Extraneous control flow error

[sandersn]

From vscode: src/vs/workbench/contrib/search/common/searchModel.ts

Code

interface F {
    d(): void
}
export function doRemove(dds: F | F[]) {
    if (!Array.isArray(dds)) {
        dds = [dds]
    }
    for (let n of dds) {
        n.d()
    }
    return dds
}

Expected behavior:

No error, and n: F at all locations, and dds: F[] at all locations.

Actual behavior:

Types are correct; bogus error: 'F | F[]' must have a 'Symbol.iterator' method that returns an iterator.

Looks like narrowing works as expected, except for errors.

[sandersn]

For future bisecting: Doesn't repro on a commit from March 2: dd6811f