Improve docs and coverage

Author: josevalim

lib/elixir/lib/kernel.ex

@@ -2463,6 +2463,10 @@ defmodule Kernel do
   @doc """
   Returns true if `term` is a struct of `name`; otherwise returns `false`.
 
+  `is_struct/2` does not check that `name` exists and is a valid struct.
+  If you want such validations, you must pattern match on the struct
+  instead, such as `match?(%URI{}, arg)`.
+
   Allowed in guard tests.
 
   ## Examples

lib/elixir/test/elixir/kernel/lexical_tracker_test.exs

@@ -357,6 +357,18 @@ defmodule Kernel.LexicalTrackerTest do
       assert URI in compile
       assert URI in exports
       refute URI in runtime
+
+      {{compile, exports, runtime, _}, _binding} =
+        Code.eval_string("""
+        defmodule Kernel.LexicalTrackerTest.StructPattern do
+          def is_uri(%URI{}), do: true
+          Kernel.LexicalTracker.references(__ENV__.lexical_tracker)
+        end |> elem(3)
+        """)
+
+      refute URI in compile
+      assert URI in exports
+      assert URI in runtime
     end
 
     test "Macro.struct! adds an export dependency" do