`error` type used as a `ternary` match type should be disallowed

[jaehyun1ee]

This is a follow-up issue from the discussion made in LDWG regarding p4lang/p4-spec#1389.

The specification mentions about ternary as:

a ternary match kind on a key field means that the field in the table specifies a set of values for the key field using a value and a mask. The meaning of the (value, mask) pair is similar to the P4 mask expressions, as described in Section 8.15.3: a key field k matches the table entry when k & mask == value & mask.

Thus, a ternary match makes sense for numeric types, but should not make sense for non-numeric types.

However, p4c allows non-numeric types for ternary matches, e.g., p4c/testdata/p4_16_samples/metrics/metrics_test_4.p4:

struct meta_t {
        /* ... omitted ... */
        error err_field; // error code
}

control ingress(
    inout headers_t hdr,
    inout meta_t meta,
    inout standard_metadata_t standard_metadata)

    table custom_table {
        key = {
            /* ... omitted ... */
            meta.err_field : ternary;
        }
        actions = {
            set_output;
            set_headers;
            my_drop;
        }
        default_action = my_drop();
    }
}

meta.err_field has error type, matched with ternary in custom_table. Because error types are neither numeric nor serializable, the semantics is the match is unclear.

Discussion within 2026-May LDWG was that p4c should disallow such program, where the example program can be amended by declaring the match kind as optional rather than ternary.