The parser will accept code like below:

case 1
when .&&(2) # Error: undefined method '&&' for Int32
when .=(3)  # ditto
when .+=(4) # ditto
end

They all produce errors in the semantic phase only because it is indeed impossible to define those methods (&& is syntactic sugar for if, = is plain assignment, and += is an OpAsssign that will be normalized shortly). It should be possible to move the error to the syntactic phase without breaking any existing code.