Fix tagged unions multiple processing in submodels

[vadim-su]

Change Summary

In some cases like that:

import enum
from typing import Annotated, Literal

from pydantic import BaseModel, Field


class ComponentType(enum.IntEnum):
        SomeComponent1 = 1
        SomeComponent2 = 2

class SomeComponent1(BaseModel):
    type: Literal[ComponentType.SomeComponent1] = ComponentType.SomeComponent1
    
class SomeComponent2(BaseModel):
    type: Literal[ComponentType.SomeComponent2] = ComponentType.SomeComponent2

Component = Annotated[SomeComponent1|SomeComponent2, Field(discriminator='type')]

class TestModel1(BaseModel):
    components: list[Component]
    
class TestModel2(BaseModel):
    components: list[Component]
    
class TestModel3(BaseModel):
    submodels: TestModel1|TestModel2

apply_discriminators function could process tagged-unions because the tagged-union contains discriminator, but doesn't take that the schema was processed in another model. I excluded tagged-unions from the recursive processing.

I'm not sure about the place of the fix. Possibly a better place for this is _ApplyInferredDiscriminator.apply

pydantic/pydantic/_internal/_discriminated_union.py

Lines 154 to 160 in 15d5e0b

	self.definitions.update(collect_definitions(schema))
	assert not self._used
	schema = self._apply_to_root(schema)
	if self._should_be_nullable and not self._is_nullable:
	schema = core_schema.nullable_schema(schema)
	self._used = True
	return schema

Related issue number

Fix #6339

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)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @lig

[vadim-su]

please review

[dmontagu]

@suharnikov any chance you could add a bit of validation here just to make sure it actually behaves correctly during validation, not just that it stops erroring while building the schema

[adriangb]

Also a json schema check would be good

[samuelcolvin]

We need to start requiring change files.

[vadim-su]

I missed all the fun 😅

[a10y]

@samuelcolvin / @dmontagu Is there going to be a minor release sometime soon with this fix? I use nested discriminated unions pretty extensively in my API so this is currently blocking my upgrade to v2

EDIT: This is now in 2.0.3, thanks all!