Error incorrectly thrown for &__ selector

[xzyfer]

Originally reported sass/node-sass#1329.

Turns out this is valid Sass.

foo {
  &-- {
    &baz {
      color: red;
    } } } 

Ruby Sass

foo--baz {
  color: red;
}

LibSass

Error: Invalid CSS after "foo {": expected "}", was "&-- {"
        on line 25 of test.scss
>> foo {
   -----^

---

It's worth noting that &__ compiles correctly.

foo {
  &__ {
    &baz {
      color: red;
    } } }

foo__baz {
  color: red;
}

Spec sass/sass-spec#670

[mgreter]

Reason why __ works, is because the following is valid

__ {
  foo: bar;
}

While the following is not:

-- {
  foo: bar;
}

Knowing that a fix should be pretty simple.

[xzyfer]

Please activate the spec @mgreter