[key in 'a' | 'b'] is not available in interface's root

[deepkolos]

TypeScript Version: 3.7.2

Code

type a = 'a' | 'b' | 'c'

interface test {
    [k in a]: any; // it doesn't works
    b: {
        [k in a]: any; // it works
    }
}

Expected behavior: in operator works under interface's root

Actual behavior: it doesn't works

Playground Link: https://www.typescriptlang.org/play/index.html?ssl=8&ssc=2&pln=1&pc=1#code/C4TwDgpgBAhlC8UDkMlQD7IEZs0gxkgFBECWAdsBAE4BmM+0VAzsFAN5FTdQDaA1lAqwAugC5Y5EAG4uPLBM49lfQcJjjJMudwC+RXUA

Related Issues:

[deepkolos]

#24220