I think that test is wrong. Per https://drafts.csswg.org/css-values/#calc-range:
Parse-time range-checking of values is not performed within math functions, and therefore out-of-range values do not cause the declaration to become invalid. However, the value resulting from an expression must be clamped to the range allowed in the target context. Clamping is performed on computed values to the extent possible, and also on used values if computation was unable to sufficiently simplify the expression to allow range-checking. (Clamping is not performed on specified values.)
Which means that
calc(2 * -4) should not be invalid, and should compute to 0.