C++: Fix ICE when adding overloaded operator via using_decl (PR c++/88699)

PR c++/88699 reports an ICE within this assertion in add_method:

  gcc_assert (!current_fns || !DECL_DESTRUCTOR_P (method));

when adding an overloaded operator to a class via a using_decl, due to
DECL_DESTRUCTOR_P requiring a FUNCTION_DECL, but "method" being a
USING_DECL.

This patch weakens the assertion to avoid testing DECL_DESTRUCTOR_P
for the case where "via_using" is true, fixing the ICE.

gcc/cp/ChangeLog:
	PR c++/88699
	* class.c (add_method): Don't use DECL_DESTRUCTOR_P on
	USING_DECLs.

gcc/testsuite/ChangeLog:
	PR c++/88699
	* g++.dg/template/pr88699.C: New test.

From-SVN: r268041

gcc/cp/ChangeLog

+2019-01-17  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/88699
+	* class.c (add_method): Don't use DECL_DESTRUCTOR_P on
+	USING_DECLs.
+
 2019-01-17  Nathan Sidwell  <nathan@acm.org>
 
 	PR c++/86610

gcc/cp/class.c

@@ -1134,7 +1134,7 @@ add_method (tree type, tree method, bool via_using)
     }
 
   /* A class should never have more than one destructor.  */
-  gcc_assert (!current_fns || !DECL_DESTRUCTOR_P (method));
+  gcc_assert (!current_fns || via_using || !DECL_DESTRUCTOR_P (method));
 
   current_fns = ovl_insert (method, current_fns, via_using);
 

gcc/testsuite/ChangeLog

+2019-01-17  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/88699
+	* g++.dg/template/pr88699.C: New test.
+
 2019-01-17  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/88800

gcc/testsuite/g++.dg/template/pr88699.C

+// { dg-do compile }
+
+template <typename>
+struct A {
+  void operator= (int);
+  template <int> class B;
+};
+template <typename C>
+template <int>
+struct A<C>::B : A {
+  using A::operator=;
+  void operator= (B);
+};