Improve error message on invalid map definition, closes #4068

Author: José Valim

lib/elixir/lib/kernel/typespec.ex

@@ -722,10 +722,16 @@ defmodule Kernel.Typespec do
     {:type, line(meta), :map, :any}
   end
 
-  defp typespec({:%{}, meta, fields}, vars, caller) do
+  defp typespec({:%{}, meta, fields} = map, vars, caller) do
     fields =
-      :lists.map(fn {k, v} ->
-        {:type, line(meta), :map_field_assoc, [typespec(k, vars, caller), typespec(v, vars, caller)]}
+      :lists.map(fn
+        {k, v} ->
+          {:type, line(meta), :map_field_assoc, [typespec(k, vars, caller), typespec(v, vars, caller)]}
+        {:|, _, [_, _]} ->
+          compile_error(caller, "invalid map specification. When using the | operator in the map key, " <>
+                                "make sure to wrap the key type in parentheses: #{Macro.to_string(map)}")
+        _ ->
+          compile_error(caller, "invalid map specification: #{Macro.to_string(map)}")
       end, fields)
 
     {:type, line(meta), :map, fields}

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

@@ -889,7 +889,6 @@ defmodule Kernel.ErrorsTest do
       '''
 
     message = "nofile:2: spec for undefined function omg/0"
-
     assert_compile_fail CompileError, message,
       '''
       defmodule Kernel.ErrorsTest.TypespecErrors2 do

lib/elixir/test/elixir/kernel/typespec_test.exs

@@ -281,6 +281,14 @@ defmodule Kernel.TypespecTest do
     end
   end
 
+  test "@type with an invalid map notation" do
+    assert_raise CompileError, ~r"invalid map specification", fn ->
+      test_module do
+        @type content :: %{atom | String.t => term}
+      end
+    end
+  end
+
   test "@type with list shortcuts" do
     module = test_module do
       @type mytype :: []
@@ -662,7 +670,7 @@ defmodule Kernel.TypespecTest do
     end
   end
 
-  test "@spec gives a nice error message when return type is missing" do
+  test "@spec shows readable error message when return type is missing" do
     assert_raise CompileError, ~r"type specification missing return type: myfun\(integer\)", fn ->
       test_module do
         @spec myfun(integer)