Nested structure without index signature is not assignable to `Jsonifiable`

[bleistift-zwei]

Consider this simple nested data structure:

interface Inner {
  foo: string
}

interface Outer {
  bar: Inner
}

A value of type Outer can be losslessly converted to and from JSON. However, assigning it to a variable of type Jsonifiable won’t compile with TS 5.5.2 and type-fest@4.20.1 Playground link:

declare const outer: Outer
let jsonifiable: Jsonifiable = outer
//  ^ Type 'Outer' is not assignable to type 'Jsonifiable'.
//      Type 'Outer' is not assignable to type '{ [x: string]: Jsonifiable | undefined; }'.
//        Index signature for type 'string' is missing in type 'Outer'.(2322)

I believe the assignment should be valid.

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[sindresorhus]

// @itsjohncs