Subject: 26.0.50; invalid-read-syntax error using Edebug on lisp/emacs-lisp/pcase.el

From: Gemini Lasswell <[email-masked]>
Date: Mon, 17 Oct 2016 08:44:58 -0700

To reproduce, start at the top of your Emacs source tree and:
1. emacs -Q
2. M-x edebug-all-defs RET
3. C-x C-f lisp/emacs-lisp/pcase.el
4. M-x eval-buffer RET

The reason this is happening is that in this line:

(def-edebug-spec pcase-MACRO pcase--edebug-match-macro)

pcase--edebug-match-macro does not have an Edebug specification, so the
Edebug specification for def-edebug-spec fails to match.

But when you aren't trying to debug pcase.el and are instead trying to
debug something that uses pcase, this Edebug specification works,
because internally to Edebug when it is trying to match an Edebug
specification, if it finds a symbol without a specification it assumes
it is one of Edebug's match functions and calls it. This is how Edebug
implements the behavior for the symbols used in Edebug specifications.
And pcase--edebug-match-macro works like one of Edebug's match
functions.

I think it's a rather poor idea for pcase.el to depend on internal
implementation details of Edebug. But since it does, replacing the line
of code above with:

(put 'pcase-MACRO 'edebug-form-spec 'pcase--edebug-match-macro)

would make it more obvious that it's using internal implementation
details of Edebug, as well as fixing the syntax error.