TypeScript：从ES5目标切换时出现编译错误

Question

以下代码在编译目标为ES5时进行编译，但是在切换到ES6或ES2016或ES2015时会产生编译错误。

Playground Link

type Grid<T> = {
    cells: T[]
}

type Cell = {
    position: number;
}

type UI_Cell = Cell & {
    thingy: number;
}

type Battle = {
    grid: Grid<Cell>
}

type UI_Battle = Battle & {
    grid: Grid<UI_Cell>
}

let battle: UI_Battle;

function mami() {
    for (const cell of battle.grid.cells) {
        console.log(cell.thingy); // Error: Property 'thingy' does not exist on type 'Cell'
    }
}

可以通过明确地执行以下操作来修复错误：

const cells: UI_Cell[] = battle.grid.cells;
for (const cell of cells) {
    console.log(cell.thingy);
}

但这很不方便，只是“怪异”。是编译器错误吗？