Fix false positive in UnusedFunctionReturnHelper

Refs #1167

Author: rrrene

lib/credo/check/warning/unused_function_return_helper.ex

@@ -230,6 +230,22 @@ defmodule Credo.Check.Warning.UnusedFunctionReturnHelper do
     end
   end
 
+  # anon_fun.()
+  defp verify_candidate(
+         {{:., _, [{fun_name, _, nil}]}, _, arguments} = ast,
+         :not_verified = acc,
+         candidate
+       )
+       when is_atom(fun_name) and is_list(arguments) do
+    # IO.inspect(ast, label: "anon_fun.() (#{Macro.to_string(candidate)} #{acc})")
+
+    if Credo.Code.contains_child?(arguments, candidate) do
+      {nil, :VERIFIED}
+    else
+      {ast, acc}
+    end
+  end
+
   # module.my_fun()
   defp verify_candidate(
          {{:., _, [{module, _, []}, fun_name]}, _, arguments} = ast,
@@ -278,14 +294,6 @@ defmodule Credo.Check.Warning.UnusedFunctionReturnHelper do
     end
   end
 
-  {:., [line: 3, column: 31],
-   [
-     {{:., [line: 3, column: 22],
-       [{:__aliases__, [line: 3, column: 5], [:CredoSampleModule]}, :runner]},
-      [line: 3, column: 23], []},
-     :do_some_function
-   ]}
-
   # module.my_fun()
   defp verify_candidate(
          {{:., _, [{module_variable, _, nil}, fun_name]}, _, arguments} = ast,

test/credo/check/warning/unused_enum_operation_test.exs

@@ -145,6 +145,21 @@ defmodule Credo.Check.Warning.UnusedEnumOperationTest do
     |> refute_issues()
   end
 
+  test "it should NOT report a violation when inside an anon function call" do
+    """
+    defmodule CredoSampleModule do
+      def fun(anon_func) do
+        anon_func.(Enum.random(1..10))
+
+        :ok
+      end
+    end
+    """
+    |> to_source_file
+    |> run_check(@described_check)
+    |> refute_issues()
+  end
+
   test "it should NOT report a violation when inside of assignment" do
     """
     defmodule CredoSampleModule do