src/eval.c: Fix bug#62419

Yup, almost 40 years after ELisp first combined them, buffer-local
and let bindings still don't work quite right :-(

The "automatically buffer-local if set" semantics should follow the
principle that it becomes buffer-local iff the var's current binding
refers to the top-level/global/non-let binding.

* src/eval.c (let_shadows_buffer_binding_p): Disregard non-global
let-bindings.

* test/src/eval-tests.el (eval-test--bug62419): New test.

1 files changed, 19 insertions, 0 deletions

diff --git a/test/src/eval-tests.el b/test/src/eval-tests.el
index 1e7edca3bac..e0a27439ba2 100644
--- a/test/src/eval-tests.el
+++ b/test/src/eval-tests.el
@@ -247,4 +247,23 @@ expressions works for identifiers starting with period."
             (should (equal (string-trim (buffer-string))
                            expected-messages))))))))
 
+(defvar-local eval-test--local-var 'global)
+
+(ert-deftest eval-test--bug62419 ()
+  (with-temp-buffer
+    (setq eval-test--local-var 'first-local)
+    (let ((eval-test--local-var t))
+      (kill-local-variable 'eval-test--local-var)
+      (setq eval-test--local-var 'second-local)
+      (should (eq eval-test--local-var 'second-local)))
+    ;; FIXME: It's not completely clear if exiting the above `let'
+    ;; should restore the buffer-local binding to `first-local'
+    ;; (i.e. reset the value of the second buffer-local binding to the
+    ;; first's initial value) or should do nothing (on the principle that
+    ;; the first buffer-local binding doesn't exists any more so there's
+    ;; nothing to restore).  I think both semantics make sense.
+    ;;(should (eq eval-test--local-var 'first-local))
+    )
+  (should (eq eval-test--local-var 'global)))
+
 ;;; eval-tests.el ends here