Fix arbitrary variables of `font-family` and `font-weight` groups being merged incorrectly

[dcastil]

How does one resolve the conflict between font-family and font-weight with arbitrary values?

EX: twMerge('font-(family-name:--custom-family) font-(--custom-weight)') -> returns: font-(--custom-weight) expected: font-(family-name:--custom-family) font-(--custom-weight) twMerge('font-(--custom-weight) font-(family-name:--custom-family)') -> returns font-(family-name:--custom-family) expected: font-(--custom-weight) font-(family-name:--custom-family)

Originally posted by @bguggie in https://github.com/dcastil/tailwind-merge/discussions/584

Just checked and you're right, this is a bug, I'll fix it.

This one is not easy to resolve due to how tailwind-merge works internally and it will take some time for me to find a way to fix it. As a workaround you can use the important modifier on one of the classes to make sure the both stay in the output, e.g. font-(family-name:--custom-family) font-(--custom-weight)!.

https://github.com/dcastil/tailwind-merge/discussions/584#discussioncomment-13398886

[dcastil]

The problem:

• font-weight group which is defined first is using isArbitraryVariable as a catch-all, preventing any subsequent groups from matching arbitrary variables
• font-family group which is defined second is using isArbitraryValue as a catch-all, preventing any subsequent groups form matching arbitrary values

No matter which group is defined first, it will prevent the other from matching either arbitrary variables or arbitrary values.

[dcastil]

Possible solution: Add optional priority number property to validator functions which will be used to insert validators in classMap.

https://github.com/dcastil/tailwind-merge/blob/47155f0ebe17188991c8cafc9fe2ae034b18265c/src/lib/class-group-utils.ts#L145-L148

Instead of pushing to the validators array, I search for an index based on the priority and splice the element into it.

[dcastil]

This would be a breaking change for users of tailwind-merge's validators since their execution order might change. I can only do this in a new major version.