[pcwalton]

> In C++ there is another reason that capture-by-reference is a sensible default: there is no garbage collector. It is up to the programmer to ensure that the lexical environment remains valid for the lifetime of the closure, and so you can only capture locals by value if you return a closure.

That's true, and it's why we applied the same reasoning to Rust. But we ended up with a very confusing situation. The definition of "closure" in an imperative language really implies capturing by reference; virtually all imperative languages that aren't C++ (or Rust) work this way (in Java it's a little muddy because of the final restriction, which is much maligned because it violates this intuition).

A closure that captures by value just isn't a closure by most programmers' definitions, and I think that the language would be better off treating it as something syntactically different from a closure.