c++: Fix ICE with invalid using enum [PR96462]

Here we ICE in finish_nonmember_using_decl -> lookup_using_decl ->
... -> find_namespace_slot because "name" is not an IDENTIFIER_NODE.
It is a BIT_NOT_EXPR because this broken test uses

  using E::~E; // SCOPE::NAME

A using-decl can't refer to a destructor, and lookup_using_decl already
checks that in the class member case.  But in C++17, we do the "enum
scope is the enclosing scope" block, and so scope gets set to ::, and
we go into the NAMESPACE_DECL block.  In C++20 we don't do it, we go
to the ENUMERAL_TYPE block.

I resorted to hoisting the check along with a diagnostic tweak: we
don't want to print "::::~E names destructor".

gcc/cp/ChangeLog:

	PR c++/96462
	* name-lookup.c (lookup_using_decl): Hoist the destructor check.

gcc/testsuite/ChangeLog:

	PR c++/96462
	* g++.dg/cpp2a/using-enum-8.C: New test.

2 files changed, 13 insertions, 7 deletions

diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index 52e4a63..1f4a7ac 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -5700,6 +5700,14 @@ lookup_using_decl (tree scope, name_lookup &lookup)
 	scope = ctx;
     }
 
+  /* You cannot using-decl a destructor.  */
+  if (TREE_CODE (lookup.name) == BIT_NOT_EXPR)
+    {
+      error ("%<%T%s%D%> names destructor", scope,
+	     &"::"[scope == global_namespace ? 2 : 0], lookup.name);
+      return NULL_TREE;
+    }
+
   if (TREE_CODE (scope) == NAMESPACE_DECL)
     {
       /* Naming a namespace member.  */
@@ -5739,13 +5747,6 @@ lookup_using_decl (tree scope, name_lookup &lookup)
 	  return NULL_TREE;
 	}
 
-      /* You cannot using-decl a destructor.  */
-      if (TREE_CODE (lookup.name) == BIT_NOT_EXPR)
-	{
-	  error ("%<%T::%D%> names destructor", scope, lookup.name);
-	  return NULL_TREE;
-	}
-
       /* Using T::T declares inheriting ctors, even if T is a typedef.  */
       if (lookup.name == TYPE_IDENTIFIER (npscope)
 	  || constructor_name_p (lookup.name, npscope))
diff --git a/gcc/testsuite/g++.dg/cpp2a/using-enum-8.C b/gcc/testsuite/g++.dg/cpp2a/using-enum-8.C
new file mode 100644
index 0000000..9a743a9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/using-enum-8.C
@@ -0,0 +1,5 @@
+// PR c++/96462
+// { dg-do compile { target c++11 } }
+
+enum E {};
+using E::~E; // { dg-error "names destructor" }