Fixes discriminated unions not working on aliased literal fields. Issue #3849.

[benwah]

Change Summary

The issue is described here: #3849

TL;DR:

When you have an aliased field, and it uses a discriminated union; you can no longer use the un-aliased field name to instantiate the model.

According to the discussion, this is not going to be a problem once Pydantic 2.x releases, but it's definitly problematic for 1.10.x which I'm currently using.

Here's the test failure without this change:

============================================================== FAILURES ==============================================================
_________________________________________ test_discriminated_union_validation_aliased_field __________________________________________

    def test_discriminated_union_validation_aliased_field():
        def to_upper(field_name: str) -> str:
            return field_name.upper()
    
    
        class PyBase(BaseModel):
            class Config:
                allow_population_by_field_name = True
                alias_generator = to_upper
    
    
        class Dog(PyBase):
            pet_type: Literal["dog"]
            first_name: str
    
    
        class Cat(PyBase):
            pet_type: Literal["cat"]
            first_name: str
    
    
        Pet = Annotated[Cat | Dog, Field(discriminator="pet_type")]
    
    
        class Model(PyBase):
            pet: Pet
    
    
>       m = Model.parse_obj({'pet': {'pet_type': 'cat', "first_name": "Benoit"}})

tests/test_discrimated_union.py:116: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pydantic/main.py:526: in parse_obj
    return cls(**obj)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

__pydantic_self__ = Model(), data = {'pet': {'first_name': 'Benoit', 'pet_type': 'cat'}}, values = {}, fields_set = {'pet'}
validation_error = ValidationError(model='Model', errors=[{'loc': ('PET',), 'msg': "Discriminator 'pet_type' is missing in value", 'type': 'value_error.discriminated_union.missing_discriminator', 'ctx': {'discriminator_key': 'pet_type'}}])

    def __init__(__pydantic_self__, **data: Any) -> None:
        """
        Create a new model by parsing and validating input data from keyword arguments.
    
        Raises ValidationError if the input data cannot be parsed to form a valid model.
        """
        # Uses something other than `self` the first arg to allow "self" as a settable attribute
        values, fields_set, validation_error = validate_model(__pydantic_self__.__class__, data)
        if validation_error:
>           raise validation_error
E           pydantic.error_wrappers.ValidationError: 1 validation error for Model
E           PET
E             Discriminator 'pet_type' is missing in value (type=value_error.discriminated_union.missing_discriminator; discriminator_key=pet_type)

pydantic/main.py:341: ValidationError
====================================================== short test summary info =======================================================
FAILED tests/test_discrimated_union.py::test_discriminated_union_validation_aliased_field - pydantic.error_wrappers.ValidationError...
========================================================= 1 failed in 0.17s ==========================================================

Related issue number

Fix #3849

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: @PrettyWood

[benwah]

please review

[benwah]

Hypothesis test suite is failing in my local tests, with and without these changes. So I don't think this is a regression.