interface type can't be assigned to index signature type with unknown

[g-plane]

Bug Report

🔎 Search Terms

interface
index signature
assignment
assignability

🕗 Version & Regression Information

From 4.0 to Nightly. (I haven't tested it with older versions.)

⏯ Playground Link

Playground link

💻 Code

interface MyObject {
  prop: string
}

declare let a: { [x in string]: unknown }
declare let b: { [x in string]: any }
declare let c: MyObject

a = c  // error at this line
b = c

🙁 Actual behavior

Type 'MyObject' is not assignable to type '{ [x: string]: unknown; }'. Index signature is missing in type 'MyObject'.

🙂 Expected behavior

No errors.

[MartinJohns]

See #42825 (comment).