c++: fix triviality of class with unsatisfied op=

cxx20_pair is trivially copyable because it has a trivial copy constructor
and only a deleted copy assignment operator; the non-triviality of the
unsatisfied copy assignment overload is not considered.

gcc/cp/ChangeLog:

	* class.cc (check_methods): Call constraints_satisfied_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/cond-triv3.C: New test.

2 files changed, 55 insertions, 2 deletions

diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index b84f422..aebcb53 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -4795,8 +4795,9 @@ check_methods (tree t)
 
   /* Check whether the eligible special member functions (P0848) are
      user-provided.  add_method arranged that the CLASSTYPE_MEMBER_VEC only
-     has the eligible ones; TYPE_FIELDS also contains ineligible overloads,
-     which is why this needs to be separate from the loop above.  */
+     has the eligible ones, unless none are eligible; TYPE_FIELDS also contains
+     ineligible overloads, which is why this needs to be separate from the loop
+     above.  */
 
   if (tree dtor = CLASSTYPE_DESTRUCTOR (t))
     {
@@ -4819,6 +4820,10 @@ check_methods (tree t)
     {
       if (!user_provided_p (fn))
 	/* Might be trivial.  */;
+      else if (TREE_CODE (fn) == TEMPLATE_DECL)
+	/* Templates are never special members.  */;
+      else if (!constraints_satisfied_p (fn))
+	/* Not eligible.  */;
       else if (copy_fn_p (fn))
 	TYPE_HAS_COMPLEX_COPY_CTOR (t) = true;
       else if (move_fn_p (fn))
@@ -4829,6 +4834,10 @@ check_methods (tree t)
     {
       if (!user_provided_p (fn))
 	/* Might be trivial.  */;
+      else if (TREE_CODE (fn) == TEMPLATE_DECL)
+	/* Templates are never special members.  */;
+      else if (!constraints_satisfied_p (fn))
+	/* Not eligible.  */;
       else if (copy_fn_p (fn))
 	TYPE_HAS_COMPLEX_COPY_ASSIGN (t) = true;
       else if (move_fn_p (fn))
diff --git a/gcc/testsuite/g++.dg/cpp2a/cond-triv3.C b/gcc/testsuite/g++.dg/cpp2a/cond-triv3.C
new file mode 100644
index 0000000..d0711cf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/cond-triv3.C
@@ -0,0 +1,44 @@
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct X
+{
+    T first{};
+
+    X& operator=(const X&) = delete;
+    X& operator=(const X&) requires requires (T& t) { t = t; } { return *this; }
+};
+
+// C++20 std::pair<const int, int>:
+using cxx20_pair = X<const int>;
+static_assert( __is_trivially_constructible(cxx20_pair, const cxx20_pair&), "" );
+static_assert( !__is_assignable(cxx20_pair&, const cxx20_pair&), "" );
+static_assert( __is_trivially_copyable(cxx20_pair), "" );
+
+template<bool, typename, typename F> struct conditional { using type = F; };
+template<typename T, typename F> struct conditional<true, T, F> { using type = T; };
+
+struct base
+{
+    base() = default;
+    ~base() = default;
+    base(const base&) = default;
+    base& operator=(const base&) = delete;
+};
+
+struct nope;
+
+template<class T>
+struct Y : base
+{
+    T first{};
+
+    Y& operator=(typename conditional<__is_assignable(T&, const T&), const Y&, const nope&>::type)
+    { return *this; }
+};
+
+// C++17 std::pair<const int, int>:
+using cxx17_pair = Y<const int>;
+static_assert( __is_trivially_constructible(cxx17_pair, const cxx17_pair&), "" );
+static_assert( ! __is_assignable(cxx17_pair&, const cxx17_pair&), "" );
+static_assert( __is_trivially_copyable(cxx17_pair), "???" );