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  <nat...@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.

gcc/cp/ChangeLog:
        2019-02-13  David Malcolm  <dmalc...@redhat.com>
        Backport of r268847 from trunk.

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

gcc/testsuite/ChangeLog:
        2019-02-13  David Malcolm  <dmalc...@redhat.com>
        Backport of r268847 from trunk.

        PR c++/89036
        * g++.dg/concepts/pr89036.C: New test.
---
 gcc/cp/class.c                          | 3 ---
 gcc/testsuite/g++.dg/concepts/pr89036.C | 8 ++++++++
 2 files changed, 8 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/concepts/pr89036.C

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index d524a98..834ba17 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -1141,9 +1141,6 @@ 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));
-
   current_fns = ovl_insert (method, current_fns, via_using);
 
   if (!COMPLETE_TYPE_P (type) && !DECL_CONV_FN_P (method)
diff --git a/gcc/testsuite/g++.dg/concepts/pr89036.C 
b/gcc/testsuite/g++.dg/concepts/pr89036.C
new file mode 100644
index 0000000..f83ef8b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/pr89036.C
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-fconcepts" }
+
+template<typename T>
+struct Y {
+  ~Y() requires(true) = default;
+  ~Y() requires(false) {}
+};
-- 
1.8.5.3

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