c++: ICE on unviable/ambiguous constrained dtors [PR96745]

Here we're crashing from check_bases_and_members due to
CLASSTYPE_DESTRUCTOR being an OVERLOAD which, due to the pruning
performed by add_method, should only happen if there is no viable
destructor or the destructor is ambiguous because of unsatisfied
or ambiguous constraints.

This patch fixes this by making check_bases_and_members naturally handle
CLASSTYPE_DESTRUCTOR being an OVERLOAD.  It's then convenient to prune
the OVERLOAD after effectively diagnosing the overload resolution
failure in check_methods.

	PR c++/96745

gcc/cp/ChangeLog:

	* class.cc (check_methods): Diagnose an unviable OVERLOAD
	set for CLASSTYPE_DESTRUCTOR differently from an ambiguous one.
	Then prune the OVERLOAD to a single function.
	(check_bases_and_members): Handle CLASSTYPE_DESTRUCTOR being
	an OVERLOAD when calling deduce_noexcept_on_destructor.
	Document why it has to be called before check_methods.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-dtor1.C: New test.

2 files changed, 37 insertions, 3 deletions

diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 40e6b73..27a7982 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -4808,9 +4808,23 @@ check_methods (tree t)
 	     in that class with an empty argument list to select the destructor
 	     for the class, also known as the selected destructor. The program
 	     is ill-formed if overload resolution fails. */
+	  int viable = 0;
+	  for (tree fn : ovl_range (dtor))
+	    if (constraints_satisfied_p (fn))
+	      ++viable;
+	  gcc_checking_assert (viable != 1);
+
 	  auto_diagnostic_group d;
-	  error_at (location_of (t), "destructor for %qT is ambiguous", t);
+	  if (viable == 0)
+	    error_at (location_of (t), "no viable destructor for %qT", t);
+	  else
+	    error_at (location_of (t), "destructor for %qT is ambiguous", t);
 	  print_candidates (dtor);
+
+	  /* Arbitrarily prune the overload set to a single function for
+	     sake of error recovery.  */
+	  tree *slot = find_member_slot (t, dtor_identifier);
+	  *slot = get_first_fn (dtor);
 	}
       else if (user_provided_p (dtor))
 	TYPE_HAS_NONTRIVIAL_DESTRUCTOR (t) = true;
@@ -6048,10 +6062,12 @@ check_bases_and_members (tree t)
   check_bases (t, &cant_have_const_ctor, &no_const_asn_ref);
 
   /* Deduce noexcept on destructor.  This needs to happen after we've set
-     triviality flags appropriately for our bases.  */
+     triviality flags appropriately for our bases, and before checking
+     overriden virtual functions via check_methods.  */
   if (cxx_dialect >= cxx11)
     if (tree dtor = CLASSTYPE_DESTRUCTOR (t))
-      deduce_noexcept_on_destructor (dtor);
+      for (tree fn : ovl_range (dtor))
+	deduce_noexcept_on_destructor (fn);
 
   /* Check all the method declarations.  */
   check_methods (t);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-dtor1.C b/gcc/testsuite/g++.dg/cpp2a/concepts-dtor1.C
new file mode 100644
index 0000000..b1f3b4e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-dtor1.C
@@ -0,0 +1,18 @@
+// PR c++/96745
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct A { // { dg-error "destructor for 'A<int>' is ambiguous" }
+  ~A() requires true;
+  ~A() requires (!!true);
+};
+
+A<int> a;
+
+template<class T>
+struct B { // { dg-error "no viable destructor for 'B<int>'" }
+  ~B() requires false;
+  ~B() requires (!!false);
+};
+
+B<int> b;