TypeScript Version:

3.6.4

Search Terms:
Partial field conditional matching
3.6.4 conditional partial
Code

export type QueryType = (
  { __typename?: 'TypeOne' } & { a: boolean }
) | (
  { __typename?: 'TypeTwo' } & { a: boolean }
) | (
  { __typename?: 'TypeThree' } & { a: boolean }
);

type PickFromUnion<Union, Typename> =
  Union extends { __typename?: Typename } ? Union : never;

type FirstType = PickFromUnion<QueryType, 'TypeOne'>;

Expected behavior:
FirstType would be of type { __typename?: 'TypeOne' } & { a: boolean }
Actual behavior:
FirstType is of type QueryType

Playground Link:
3.6.3 where it works
http://www.typescriptlang.org/play/?ts=3.6.3#code/KYDwDg9gTgLgBDAnmYcCKBXYVEBVmoC8cAFAFBxwDecA+rUigHYCGAtsAPwBccA5PhQB5JsD5wAvnABk1OC14AjCBAA2wFk0lkAlHAA+pCnPqNgrDj36DguAO4RxU2TQVxlajVom6DRyjSmBBZcvAIEuAAWUMBikjJybh7qmto6ANxkZGZwAAoAlgDGANYAYlAQbACqTPkQTAA8NXVMADRwNiEAfHCExs31cKAw5gAmAM4mDMHsoR0zHPGccANavKIAbtiZ2QRwpflQ4zA2vXlFZRXVtfUNmNh4BO3hwqJ8XelAA

3.7 where it does not work (3.6.4 not provided in playground)
http://www.typescriptlang.org/play/?ts=3.7-Beta#code/KYDwDg9gTgLgBDAnmYcCKBXYVEBVmoC8cAFAFBxwDecA+rUigHYCGAtsAPwBccA5PhQB5JsD5wAvnABk1OC14AjCBAA2wFk0lkAlHAA+pCnPqNgrDj36DguAO4RxU2TQVxlajVom6DRyjSmBBZcvAIEuAAWUMBikjJybh7qmto6ANxkZGZwAAoAlgDGANYAYlAQbACqTPkQTAA8NXVMADRwNiEAfHCExs31cKAw5gAmAM4mDMHsoR0zHPGccANavKIAbtiZ2QRwpflQ4zA2vXlFZRXVtfUNmNh4BO3hwqJ8XelAA