Fix const validator not running when class validators are present

[hmvp]

Change Summary

Fix issue where const validator was not used

Related issue number

closes #1956

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1957 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1957   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3909      3909           
  Branches       788       788           
=========================================
  Hits          3909      3909           

Impacted Files	Coverage Δ
pydantic/fields.py	100.00% <100.00%> (ø)

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[hmvp]

@samuelcolvin If you find the time, would you consider reviewing and merging this bugfix?

[samuelcolvin]

This looks fine to me, but I'm worried there must have been a good reason the += was omitted.

Is there any case you can think of that this could break?

[hmvp]

Well it breaks code that relies on const validators not running when custom validators are present which I find odd behaviour to want.. The added test is an example of that.

BTW in 1.5.x this did work and the added test would not fail (that is why I found the issue in the first place, when upgrading to 1.6.1 and having a test fail)

I suspect the plus was left out because this code moved around a bit and that line use to be the first place the post_validators where filled..

[samuelcolvin]

makes sense, thank you.