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

从链接的 TypeScript Playground 复现开始，并确认两个合并的 PropsContainer 声明产生的诊断。然后跟踪后续属性声明和索引签名的类型检查路径。完成的标准是：所示的合并被接受，specialProp 收窄为 string，并且现有的不兼容重复声明仍被拒绝。