Improve UndefinedFunctionError for mis-cased module (#12839)

Author: davydog187

lib/elixir/lib/exception.ex

@@ -1342,8 +1342,33 @@ defmodule UndefinedFunctionError do
       hint_for_loaded_module(module, function, arity, nil)
   end
 
-  defp hint(_module, _function, _arity, _loaded?) do
-    ""
+  defp hint(module, function, arity, _loaded?) do
+    downcased_module = downcase_module_name(module)
+
+    candidate =
+      Enum.find(:code.all_available(), fn {name, _, _} ->
+        downcase_module_name(name) == downcased_module
+      end)
+
+    with {_, _, _} <- candidate,
+         {:module, module} <- load_module(candidate),
+         true <- function_exported?(module, function, arity) do
+      ". Did you mean:\n\n      * #{Exception.format_mfa(module, function, arity)}\n"
+    else
+      _ -> ""
+    end
+  end
+
+  defp load_module({name, _path, _loaded?}) do
+    name
+    |> List.to_atom()
+    |> Code.ensure_loaded()
+  end
+
+  defp downcase_module_name(module) do
+    module
+    |> to_string()
+    |> String.downcase(:ascii)
   end
 
   @doc false

lib/elixir/test/elixir/exception_test.exs

@@ -595,6 +595,17 @@ defmodule ExceptionTest do
       assert message =~ "* set_cookie/2"
     end
 
+    test "annotates undefined function error with module suggestions" do
+      assert blame_message(ENUM, & &1.map(&1, 1)) == """
+             function ENUM.map/2 is undefined (module ENUM is not available). Did you mean:
+
+                   * Enum.map/2
+             """
+
+      assert blame_message(ENUM, & &1.not_a_function(&1, 1)) ==
+               "function ENUM.not_a_function/2 is undefined (module ENUM is not available)"
+    end
+
     test "annotates undefined function clause error with macro hints" do
       assert blame_message(Integer, & &1.is_odd(1)) ==
                "function Integer.is_odd/1 is undefined or private. However, there is " <>