[css-values-4] Should simplification of Sum nodes convert to canonical units?

[smfr]

At https://drafts.csswg.org/css-values-4/#calc-simplification Step 7.2, it says:

For each set of root’s children that are numeric values with identical units, remove those children and replace them with a single numeric value containing the sum of the removed nodes, and with the same unit.

Note the "identical units". This is different from Product nodes, which simplify down (when possible) to a value represented in the result's canonical unit.

Step 4 says:

If root is an operator node that’s not one of the calc-operator nodes, and all of its children are numeric values with enough information to computed the operation root represents, return the result of running root’s operation using its children, expressed in the result’s canonical unit.

but Sum is one of the calc-operator nodes. So following the steps, I'm not sure how calc(1rad + 45deg) gets reduced down to 102.29578deg.

[smfr]

There's also some ambiguity about whether the function name is preserved on the root if it simplifies down to a numerical value. Should calc(1px) simplify to 1px, and min(1px) to 1px? I think the combination of 10.7 step 6 and 10.7.1 step 4 suggest that they do, but browsers don't seem to implement that.

[smfr]

I think reduction of calc(1rad + 45deg)` to 102.29578deg`` was me just misunderstanding a web platform test.

[smfr]

Some concrete examples where implementations seem to disagree. How do the following serialize?

• min(1): min(1) or 1?
• min(1, 2): min(1, 2) or 1?
• min(min(1)): min(min(1)), min(1) or 1?
• calc(min(1)): calc(min(1)), min(1) or 1 or other?

Questions:

1. Should simplification ever remove all the math functions?
2. Should nested math functions of different types be simplified, i.e. is it OK to simplify calc(calc(1)) but not calc(min(1))

@tabatkins the note at https://drafts.csswg.org/css-values-4/#calc-serialize says this is under discussion, but is there anything more recent than 2016?

[tabatkins]

So following the steps, I'm not sure how calc(1rad + 45deg) gets reduced down to 102.29578deg.

That's triggered by step 3, where you simplify all the children of the node; 1rad and 45deg then hit step 1.2 and return themselves converted to deg. Then, back in simplifying the Sum node, step 7.2 sees that it has two deg children and combines them.

There's also some ambiguity about whether the function name is preserved on the root if it simplifies down to a numerical value. Should calc(1px) simplify to 1px, and min(1px) to 1px? I think the combination of 10.7 step 6 and 10.7.1 step 4 suggest that they do, but browsers don't seem to implement that.

It's definitely not clear, and I'm not sure what's ideal, actually! That's why I opened #4399 to talk about it. ^_^ I'll move your questions/examples over to that issue, and since the original question from this issue has been answered, I'll close this one.