } -> convertible_to<bool>;
          }
        friend constexpr bool
-       operator==(const expected& __x, const _Up& __v)
+       operator==(const expected<_Vp, _Er>& __x, const _Up& __v)
        noexcept(noexcept(bool(*__x == __v)))
        { return __x.has_value() && bool(*__x == __v); }
 

diff --git a/libstdc++-v3/testsuite/20_util/expected/119714.cc b/libstdc++-v3/testsuite/20_util/expected/119714.cc
new file mode 100644 (file)
index 0000000..a8dc6e8
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/expected/119714.cc
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++23 } }
+
+// PR libstdc++/119714 - constraint recursion with std::expected::operator==
+
+#include <expected>
+#include <vector>
+
+using I = std::vector<std::expected<int,int>>::iterator;
+static_assert(std::totally_ordered<I>);