False positive error report for interface type missing index signature, thus not assignable to object type with `unknown` values.

TypeScript Version: 3.6.0-dev.20190704 (3.4.3 doesn't have this bug)

Search Terms:
index signature, unknown, assignable, interface

Code

interface A {
  properties?: { [Key: string]: unknown };
}

type Bar = {
  key?: "value";
};

interface Baz {
  key?: "value";
}

interface B extends A { // ok
  properties?: Bar;
}

interface C extends A { // error: Baz is not assignable to { [Key: string]: unknown } -> Index signature is missing
  properties?: Baz;
}

Expected behavior:
No error expected, interface C extends A correctly.

Actual behavior:
Error because interface C does not extend interface A

Playground Link:
http://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgILIN4ChnIA5QD2e0YwEAzgPwBcmyA2gNIQCedFYUoA5gLp0AriADWIQgHcQyAL4BuLDKxYwrEsgBCcKMgC8mHMhFtayAEQA3OABtBEMwvnLQkWIhRaAXgdzHWpyxs7B0VncGh4JE1kCAAPSBAAEwo0egB6NORCEUMCYlJyajotKEcw10iUAGEY+IgklPQMZAyYqCIoYrhvYBTxMGQ4CgpgHhA4ACNrFDBCemY2Di5eAWRhMUlpGWQAWgA+ZABJJLjkEbG4MEEoFF7kAFtekZAeXKISKDJKUy8yoA

Related Issues:
#30977 (this one is specifically for enums)