null checked variables in getter's closure

[vmit]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)

Code

export function foo(z: string | null) {
    const zConst = z;
    if (zConst !== null) {
        return {
            get bar() {
                return zConst.length;
            }
        }
    }
}

Expected behavior:
The code is compiled with "strictNullChecks=true" option as zConst is null-checked and could not be changed.
Actual behavior:
TS is complaining about zConst being possibly null.

The problem seems is relevant to getters only since with the same zConst in a simple function scope it is fine.

[FoxShaunR]

This isn't limited to null checks. Ran into the same issue with a discriminating union. Example here

[jakebailey]

Just checking old issues; this one was fixed by #45020 (#45006 was the same as this issue).