Change Summary

Extended optionvalidate_default so that it's possible to validate default values when a model schema is being built.

This option now can be a boolean or 3 possible string values:'never','definition', or'init'.

• never: no validation for default values at all. This is the same asFalse.
• init: validation at model instantiation. This is the same asTrue.
• definition: validation at model definition.

The behaviour ofTrue/False does not change with this PR.

Note that even thoughinit anddefinition are not mutually exclusive and I implemented these as flags that can co-exist, I decided not to add another option, e.g.always, that cover both flags because if a validation is raised at model definition, it's pretty much impossible to trigger such error at model instantiation because the model does not built at all and therefore we don't really needalways.

Related issue number

Part ofpydantic/pydantic#8722

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with thispydantic-core (except for expected changes)
• My PR is ready to review,please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer:@davidhewitt