Raise clearer error when reserved word used at the top level of defmodule (#15123)

Author: sabiwara

lib/elixir/lib/kernel.ex

@@ -5220,6 +5220,11 @@ defmodule Kernel do
     end
   end
 
+  defmacro defmodule(alias, [{:do, _block}, {atom, _} | _]) when is_atom(atom) do
+    raise ArgumentError,
+          "unexpected reserved word at the top-level of the \"defmodule #{Macro.to_string(alias)}\" do-block: #{atom}"
+  end
+
   defp module_meta({_, meta, _}), do: meta
   defp module_meta(_), do: []
 

lib/elixir/test/elixir/kernel/errors_test.exs

@@ -1003,6 +1003,18 @@ defmodule Kernel.ErrorsTest do
     )
   end
 
+  test "reserved word used at module top-level" do
+    assert_eval_raise(
+      ArgumentError,
+      ["unexpected reserved word at the top-level of the \"defmodule Foo\" do-block: catch"],
+      """
+      defmodule Foo do
+        def foo, do: :foo catch :bar
+      end
+      """
+    )
+  end
+
   defp bad_remote_call(x), do: x.foo()
 
   defmacro sample(0), do: 0