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

TypeScript Playground での再現から始め、現在の結果を想定される弱い型の診断結果と比較します。既存のルールとその根拠を確認するため、関連する issue #9900 と pull request #16047 の議論を読んでください。サンプルが指定されたエラーを報告し、関連するインデックスシグネチャへの代入にリグレッションを発生させなければ完了です。