Question on comparison to negative constants

[fruffy]

So I am a little bit confused on how to treat the following expression:
Is this true, false, or undefined?

control ingress(inout Headers h, inout Meta m, inout standard_metadata_t sm) {
    apply {
        if (8w255 == -1) {
            h.eth_hdr.eth_type = 1;
        } else {
            h.eth_hdr.eth_type = 2;
        }
    }
}

After FrontEnd_6_ConstantFolding this evaluates to false.
The compiler naturally also gives a warning:

bugs/negative_equality.p4(30): [--Wwarn=mismatch] warning: -1: negative value with unsigned type
        if (8w255 == -1) {

I assumed this equality would evaluate to true since both expressions are 8w255 after the implicit cast. Unless such a comparison always evaluates to false, regardless of value, because of the type mismatch. I was digging through the constantFolding.cpp code and saw that the result is evaluate before the cast: https://github.com/p4lang/p4c/blob/master/frontends/common/constantFolding.cpp#L419
Is this intentional?

negative_equality.p4.txt
negative_equality.stf.txt

[mihaibudiu]

Good question, I think this is a bug, because the spec says that -1 should be cast to whatever type is on the left, and then this should be true.

[mihaibudiu]

We will discuss this in the design working group nevertheless.