Object spread drops index signature

TypeScript Version: 3.0.1

Search Terms:
spread index signature, spread indexer

Code

declare var v: Record<string, string>;

let v2 = {...v}; // works
let v3 = {a: '', ...v}; // index signature lost
let v4 = {...v, a: ''}; // same as above
let v5 = {a: 1, ...v}; // index signature lost, should be '{a: number, [x: string]: number | string}'
let v6 = {[Number()]: 1, ...v}; // empty object type ò,Ó, should be '{[x: string]: number | string}'

Expected behavior:
Index signature exists on all object types.

Actual behavior:
Index signature is lost if object literal contains another property assignment. I'm not entirely sure what the type of v6 should be.

Playground Link: https://agentcooper.github.io/typescript-play/#code/CYUwxgNghgTiAEA3WSBc8BK4D2NgB4BnAFxgEsA7AcwBp4TzqA+AbgCg2IRikAmeALzwA3gDpxiAL4t4AelnwA7rgDWhTtyQBmQSKjoA5AbrjRUmfPiVQAD3pkqFKMQCuceBGwkNPRABZdMQk6fXgjaTkFQigAWwQoQngoACNsRBAfJABWQNCARhMJCMtrEDtCBydXd08SOkIAC2wXCGB4ZIQDYVCKFxiOmDoAbRt0BkoqAF10Xv6QGHgAH3pSCckDTMQANkChgDk+gYAKAEpp+AL4U3NI+BAYgAdiAE94bGSAK3AeF4eEACeaABl+pNFptDphYQjMarajnWYDJYrRhUdZAA

Related Issues:
#16694
#16373

从 issue 中的 TypeScript 3.0.1 示例开始，通过链接的 Playground 重现推断出的类型。查看相关 issue #16694 和 #16373，了解之前的讨论和预期行为，尤其是 v5 和 v6。对于报告的对象展开案例，建立并实现一致的索引签名行为，并为这些示例提供覆盖，即视为完成。