Inconsistent behavior of functions that contains 'calc' in their names

[kaishin]

I am aware that calc() is handled differently, but the behavior below is inconsistent:

@function my-calc($value) {
  @return false;
}

@function -my-calc($value) {
  @return false;
}

@function -my-string($value) {
  @return false;
}

.test {
  color: -my-calc(12);
  color: my-calc(12);
  color: -my-string(12);
}

The output is:

.test {
  color: -my-calc(12);
  color: false;
  color: false;
}

instead of:

.test {
  color: false;
  color: false;
  color: false;
}

I tried with LibSass and I got the expected behavior above.

[KittyGiraudel]

Cannot reproduce it with -calc, nor with --calc. Only with -*-calc where * is a string. Prefix-something mess?

[chriseppstein]

Sass doesn't know what are the set of legal vendor prefixes (on purpose) so we have special cased -*-calc(...) to have the same semantics as calc(...)

[nex3]

More background information: calc() has special parsing rules that allow it to have its own mini-language of arithmetic that's entirely separate from the rest of CSS. In order to support this, Sass needs to know that it's a calc() variant at parse time, before it knows what user-defined functions exist.

I've filed #1598 to track deprecating the ability to define confusing functions like this.