C++ concepts: fix ICE with requires on dtors (PR c++/89036)

PR c++/89036 reports an ICE due to this assertion failing

1136	  /* A class should never have more than one destructor.  */
1137	  gcc_assert (!current_fns || via_using || !DECL_DESTRUCTOR_P (method));

on this template with a pair of dtors, with
mutually exclusive "requires" clauses:

template<typename T>
struct Y {
    ~Y() requires(true) = default;
    ~Y() requires(false) {}
};

Nathan introduced this assertion as part of:

  ca9219bf18c68a001d62ecb981bc9176b0feaf12 (aka r251340):
    2017-08-24  Nathan Sidwell  <nathan@acm.org>
       Conversion operators kept on single overload set

which, amongst other changes to add_method had this:
     /* A class should never have more than one destructor.  */
  -  if (current_fns && DECL_MAYBE_IN_CHARGE_DESTRUCTOR_P (method))
  -    return false;
  +  gcc_assert (!current_fns || !DECL_DESTRUCTOR_P (method));

The following patch drops the assertion (I already had to generalize
the assertion in r268041 to fix PR c++/88699).

gcc/cp/ChangeLog:
	PR c++/89036
	* class.c (add_method): Drop destructor assertion.

gcc/testsuite/ChangeLog:
	PR c++/89036
	* g++.dg/concepts/pr89036.C: New test.

From-SVN: r268847

gcc/cp/ChangeLog

+2019-02-13  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/89036
+	* class.c (add_method): Drop destructor assertion.
+
 2019-02-13  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/88986

gcc/cp/class.c

@@ -1134,9 +1134,6 @@ add_method (tree type, tree method, bool via_using)
 	}
     }
 
-  /* A class should never have more than one destructor.  */
-  gcc_assert (!current_fns || via_using || !DECL_DESTRUCTOR_P (method));
-
   current_fns = ovl_insert (method, current_fns, via_using);
 
   if (!COMPLETE_TYPE_P (type) && !DECL_CONV_FN_P (method)

gcc/testsuite/ChangeLog

+2019-02-13  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/89036
+	* g++.dg/concepts/pr89036.C: New test.
+
 2019-02-13  Tamar Christina  <tamar.christina@arm.com>
 
 	PR target/88847

gcc/testsuite/g++.dg/concepts/pr89036.C

+// { dg-do compile { target c++11 } }
+// { dg-options "-fconcepts" }
+
+template<typename T>
+struct Y {
+  ~Y() requires(true) = default;
+  ~Y() requires(false) {}
+};