Matching control types that have default parameters

[jaehyun1ee]

P4 allows default control parameters.

The below program default-control-argument.p4 from the p4c test suite expects that control declaration MyC matches the control type signature C<H, M>. (The compiler frontend treats this program as valid.)

control C<H, M>(
    inout H hdr,
    inout M meta,
    in intrinsic_metadata_t intr_md = {0, 0});

package P<H, M>(C<H, M> c);

struct hdr_t { }
struct meta_t { }

control MyC(inout hdr_t hdr, inout meta_t meta) {
   apply {}
}

P(MyC()) main;

However, I wonder if the above program should actually be accepted as valid, as per the spec. Default parameters can be omitted when invoking the control instance, but I believe they cannot be omitted when declaring a control object.

The above code is also misleading because,

• The control type declaration control<H, M> implies that we can explicitly supply an argument corresponding to the intr_md parameter.
• However, if we were to supply a third argument for MyC, it would result in an error since the control declaration MyC expects only two.

A valid program would write something like:

control MyC(inout hdr_t hdr, inout meta_t meta, in instrinsic_metadata_t intr_md = { 0, 0 }) {
   apply {}
}