Enum reset x

[nblei]

Allows EnumMembers with 'x' or 'z' values to be Factors in assignment operations inside if_reset blocks.

Previously,

module top (
    i_clk: input clock,
    i_rst: input reset,
) {

    enum State: logic<3> {
        Idle = 3'bxx1,
        Run0 = 3'b000,
        Run1 = 3'b010,
        Run2 = 3'b100,
        Done = 3'b110,
    }

    var state: State;

    always_ff {
        if_reset {
            state = State::Idle;
        }
    }

}

would result in an error because of the don't care bits in State::Idle.

[nblei]

@dalance

One thing that is strange about this is that EnumItems are not evaluated. When are symbols supposed to be evaluated?

[dalance]

Evaluation is executed at symbol table resolve.

veryl/crates/analyzer/src/symbol_table.rs

Line 173 in 68c3593

symbol.evaluate();

But it may be unnecessary. I think calling symbol.found.evaluate() is sufficient when it is necessary.
If the symbol is already evalated, it returns the cached evaluated, and if not, it evaluates at the call time.

Therefore, the code of evaluator.rs can be simplified like below:

// from
if let Some(evaluated) = symbol.found.evaluated.get() {
    evaluated
} else {
    symbol.found.evaluate()
}

// to
symbol.found.evaluate()

[taichi-ishitani]

For SystemVerilog, there is the limitation below for unassigned enum labels following the enum label with x/z value.

From IEEE 1800-2023 6.19.2 Enumerated type ranges

I think you need to check if this limitation is satisfied.

[dalance]

@nblei
I think this PR can be merged.
Is it ok with you?

@taichi-ishitani
I think this change is about evaluating enum value,
and your suggestion seems to be about requirement of additional check for enum declaration.
So could you create a new issue for it?

[taichi-ishitani]

@taichi-ishitani I think this change is about evaluating enum value, and your suggestion seems to be about requirement of additional check for enum declaration. So could you create a new issue for it?

Sure, I will create a new ticket.

[nblei]

@nblei I think this PR can be merged. Is it ok with you?

Yes.