Edebug: Disable backtracking when hitting a &define keyword.

Edebug doesn't deal well with backtracking out of definitions, see
Bug#41988.  Rather than trying to support this rare situation (e.g. by
implementing a multipass parser), prevent it by adding an implicit
gate.

* lisp/emacs-lisp/edebug.el (edebug--match-&-spec-op): Disable
backtracking when hitting a &define keyword.

* test/lisp/emacs-lisp/edebug-tests.el
(edebug-tests-duplicate-&define): New unit test.
(edebug-tests--duplicate-&define): New helper macro.

* doc/lispref/edebug.texi (Backtracking): Mention &define in the list
of constructs that disable backtracking.

* etc/NEWS: Document new behavior.

1 files changed, 5 insertions, 5 deletions

diff --git a/doc/lispref/edebug.texi b/doc/lispref/edebug.texi
index 8942f55affb..323130f2378 100644
--- a/doc/lispref/edebug.texi
+++ b/doc/lispref/edebug.texi
@@ -1510,11 +1510,11 @@ form specifications (that is, @code{form}, @code{body}, @code{def-form}, and
 must be in the form itself rather than at a higher level.
 
 Backtracking is also disabled after successfully matching a quoted
-symbol or string specification, since this usually indicates a
-recognized construct.  But if you have a set of alternative constructs that
-all begin with the same symbol, you can usually work around this
-constraint by factoring the symbol out of the alternatives, e.g.,
-@code{["foo" &or [first case] [second case] ...]}.
+symbol, string specification, or @code{&define} keyword, since this
+usually indicates a recognized construct.  But if you have a set of
+alternative constructs that all begin with the same symbol, you can
+usually work around this constraint by factoring the symbol out of the
+alternatives, e.g., @code{["foo" &or [first case] [second case] ...]}.
 
 Most needs are satisfied by these two ways that backtracking is
 automatically disabled, but occasionally it is useful to explicitly