Misleading error elaboration #25971

TypeScript Version: 3.1.0-dev

Search Terms: Error elaboration

Code

function foo<T extends { a: string }>(x: T) {
    x = { a: "abc", b: 20, c: 30 };
}

Expected behavior: An error stating { a: string, b: number, c: number } is not assignable to T.

Actual behavior: Elaboration errors on the b and c property assignments which misleads the user to think the issue is with those properties. The real issue is that no object literal and only a T is assignable to T, and we should not attempt to elaborate here. Generally, the elaboration logic can't rely on the behavior of getIndexedAccess without first validating that a meaningful type is being requested because getIndexedAccess will happily create deferred indexed access types that technically are errors. The elaboration logic needs to be more selective about which target types it descends into. Specifically, I would suggest only descending into non-generic object types and non-generic intersection types.