bug#24402: More Information

[Gemini Lasswell]

I’ve done some investigating of why this is happening. testcover-start
transforms:
    (should-error (my-error))
into:
    (should-error (testcover-after 2 (my-error)))

Then the macro expansion of should-error separates the form which it
is passed into a function symbol and list of arguments, and applies
the function to the arguments inside of a condition-case that traps
errors. The problem is that the arguments are evaluated first, outside
of the condition-case, so errors in their evaluation do not get
caught. This problem is not specific to testcover. In this example,
the first test passes and the second fails:

(defun div-by (n)
  (/ 100 n))

(defmacro log-div-by (n)
  `(message "div-by: %d" (div-by ,n)))

(ert-deftest test-div-by ()
  (should-error (div-by 0)))

(ert-deftest test-log-div-by ()
  (should-error (log-div-by 0)))