Cannot assign property to same type with generic key

TypeScript Version: 3.6.0-dev.20190803

Search Terms: assigning keyof generic key

Code

type A = { a: number }
type B = { b: number }

interface Foo {
    a: A
    b: B
}

declare let target: Foo
declare let source: Foo
declare let key: keyof Foo

target[key] = source[key]

Expected behavior:

I expect it to allow the assignment. Since typeof target and typeof source are the same, I expect typeof target[key] to always be the same as typeof source[key].

Actual behavior:

error TS2322: Type 'A | B' is not assignable to type 'A & B'.

Playground Link: https://www.typescriptlang.org/play/#code/C4TwDgpgBAglC8UDeUCGAuKA7ArgWwCMIAnKAXwChRIoAhBZKAzXQk8iigSy2BIDNUAY2gAxAPbjkFKLLSYYMuczoVKFACYQhAG1TFoOiMCjB9Ac2OYJ4zdr0GoRkwGdxOYiOuS7u-YeMoAGsIEEwQkHF+KBtOM2JLYABtCIBdBjcPERTQ1IogA

Related Issues: #31665 (but it was closed and the comments seem to indicate that this should work 🤔) ping @RyanCavanaugh

Comienza con el ejemplo mínimo de TypeScript del issue y reproduce el diagnóstico TS2322 en el Playground enlazado. Lee el issue relacionado #31665 e investiga el recorrido de comprobación de tipos para la asignación indexada con una clave genérica keyof. Se considera terminado cuando se acepta la asignación mostrada, manteniendo la seguridad de tipos para tipos de origen y destino diferentes.