Fixes discriminated unions not working on aliased literal fields. Issue

#3849.

pydantic/fields.py

@@ -1107,7 +1107,10 @@ def _validate_discriminated_union(
         assert self.discriminator_alias is not None
 
         try:
-            discriminator_value = v[self.discriminator_alias]
+            try:
+                discriminator_value = v[self.discriminator_alias]
+            except KeyError:
+                discriminator_value = v[self.discriminator_key]
         except KeyError:
             return v, ErrorWrapper(MissingDiscriminator(discriminator_key=self.discriminator_key), loc)
         except TypeError:

tests/test_discrimated_union.py

@@ -85,6 +85,39 @@ class Pet(BaseModel):
             __root__: Union[Cat, Dog] = Field(..., discriminator='pet_type')
 
 
+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"}})
+
+    assert isinstance(m.pet, Cat)
+
+
 def test_discriminated_union_validation():
     class BlackCat(BaseModel):
         pet_type: Literal['cat']