Params.fetch covers up any KeyError raised in block.

test/parameters_basic_test.rb

@@ -0,0 +1,12 @@
+require 'test_helper'
+require 'action_controller/parameters'
+
+class BasicParametersTest < ActiveSupport::TestCase
+  test 'KeyError in fetch block should not be coverd up' do
+    params = ActionController::Parameters.new()
+    err = assert_raises(KeyError){
+      params.fetch(:missing_key){ raise(KeyError, "key not found: :also_missing") }
+    }
+    assert_match /also_missing/, err.message
+  end
+end