Fix byte-compiler crash for legal dynamic-binding code

This should really be taken care of by a syntax normalisation step in
the frontend, but there is no such step for non-lexbind code yet.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-letX): Tolerate bindingsa
without initialising expressions.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.

1 files changed, 12 insertions, 5 deletions

diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index c8a96fa22a9..c8990f23531 100644
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -1367,17 +1367,24 @@ See Info node `(elisp) Integer Basics'."
                              (and (consp binding) (cadr binding)))
                            bindings)
                  ,const)
-       `(let* ,(butlast bindings) ,(cadar (last bindings)) ,const)))
+       `(let* ,(butlast bindings)
+          ,@(and (consp (car (last bindings)))
+                 (cdar (last bindings)))
+          ,const)))
 
     ;; Body is last variable.
-    (`(,head ,bindings ,(and var (pred symbolp) (pred (not keywordp))
-                             (pred (not booleanp))
-                             (guard (eq var (caar (last bindings))))))
+    (`(,head ,(and bindings
+                   (let last-var (let ((last (car (last bindings))))
+                                   (if (consp last) (car last) last))))
+             ,(and last-var             ; non-linear pattern
+                   (pred symbolp) (pred (not keywordp)) (pred (not booleanp))))
      (if (eq head 'let)
          `(progn ,@(mapcar (lambda (binding)
                              (and (consp binding) (cadr binding)))
                            bindings))
-       `(let* ,(butlast bindings) ,(cadar (last bindings)))))
+       `(let* ,(butlast bindings)
+          ,@(and (consp (car (last bindings)))
+                 (cdar (last bindings))))))
 
     (_ form)))