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 ?

Inizia con il TypeScript Playground collegato e il repro fornito per enum, mapped type e Partial, quindi confronta il comportamento segnalato tra le versioni di TypeScript elencate. Il lavoro è completato quando l’esempio viene compilato come previsto se il mapped type viene costruito dall’unione di enums, con una copertura di regressione per il caso di un membro enum condiviso.