Type narrowed non-nullness lost in array method callback

[aaronbeall]

TypeScript Version: 2.5.3

Code

declare const obj: { props?: string[] }

obj.props // string[] | undefined
if (obj.props) {
    obj.props // string[]
    obj.props.forEach(p =>
        obj.props // string[] | undefined <-- Expected string[]
    )
}

Expected behavior:
obj.props inside forEach callback to be string[]

Actual behavior:
obj.props inside forEach callback goes back to possibly undefined, leading to errors if not again type guarded

[mhegazy]

Please see #10982

[aaronbeall]

Thanks, seems TS was telling me something I missed after all :)