Union in a computed property allows any assignment to property value

TypeScript Version: 4.0.0-dev.20200518

Search Terms: computed property, union

Code

type Key = 'a' | 'b';

const index1: Record<Key, string> = { a: '', b: 0 }; // Errors as expected, ok
const index2: Record<Key, string> = { a: '', b: '' };

const b: Key = 'b';

const index3: Record<Key, string> = { ...index2, [b]: 0 }; // Errors as expected, ok

declare var k: Key;

// No errors, allowing anything to be assigned to string
const index4: Record<Key, string> = { ...index2, [k]: [] };
const index5: Record<Key, string> = { a: '', b: '', [k]: 0 };

Expected behavior:
Expected error for invalid assignments in index4 and index5.

Actual behavior:
No error, allowing anything to be assigned to string

Playground Link: here

Related Issues:
https://github.com/microsoft/TypeScript/issues/36920: perhaps the computed property is deemed an excess property, though I don't think it should be

Start by running the supplied Playground reproduction and compare the diagnostics for index4 and index5 with the expected errors. Investigate computed-property handling and excess-property checking, including related issue #36920; done means both invalid assignments are rejected without regressing the valid examples.