Index signature is assignable to weak type whose properties don't match the signature type

From https://stackoverflow.com/q/52368008 . Based on https://github.com/Microsoft/TypeScript/pull/16047#issue-122101801, it appears that @sandersn may have thought about the rule, but it still makes no sense to me and no rationale is stated.

TypeScript Version: master (394ee31a56c40033f31a3a8461ec267463033194)

Search Terms: weak type index signature

Code

interface Foo {
    a?: string;
}
interface Bar {
    [n: string]: number;
}
declare let b: Bar;
// No error; expected "Type 'Bar' has no properties in common with type 'Foo'."
let a: Foo = b;

Expected behavior: Error: "Type 'Bar' has no properties in common with type 'Foo'."

Actual behavior: No error.

Playground Link: link

Related Issues: Possibly #9900

Inizia con la riproduzione in TypeScript Playground e confronta il risultato attuale con la diagnostica prevista per i tipi deboli. Leggi la discussione nell’issue #9900 e nella pull request #16047 per conoscere la regola esistente e la relativa motivazione. Il lavoro è completato quando l’esempio segnala l’errore indicato senza introdurre regressioni nelle assegnazioni correlate alle firme di indice.