It seems to check a bit too many cases, though:

$ echo -e "->\n  <a></a>\n  foo\n  <b></b>" | ./bin/coffee -bcs
[stdin]:4:4: error: Adjacent JSX elements must be wrapped in an enclosing tag
  <b></b>
   ^^^^^^

This is probably OK (why would you put a random toplevel element), but means we check every child of a block to check if it's a JSX element, if the returned value is a JSX element, which might be a bit wasteful.

It only checks siblings if the value to be returned is a JSX element. So yes, it’s a little wasteful for whenever you’re returning a JSX element, but I don’t know how to avoid checking every element if siblings are disallowed.

But I think the bigger problem is that this:

var render = () => {
  <a></a>
  <b></b>
}

throws the Adjacent JSX elements must be wrapped in an enclosing tag error in Babel, whereas this:

var render = () => {
  <a></a>;
  <b></b>;
}

does not. I guess it’s because the first example’s lack of semicolons makes it equivalent to this:

var render = () => {
  <a></a><b></b>
}

which throws the error.

So what to do? It would appear that this PR has made our error checking too strict, since this:

var render = () => {
  <a></a>;
  return <b></b>;
}

should be allowable JSX, at least if Babel is to be trusted.

Well, we can loop from 1 to length, and if both nodes[i]...csx && nodes[i - 1]...csx, we have found adjacent tags.