For an "in place" sequence operation expression, in which the first parameter of the called behavior is of direction inout, the primary expression must be acceptable as a left-hand side. However, when this is not the case (e.g., with an expression like Integer[]{1,2}->add(1)), there is currently no constraint violation, but mapping is not possible, resulting in a mapping error.
seidewitz
commented
5 years ago
For an "in place" sequence operation expression, in which the first parameter of the called behavior is of direction
inout, the primary expression must be acceptable as a left-hand side. However, when this is not the case (e.g., with an expression likeInteger[]{1,2}->add(1)), there is currently no constraint violation, but mapping is not possible, resulting in a mapping error.