Warn about unmatchable constant args to `eq`, `memq` etc

Add a byte-compiler warning about attempts to compare literal values
with undefined identity relation to other values.  For example:

  (eq x 2.0)
  (memq x '("a" (b) [c]))

Such incomparable values include all literal conses, strings, vectors,
records and (except for eql and memql) floats and bignums.
The warning currently applies to eq, eql, memq, memql, assq, rassq,
remq and delq.

* lisp/emacs-lisp/bytecomp.el (bytecomp--dodgy-eq-arg)
(bytecomp--value-type-description, bytecomp--arg-type-description)
(bytecomp--warn-dodgy-eq-arg, bytecomp--check-eq-args)
(bytecomp--check-memq-args): New.
(eq, eql, memq, memql, assq, rassq, remq, delq):
Set compiler-macro property.
* lisp/emacs-lisp/byte-run.el (with-suppressed-warnings):
Amend doc string.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp--with-warning-test): Fix text-quoting-style and expand
re-warning so that it doesn't need to be a literal.
(bytecomp-warn-dodgy-args-eq, bytecomp-warn-dodgy-args-memq):
New tests.

1 files changed, 2 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/byte-run.el b/lisp/emacs-lisp/byte-run.el
index 1babf3ec2c4..b5e887db836 100644
--- a/lisp/emacs-lisp/byte-run.el
+++ b/lisp/emacs-lisp/byte-run.el
@@ -653,7 +653,8 @@ types.  The types that can be suppressed with this macro are
 `suspicious'.
 
 For the `mapcar' case, only the `mapcar' function can be used in
-the symbol list.  For `suspicious', only `set-buffer' and `lsh' can be used."
+the symbol list.  For `suspicious', only `set-buffer', `lsh' and `eq'
+can be used."
   ;; Note: during compilation, this definition is overridden by the one in
   ;; byte-compile-initial-macro-environment.
   (declare (debug (sexp body)) (indent 1))