c++: Fix ICE after ambiguous inline namespace reopen [PR94257]

Following DR2061, 'namespace F', looks for 'F's inside inline namespaces.
That can result in ambiguous lookups that we failed to diagnose early enough,
leading us to push a new namespace and ICE later.  Diagnose the ambiguity
earlier, and then pick one.

	PR c++/94257
	* name-lookup.c (push_namespace): Triage ambiguous lookups that
	contain namespaces.

gcc/cp/ChangeLog

+2020-03-27  Nathan Sidwell  <nathan@acm.org>
+
+	PR c++/94257
+	* name-lookup.c (push_namespace): Triage ambiguous lookups that
+	contain namespaces.
+
 2020-03-27  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/94326

gcc/cp/name-lookup.c

@@ -7372,23 +7372,59 @@ push_namespace (tree name, bool make_inline)
     name_lookup lookup (name, 0);
     if (!lookup.search_qualified (current_namespace, /*usings=*/false))
       ;
-    else if (TREE_CODE (lookup.value) != NAMESPACE_DECL)
-      ;
-    else if (tree dna = DECL_NAMESPACE_ALIAS (lookup.value))
+    else if (TREE_CODE (lookup.value) == TREE_LIST)
       {
-	/* A namespace alias is not allowed here, but if the alias
-	   is for a namespace also inside the current scope,
-	   accept it with a diagnostic.  That's better than dying
-	   horribly.  */
-	if (is_nested_namespace (current_namespace, CP_DECL_CONTEXT (dna)))
+	/* An ambiguous lookup.  If exactly one is a namespace, we
+	   want that.  If more than one is a namespace, error, but
+	   pick one of them.  */
+	/* DR2061 can cause us to find multiple namespaces of the same
+	   name.  We must treat that carefully and avoid thinking we
+	   need to push a new (possibly) duplicate namespace.  Hey,
+	   if you want to use the same identifier within an inline
+	   nest, knock yourself out.  */
+	for (tree *chain = &lookup.value, next; (next = *chain);)
+	  {
+	    tree decl = TREE_VALUE (next);
+	    if (TREE_CODE (decl) == NAMESPACE_DECL)
+	      {
+		if (!ns)
+		  ns = decl;
+		else if (SCOPE_DEPTH (ns) >= SCOPE_DEPTH (decl))
+		  ns = decl;
+
+		/* Advance.  */
+		chain = &TREE_CHAIN (next);
+	      }
+	    else
+	      /* Stitch out.  */
+	      *chain = TREE_CHAIN (next);
+	  }
+
+	if (TREE_CHAIN (lookup.value))
 	  {
-	    error ("namespace alias %qD not allowed here, "
-		   "assuming %qD", lookup.value, dna);
-	    ns = dna;
+	    error ("%<namespace %E%> is ambiguous", name);
+	    print_candidates (lookup.value);
 	  }
       }
-    else
+    else if (TREE_CODE (lookup.value) == NAMESPACE_DECL)
       ns = lookup.value;
+
+    if (ns)
+      if (tree dna = DECL_NAMESPACE_ALIAS (ns))
+	{
+	  /* A namespace alias is not allowed here, but if the alias
+	     is for a namespace also inside the current scope,
+	     accept it with a diagnostic.  That's better than dying
+	     horribly.  */
+	  if (is_nested_namespace (current_namespace, CP_DECL_CONTEXT (dna)))
+	    {
+	      error ("namespace alias %qD not allowed here, "
+		     "assuming %qD", ns, dna);
+	      ns = dna;
+	    }
+	  else
+	    ns = NULL_TREE;
+	}
   }
 
   bool new_ns = false;