[gcc r13-5683] c++: ICE on unviable/ambiguous constrained dtors [PR96745]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:ed2b519e02eac99fadfa51adc7b11f8854c24575

commit r13-5683-ged2b519e02eac99fadfa51adc7b11f8854c24575
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 3 09:15:29 2023 -0500

    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.

Diff:
---
 gcc/cp/class.cc                             | 22 +++++++++++++++++++---
 gcc/testsuite/g++.dg/cpp2a/concepts-dtor1.C | 18 ++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 40e6b7365a8..27a79829737 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 00000000000..b1f3b4e579f
--- /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;