`assert_changes` should always assert some change

While using `assert_changes`, I came across some unexpected behavior:
if you provide a `to:` argument, and the expression matches but didn't
actually change, the assertion will pass.

The way `assert_changes` reads, I assumed that it would both assert
that there was any change at all, _and_ that the expression changed to
match my `to:` argument.

In the case of just a `from:` argument, `assert_changes` does what I
expect as well. It asserts that the before value `=== from` and that
the after value changed.

My key change is that `assert_changes` will now _always_ assert that
expression changes, no matter what combination of `from:` and `to:`
arguments

1 files changed, 11 insertions, 0 deletions

diff --git a/activesupport/test/test_case_test.rb b/activesupport/test/test_case_test.rb
index 84e4953fe2..79b75a001a 100644
--- a/activesupport/test/test_case_test.rb
+++ b/activesupport/test/test_case_test.rb
@@ -156,6 +156,16 @@ class AssertDifferenceTest < ActiveSupport::TestCase
     end
   end
 
+  def test_assert_changes_with_to_option_but_no_change_has_special_message
+    error = assert_raises Minitest::Assertion do
+      assert_changes "@object.num", to: 0 do
+        # no changes
+      end
+    end
+
+    assert_equal "\"@object.num\" didn't change. It was already 0", error.message
+  end
+
   def test_assert_changes_with_wrong_to_option
     assert_raises Minitest::Assertion do
       assert_changes "@object.num", to: 2 do
@@ -218,6 +228,7 @@ class AssertDifferenceTest < ActiveSupport::TestCase
   def test_assert_changes_with_message
     error = assert_raises Minitest::Assertion do
       assert_changes "@object.num", "@object.num should 1", to: 1 do
+        @object.decrement
       end
     end