If you have a Claris R2000 front chainwheel designed for 8-speed systems and you use it with a 7-speed cassette, it should still work without significant issues. The front chainwheel compatibility is primarily related to the rear derailleur and the chain width rather than the specific number of speeds on the cassette.
The main reason cranksets are often described as speed-specific is due to the chain width associated with different drivetrain speeds. As the number of speeds increases, the spacing between the cogs on the cassette becomes narrower, requiring a correspondingly narrower chain. This narrower chain is designed to mesh properly with the narrower spacing of the cassette's sprockets.
When using a crankset designed for a higher number of speeds (e.g., an 8-speed crankset) with a lower number of speeds (e.g., a 7-speed cassette), the chain width typically remains the same. Therefore, the chain will still be able to engage the front chainwheel and the rear cassette, though it may have slightly wider gaps between the individual sprockets.
In practical terms, this mismatch in speeds shouldn't cause significant performance issues. However, it's worth noting that using a lower-speed crankset with a higher-speed cassette, such as a 7-speed crankset with an 8-speed cassette, could result in problems. The narrower chain on the higher-speed cassette may have difficulty engaging properly with the wider chainrings of the lower-speed crankset.
In summary, using an 8-speed front chainwheel with a 7-speed cassette should generally work fine, as the chain width remains the same. However, using a lower-speed crankset with a higher-speed cassette could result in compatibility issues due to the narrower chain width required by the higher-speed system.