Specify behavior of `+:` slice used as l-value

[pacokwon]

The current working draft of the specification does not specify the behavior of +: style slices when they are used as l-values.

Here is the relevant description of +: slices in the specification.

A slice may also be specified as [L+:W] where L is the lowest bit of the slice and W is the width of the slice. In this case, only W must be a non-negative local compile-time known numeric value. L must also be a numeric value and some architectures may require it to be compile-time known. It must also be non-negative and in-range for the type. Some architectures may allow variable indexing, in which case an out of range value will be equivalent to a shift of that amount. An index that is not compile-time known must be a bit type in order to ensure that it is non-negative.

Earlier in the same paragraph in the specification is the behavior of H:L slices used as l-values described as follows:



Slices are also l-values, which means that P4 supports assigning to a slice: e[H:L] = x. The effect of this statement is to set bits H through L (inclusive of both) of e to the bit-pattern represented by x, and leaves all other bits of e unchanged.

Unlike H:L slices, the range referenced by +: could potentially go out of bounds. Hence a clear description of how it behaves in such edge cases also seems necessary.