This started as an attempt to fix a GCC 11 warning of misplaced parentheses.
I then noticed that trying to fix the parentheses warning actually triggered
errors in the tests, showing that we were incorrectly assuming that the
implementation of ranges::advance was using operator+= or operator-=.
This commit fixes that issue and makes the tests easier to follow by
localizing the assertions it makes.
Differential Revision: https://reviews.llvm.org/D103272
Implements part of P0896 'The One Ranges Proposal'.
Implements [range.iter.op.prev].
Depends on D102563.
Differential Revision: https://reviews.llvm.org/D102564
Implements part of P0896 'The One Ranges Proposal'.
Implements [range.iter.op.next].
Depends on D101922.
Differential Revision: https://reviews.llvm.org/D102563