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Although the Haddock's for `shiftL` and `shiftR` do require the number of bits to be non-negative, we should still check this before calling out to primitives (which also have undefined behaviour for negative bit shifts). If a user _really_ wants to bypass checks that the number of bits is sensible, they already have the aptly-named `unsafeShiftL`/`unsafeShiftR` at their disposal. See #16111.
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