the value shift operators only reject a count >= bigint_bits, so MathLib::value::shiftLeft still left-shifts a negative value and both shiftLeft and shiftRight still shift by a negative count, which is undefined behaviour. it is reachable when folding a template argument like 0x8000000000000000 << 1 in simplifyNumericCalculations, since MathLib::isNegative only checks for a leading minus while a large hex literal parses to a negative bigint and the guard there lets it through. mirror the negative-operand check calculate.h already uses and return the operand unchanged, as is already done for oversized counts. ubsan flags the left shift at mathlib.cpp:272 on that input.