It's a tricky case but I'm not sure I find the fixed behaviour intuitive. I would expect the escape in this place if "#{{...}}" was it's own kind of expression and not equivalent to either StringInterpolation(TupleLiteral(...)) or #MacroInterpolation(...). Maybe we should just make it a parse error and tell people to use @bew's workaround?

@jhass I think to implement it is hard, at least harder than this, and the benefit is so little.

And, this behavior makes sense to me really. Currently "#{{ ... }}" inside macro is parsed as macro literal "#{{ ... }}" as-is because #{ inside string literal leads string interpolation even if inside macro. "#{{{ ... }}}" is parsed as literal "#{, macro expression {{ ... }} and macro literal }" due to the same reason. Thus, "\#{{ ... }}" should be parsed as macro literal "\#, macro expression {{ ... }} and literal " because \#{ inside string literal doesn't lead string interpolation.

Why not make #{{foo}} a macro interpolation plus # by default? If it is supposed to be a string interpolation, you'd just need to add whitespace. So there would be no need for escaping.

/cc @asterite