Type check loosening when using Record with union types.

[elementum]

Bug Report

Under certain conditions typescript compiler allows setting any value for a record that has union type key. See code section below.

🕗 Version & Regression Information

Tested in 4.8.4

⏯ Playground Link

Playground link with relevant code

💻 Code

type UnionType = "ABC" | "XYZ"
type RecordType = Partial<Record<UnionType, string>>

// no errors, ok
const value1: RecordType = {
    XYZ: "some value",
}

// error, ok, number where string is expected
const value2: RecordType = {
    XYZ: 123,
}

// error, ok, number where string is expected
const value3: RecordType = {
    ["XYZ"]: 123
}

// error, ok, key test is not allowed
const value5: RecordType = {
    ["test"]: 123
}


// as UnionType clause is important
// it could be a function that returns UnionType, e.g. const key = getUnionKey(), same problem
const key: UnionType = "XYZ" as UnionType

// no error, but it should be there
const value4: RecordType = {
    [key]: 123
}

🙁 Actual behavior

No error when using square brackets. Last example.

🙂 Expected behavior

It should throw an error.

[MartinJohns]

Duplicate of #38663.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[MartinJohns]

Thank you mister typescript-not sir.