[css-values-5] should 'calc-size(0)' work?

[dbaron]

In https://issues.chromium.org/341564374 @yisibl pointed out that calc-size(0) doesn't work. I think this matches the current spec which defines the behavior to work like calc(), where it was intentional that calc(0) doesn't work.

I think despite the parallels with calc() we may want to consider making calc-size(0) work correctly, because:

• calc-size() is explicitly for lengths only so there's less concern about unit algebra making sense
• it would be nice to preserve the principle that "wrap an animation endpoint in calc-size()" works in general, and 0 is one of the most common length animation endpoints.

[Loirooriol]

To clarify, this is about unitless lengths. calc(0) works fine as an <integer>.

[tabatkins]

Yes, I think allowing a bare 0 is fine; as you say, we have the necessary context to infer the type (the calculation must be a length).

We probably want to do it as a grammar branch, so only a literal 0 with nothing else works. (Such a calculation would otherwise be invalid since it would type as <number>, so this is safe.) If the calculation is any more complex than that, we continue to treat it as a number.