Improve UndefinedFunctionError for unqualified module

lib/elixir/lib/exception.ex

@@ -1345,17 +1345,25 @@ defmodule UndefinedFunctionError 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
+    candidates =
+      Enum.filter(:code.all_available(), fn {name, _, _} = candidate ->
+        if downcase_module_name(name) == downcased_module or
+             String.ends_with?(to_string(name), strip_elixir_prefix(module)) do
+          with {:module, module} <- load_module(candidate),
+               true <- function_exported?(module, function, arity) do
+            true
+          else
+            _ -> false
+          end
+        else
+          false
+        end
       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"
+    if candidates != [] do
+      ". Did you mean:\n#{Enum.map(candidates, fn {module, _, _} -> "\n      * #{Exception.format_mfa(String.to_atom(to_string(module)), function, arity)}" end)}\n"
     else
-      _ -> ""
+      ""
     end
   end
 
@@ -1365,6 +1373,12 @@ defmodule UndefinedFunctionError do
     |> Code.ensure_loaded()
   end
 
+  defp strip_elixir_prefix(module) do
+    module
+    |> to_string()
+    |> String.replace_leading("Elixir.", "")
+  end
+
   defp downcase_module_name(module) do
     module
     |> to_string()

lib/elixir/test/elixir/exception_test.exs

@@ -596,6 +596,25 @@ defmodule ExceptionTest do
     end
 
     test "annotates undefined function error with module suggestions" do
+      import PathHelpers
+
+      modules = [
+        Namespace.A.One,
+        Namespace.A.Two,
+        Namespace.A.Three,
+        Namespace.B.One,
+        Namespace.B.Two,
+        Namespace.B.Three
+      ]
+
+      for module <- modules do
+        write_beam(
+          defmodule module do
+            def foo, do: :bar
+          end
+        )
+      end
+
       assert blame_message(ENUM, & &1.map(&1, 1)) == """
              function ENUM.map/2 is undefined (module ENUM is not available). Did you mean:
 
@@ -604,6 +623,18 @@ defmodule ExceptionTest do
 
       assert blame_message(ENUM, & &1.not_a_function(&1, 1)) ==
                "function ENUM.not_a_function/2 is undefined (module ENUM is not available)"
+
+      assert blame_message(One, & &1.foo()) == """
+             function One.foo/0 is undefined (module One is not available). Did you mean:
+
+                   * Namespace.A.One.foo/0
+                   * Namespace.B.One.foo/0
+             """
+
+      for module <- modules do
+        :code.delete(module)
+        :code.purge(module)
+      end
     end
 
     test "annotates undefined function clause error with macro hints" do