[css-values-5][css-mixins-1] Add an if-test for local variables

[andruud]

Within a custom function, var() can refer to a local variable / parameter, e.g:

@function --add-one(--x) {
  result: calc(var(--x) + 1px);
}

The if() function has a style() test, which accepts a <style-query>. We should have a test which can reach local variables / parameters as well, such that you can do e.g.:

@function --select(--option) {
  result: if(
    test(--option: one): foo; 
    test(--option: two): bar;
    else: baz;
  );
}

One option could be to spec that if(style()) targets locals within a function context (i.e. it sees the same thing var() sees), but this might stretch the definition of "style". Otherwise, I guess we'll need to invent a new test.

[tabatkins]

I think just using style() is the right way to go. Local variables fully shadow custom properties in other contexts; it feels weird for style() to be able to pierce thru that and see all the way to the element. Also, the variable shadowing behavior is very intentionally meant to be able to let a caller override what a callee sees for custom props.