Anyone knows how to use this constraint:
(constraint skew (within_diff_pairs))
Ah I think we need to improve the skew constraint documentation for this. The announcement is here, which does outline what it does: Post-V8 New Features and Development News - #34 by JamesJ
Essentially, if you don’t have (within_diff_pairs), the skew will be tested across all matching nets. This is what you’d want for testing the skew of a bus, but not the skew of multiple differential pairs which all have the same net class.
Adding (within_diff_pairs) means that the skew is tested independently for each differential pair in the set of tracks which match the rule conditions.