Clarification on directionless action arguments behaving like direction in

[jaehyun1ee]

The spec mentions about directionless action arguments:

For an action, a directionless parameter indicates that it is “action data”. See Section 14.1 for the meaning of action data, but its meaning includes the following possibilities:

• The parameter's value is provided in the P4 program. In this case, the parameter behaves as if the direction were in.

The phrase "behaves as if the direction were in", I believe, is not quite clear, particularly regarding compile-time known-ness.

The differences between an in parameter and a directionless parameters, aside their copy-in/out semantics, is:

• in can be implicitly cast, while directionless cannot (as per the current spec)
• directionless argument should be compile-time known, while no such restriction exists for in argument.

So, I wonder if "behaves as if the direction were in" also imply that the directionless arguments supplied to actions in a P4 program (not by the control plane) need not be a compile-time known value?

Relevant to this issue, issue473.p4 in the p4c test suite is expected to be rejected by the frontend.

action b(inout bit<32> x, bit<8> data) {
    meta.a = meta.a ^ (bit<4>) x ^ (bit<4>) data;
}
table t1 {
    key = { hdr.ethernet.srcAddr : exact; }
    actions = { b(meta.c); }
    default_action = b(meta.c, (bit<8>) meta.d);
}

issue473.p4(70): [--Werror=type-error] error: (bit<8>)meta.d: action argument must be a compile-time constant
        default_action = b(meta.c, (bit<8>) meta.d);
                                   ^^^^^^^^^^^^^^^