Fix Issue 24541 - cartesianProduct should have a length for finite ranges

[nicolo-mn]

This Draft PR aims to address issue #9871 (Issue 24541 on BugZilla). As of now, I have added a length field to the cartesianProduct implementation with two finite forward ranges. Since I've already built the unittests for it I'm publishing this Draft PR in case anyone wants to give some feedback. I'm still working on the case of two finite ranges where one of them is not a forward range, particularly to replace the joiner returned in cartesianProduct with a sized range.

Changelog as of now:

• Modified the Result struct inside cartesianProduct to include a length field.
• Added checks on length in unittest regarding cartesianProduct with finite forward ranges.
• Added a unittest to ensure the correct calculation of length when popping elements.

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Testing this PR locally

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[thewilsonator]

Remove the trailing whitespace to fix the style failures

[nicolo-mn]

length field in cartesianProduct with one finite input range and one finite forward range has been added

[nicolo-mn]

@thewilsonator Could you check my changes when you got some time? Thanks in advance!

[thewilsonator]

This appears to turn cartesianProduct from a lazy range into an eager range. Surely we should check if hasLength!R1 && hasLength!R2 (after taking care of infinite ranges) and compute that.

@CyberShadow do you have a better solution?

[CyberShadow]

I was thinking of the same solution. Maybe we can use takeExactly to set the length if we can know it.

[CyberShadow]

A hypothetical assumeLength range wrapper might be even better (like assumeSorted).