mergeDeep does not allow interface types

[RedHatter]

mergeDeep does not allow interface types. The following

interface Example {
  b: string
}

const a: Example = { b: 'c' }

R.mergeDeep(a, { d: 'e' })

fails with

Argument of type 'Example' is not assignable to parameter of type 'Record<string, unknown>'.
  Index signature for type 'string' is missing in type 'Example'.

[cjquines]

i think this is microsoft/TypeScript#42825—interfaces are a target for declaration merging and so can't extend Record<string, unknown>

[eranhirsch]

I can see if we can extend the types to support interfaces, too; regardless, take a look at the following explanation of what kind of challenges mergeDeep introduces (here, about Classes):

#317 (comment)