Major Description
INSTR's documentation reads as though it should support an expression or a literal for either argument. At least, it doesn't say that it only supports one or the other.
However, the implementation only handles the case of INSTR(expr, literal). We can pretty easily make this better and work with any combination:
e.g. INSTR(literal, expr), INSTR(expr, expr), INSTR(literal, literal)