bug#5950: defvaralias after defvar should be warned in runtime

[Noam Postavsky]

[forwarding to list, original didn't make it because bug was archived]

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> Subject: 
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> Re: bug#5950: defvaralias after defvar should be warned in runtime
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> Date: 
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> Wed, 1 Aug 2018 14:55:56 -0400
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> Hi Noam,
>
> I just updated my Emacs and ran into this warning, but I have no idea
> what's wrong with the following code :)
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>   (defvaralias 'flycheck-python-pylint-executable 'python-shell-interpreter)
>   (defvaralias 'flycheck-python-flake8-executable 'python-shell-interpreter)
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> These two variables `flycheck-&-executable' are configuration knobs
> created by Flycheck, and I never want to set them individually; instead,
> I want them to have the same value as `python-shell-interpreter'.  While
> the code above works, it also pops a "warning" buffer now:
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> Warning (defvaralias): Overwriting value of ‘flycheck-python-pylint-executable’ 
> by aliasing to ‘python-shell-interpreter’
> Warning (defvaralias): Overwriting value of ‘flycheck-python-flake8-executable’ 
> by aliasing to ‘python-shell-interpreter’
>
> I've set warning-suppress-log-types to '(defvaralias losing-value) to
> work around this, but was the warning actually intended in this case?
>
> Clément.
>
> --- End Message ---

It's warning because you overwrite the original value of
flycheck-python-pylint-executable when you call defvaralias.  In this
case you intended that, but usually it's a mistake.

You could make the warning suppression more selective by adding
(defvaralias losing-value flycheck-python-pylint-executable) and
(defvaralias losing-value flycheck-python-flake8-executable) to
warning-suppress-log-types.

Alternatively, you could do

  (setq flycheck-python-pylint-executable python-shell-interpreter)
  (defvaralias 'flycheck-python-pylint-executable 'python-shell-interpreter)
  (setq flycheck-python-flake8-executable python-shell-interpreter)
  (defvaralias 'flycheck-python-flake8-executable 'python-shell-interpreter)

(the warning doesn't trigger if the value being overwritten is `eq' to
the new one).