Re: Compiled code in Emacs-26 will fail in Emacs-25 if use pcase [was: Add new bytecode op `switch' for implementing branch tables.]

[Stefan Monnier]

> I) Compile under master branch.
> II) Load the resultant .elc with Emacs-25

As explained by Richard, this is usually considered acceptable.
We don't really try to provide forward compatibility of byte-code files
between major versions.

This said, occasionally we try to reduce the pain a little: e.g. in
Emacs-25 a similar incompatibility was introduced for the compilation of
catch&condition-case, but the bytecodes were introduced a bit earlier.
IOW the incompatibility was fundamentally introduced in 24.4 (IIRC), but
the use of the this new feature was only enabled in Emacs-25.1, so files
compiled with 25.1 will usually work in 24.5 as well.


        Stefan