bug#54747: 28.1.50; package.el - Better error message if newer Emacs is required

[Daniel Mendler]

On 4/7/22 13:46, Lars Ingebrigtsen wrote:
> I think it makes sense to make this error message better -- people may
> be calling `package-install' explicitly.
> 
> It'd be easier to debug this if I had a recipe for reproduction, though.
> Do you have one handy?

Hello Lars,

unfortunately I don't have a good recipe at hand, but you can probably
reproduce this with Emacs 26 when you try to install one of the newer
packages, e.g., my vertico package from GNU ELPA.

I am running Emacs 28 and I just went to `list-packages` and did `M-x
package-install` on a package marked as "incompat". For some weird
reason Emacs still installed the package while I would have expected an
error now? Is this another bug?

Regarding the error messages - I think even experienced users could
profit from slightly improved error messages which explain dependency
resolution failure.

- Package X version V requires package Y version W, which is
unavailable. The newest available version of Y is W'.
- Package X version V requires package Y, which is unavailable. The
package Y cannot be found in the archives.
- Package X version V requires package Y version W, which cannot be
installed. The dependency Z of Y cannot be satisfied.
- ...

Daniel