Warn when invalid fun typespec is used

Author: josevalim

lib/elixir/lib/kernel/typespec.ex

@@ -877,7 +877,15 @@ defmodule Kernel.Typespec do
 
   defp typespec({:fun, meta, args}, vars, caller, state) do
     {args, state} = :lists.mapfoldl(&typespec(&1, vars, caller, &2), state, args)
-    {{:type, location(meta), :fun, args}, state}
+
+    if args != [] do
+      IO.warn(
+        "fun/#{length(args)} is not valid in typespecs. Either specify fun() or use (... -> return) instead",
+        caller
+      )
+    end
+
+    {{:type, location(meta), :fun, []}, state}
   end
 
   defp typespec({:..., _meta, _args}, _vars, caller, _state) do

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -1818,6 +1818,20 @@ defmodule Kernel.WarningTest do
       purge([Sample1, Sample2, Sample3])
     end
 
+    test "invalid fun" do
+      assert_warn_eval(
+        [
+          "nofile:2: ",
+          "fun/1 is not valid in typespecs. Either specify fun() or use (... -> return) instead"
+        ],
+        """
+        defmodule InvalidFunType do
+          @type my_type :: fun(integer())
+        end
+        """
+      )
+    end
+
     test "invalid type annotations" do
       assert_warn_eval(
         [