Partial of a mapped type from a union of enums fails when these enums have common members

TypeScript Version: 4.0.5, 4.1.2, 4.2.0-dev.20201126

Works fine with TS 3.X

Search Terms: mapped type enum partial

Code

enum EnumA {
    A = 'A',
    B = 'B',
}

// A second enum with at least one key also in EnumA
enum EnumB {
    B = 'B',
    C = 'C',
}

type Mapped = {
    [k in EnumA|EnumB]: string;
};

const partial: Partial<Mapped> = {
    [EnumA.B]: 'value',
};

Expected behavior:
Should compile

Actual behavior:
Typescript complains that EnumA.B does not exist in Partial<Mapped> when declaring the partial variable:

Type '{ B: string; }' is not assignable to type 'Partial<Mapped>'.
  Object literal may only specify known properties, and '[EnumA.B]' does not exist in type 'Partial<Mapped>'.

Playground Link: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201126#code/KYOwrgtgBAouEEEoG8BQUNSQXigcgTwBp1MAhKXPM41AX1VVElngrUygqppM4GFK+frQaoALgE8ADsCgBZAIbTZAEyEdMAbQDWUAJYhWkBAB84kMgF0AXFADO4gE6GA5gG56n1AGMA9iCOUADu+uIAFn5g4vx+EBABAGqKADZgwHYACopO4vqpADxKKsCqAHwapBhaFogAdAi2+LScNWx1-E14ot7+geJQ-vFJqekAYk5xCFk5eYXFahW4mtW1CHXWdt0kdL0BQUMJIMlpwBNxZDO5+SlFyouVrbVkG109jH1BoRFR4tnXqTsC1Kj20awaXXWhD4YPg602+HhLVhlleWxevCqUDaqM66I67yAA

Related Issues: maybe https://github.com/microsoft/TypeScript/issues/37859 ?

Commencez par le TypeScript Playground lié et le repro fourni pour enum, mapped type et Partial, puis comparez le comportement signalé entre les versions de TypeScript indiquées. C’est terminé lorsque l’exemple se compile comme prévu quand le mapped type est construit à partir de l’union d’enums, avec une couverture de régression pour le cas d’un membre d’enum partagé.