Type inference fails on int type

[jaehyun1ee]

// issue2260-1.p4

control C();
package S(C c);
T f<T>(T x) {
    return x;
}
control MyC() {
    apply {
        bit<8> y = f(255);
   }
}
S(MyC()) main;

p4c rejects issue2260-1.p4 with this error log.

However, shouldn't it infer int as the type argument for calling f? i.e.,

bit<8> y = f<int>(255); // f returns int, and it is implicitly cast to bit<8>

[vlstill]

From the log it seems that T is actually inferred to be int.

Where 'T' is bound to 'int'

So this might be an error in processing return type of generic functions. Maybe it was not tested because it is hard to write useful generic functions in P4 (see e.g. #5067), except for extern functions.

[jaehyun1ee]

@vlstill Maybe it is because all int should be compile-time known, as mentioned in #5091?