evalv3: another "undefined field" regression

[mvdan]

# With the old evaluator.
env CUE_EXPERIMENT=evalv3=0
exec cue export

# With the new evaluator.
env CUE_EXPERIMENT=evalv3=1
exec cue export

-- input.cue --
package p

_res: {
	_fooTrue: {} | *{foo: true}
	_msgSwitch: [
		if _fooTrue.foo { msg: "foo" },
		{ msg: "bar" },
	][0]
	out: _msgSwitch.msg
}
let res = {
	out: (_res & {}).out
}
out: res.out

As of f53f655:

# With the old evaluator. (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue export
[stdout]
{
    "out": "foo"
}
# With the new evaluator. (0.038s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue export
[stderr]
res.out: undefined field: msg:
    ./input.cue:9:18
[exit status 1]

[mpvl]

I think this is the core of the issue:

y: { if x.z { 1 } }
x: *{z: true} | {}

[myitcv]

I think this is the core of the issue:

My understanding is that your reproduction is equivalent to:

if x.z {
	y: 1
}
x: *{z: true} | {}

Is this analogous to the following?

if x+1 == 1 {
	y: 1
}
x: *0 | "dummy"