Interface merging for any property names does not allow type narrowing

TypeScript Version: 4.1.0-dev.20201002

Search Terms: interface merging merge object any name subsequent property declarations

Code

interface PropsContainer {
  props: {
    [propName: string]: any;
  }
}

interface PropsContainer {
  props: {
    [propName: string]: any;
    specialProp: string;
  }
}

Expected behavior:
This should be allowed. The merged interface should result in a type with props that have any prop name as any except for specialProp which is a string.

This would be useful in a case where a third party library (such as @types/react-test-renderer) declare an object type with { [propName: string]: any } syntax, but you want to narrow the value of the property for specific types via interface merging.

Note that even { specialProp?: any, [propName: string]: any } is not allowed even though the types are functionally identical in this case.

Actual behavior:
This results in a Subsequent property declarations must have the same type.

Playground Link: https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgApQPYAcDOBhDcOUaZAbwChlktNcAucq65AbVuwDk4BbCRnGCigA5gF1GcEAE8A3MwC+FJRVCRYiFOmz5CYYiFKVqHBkxbIcWCAmBwANtqwB+STPkX2dbnwFDREshScorKQA

リンクされた TypeScript Playground の再現コードから始め、マージされた 2 つの PropsContainer 宣言について診断を確認します。次に、後続のプロパティ宣言とインデックスシグネチャに対する型チェックの経路を追跡します。示されたマージが受け入れられ、specialProp が string に絞り込まれ、既存の互換性のない再宣言が引き続き拒否されれば完了です。