failure to parse params should trigger a 400 Bad Request

actionpack/lib/action_dispatch/middleware/exception_wrapper.rb

@@ -12,6 +12,7 @@ class ExceptionWrapper
       'ActionController::NotImplemented'           => :not_implemented,
       'ActionController::UnknownFormat'            => :not_acceptable,
       'ActionController::InvalidAuthenticityToken' => :unprocessable_entity,
+      'ActionDispatch::ParamsParser::ParseError'   => :bad_request,
       'ActionController::BadRequest'               => :bad_request,
       'ActionController::ParameterMissing'         => :bad_request
     )

actionpack/test/dispatch/show_exceptions_test.rb

@@ -8,6 +8,8 @@ def call(env)
       case req.path
       when "/not_found"
         raise AbstractController::ActionNotFound
+      when "/bad_params"
+        raise ActionDispatch::ParamsParser::ParseError.new("", StandardError.new)
       when "/method_not_allowed"
         raise ActionController::MethodNotAllowed
       when "/not_found_original_exception"
@@ -33,6 +35,10 @@ def call(env)
     get "/", {}, {'action_dispatch.show_exceptions' => true}
     assert_response 500
     assert_equal "500 error fixture\n", body
+    
+    get "/bad_params", {}, {'action_dispatch.show_exceptions' => true}
+    assert_response 400
+    assert_equal "400 error fixture\n", body
 
     get "/not_found", {}, {'action_dispatch.show_exceptions' => true}
     assert_response 404

actionpack/test/fixtures/public/400.html

+400 error fixture