Dart is missing &&= and ||= operators

[Hixie]

It's really useful to be able to write code like:

    result.canTap ||= onCanTap != null;
    result.canDoubleTap ||= onDoubleTap != null;
    result.canLongPress ||= onLongPress != null;

...but currently we can't because ||= and &&= don't work in Dart. |= and &= work, but | and & aren't implemented by bool so that doesn't help here.

[lrhn]

I agree that it would be nice.

I think ||= and &&= would have slightly different semantics than other op= assignments. Those effectively do lhs = lhs op expr. In the case of ||= and &&= you probably don't even want to do the assignment if it is already true/false, so the translation should be something like:

lhs &&= expr   ==>   let v = lhs in (v ? lhs = expr : v)
lhs ||= expr   ==>   let v = lhs in (!v ? lhs = expr : v)

For non-short-circuit boolean operators, see #1080
Adding & and | to bool, like in Java and C++, was voted down at the time (the change is miniscule, it's two one-line functions in the core library, but the spec might want to make them compile-time constants as well).

[floitschG]

Fwiw we already opened that can of worms with ??=. Since that one already exists we should just allow ||= and &&=.

[munificent]

Tracked by: #26996. I know this one's older, but that one has more details.