evalv3: index out of range panic in overlayContext.allocCC

[mvdan]

# With the old evaluator.
env CUE_EXPERIMENT=evalv3=0
exec cue vet -c=false

# With the new evaluator.
env CUE_EXPERIMENT=evalv3=1
exec cue vet -c=false

-- input.cue --
package p

#Schema: {
	_ok: bool
	{} | {
		_ok: true
		let Ok = _ok
		{} | {x: Ok}
	}
}

[string]: #Schema

foo: bar
bar: {}

As of c29fd02:

# With the old evaluator. (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue vet -c=false
# With the new evaluator. (0.035s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue vet -c=false
[stderr]
panic: runtime error: index out of range [1] with length 1 [recovered]
	panic: runtime error: index out of range [1] with length 1 [recovered]
	panic: runtime error: index out of range [1] with length 1

goroutine 1 gp=0xc000002380 m=0 mp=0x19dde00 [running]:
panic({0x1073420?, 0xc00003a180?})
	/home/mvdan/tip/src/runtime/panic.go:806 +0x168 fp=0xc000404890 sp=0xc0004047e0 pc=0x472b68
cuelang.org/go/internal/core/adt.runTask.func1()
	/home/mvdan/src/c/cue/internal/core/adt/sched.go:695 +0x76 fp=0xc0004048b8 sp=0xc000404890 pc=0x739c96
panic({0x1073420?, 0xc00003a180?})
	/home/mvdan/tip/src/runtime/panic.go:787 +0x132 fp=0xc000404968 sp=0xc0004048b8 pc=0x472b32
cuelang.org/go/internal/core/adt.runTask.func1()
	/home/mvdan/src/c/cue/internal/core/adt/sched.go:695 +0x76 fp=0xc000404990 sp=0xc000404968 pc=0x739c96
panic({0x1073420?, 0xc00003a180?})
	/home/mvdan/tip/src/runtime/panic.go:787 +0x132 fp=0xc000404a40 sp=0xc000404990 pc=0x472b32
runtime.goPanicIndex(0x1, 0x1)
	/home/mvdan/tip/src/runtime/panic.go:115 +0x74 fp=0xc000404a80 sp=0xc000404a40 pc=0x4383f4
cuelang.org/go/internal/core/adt.(*overlayContext).allocCC(0xc0004055d0, 0xc0018ba800)
	/home/mvdan/src/c/cue/internal/core/adt/overlay.go:286 +0x5f6 fp=0xc000404b98 sp=0xc000404a80 pc=0x737476
cuelang.org/go/internal/core/adt.(*overlayContext).allocCC(0xc0004055d0, 0xc0018ba900)
	/home/mvdan/src/c/cue/internal/core/adt/overlay.go:271 +0xbe fp=0xc000404cb0 sp=0xc000404b98 pc=0x736f3e
cuelang.org/go/internal/core/adt.(*overlayContext).allocCC(0xc0004055d0, 0xc0018baa00)
	/home/mvdan/src/c/cue/internal/core/adt/overlay.go:271 +0xbe fp=0xc000404dc8 sp=0xc000404cb0 pc=0x736f3e
cuelang.org/go/internal/core/adt.(*overlayContext).allocCC(0xc0004055d0, 0xc0018bab00)
	/home/mvdan/src/c/cue/internal/core/adt/overlay.go:271 +0xbe fp=0xc000404ee0 sp=0xc000404dc8 pc=0x736f3e
[...]

Reduced from https://github.com/vectordotdev/vector in Unity.

[mpvl]

A somewhat simplified example (removed pattern constraint and such):

Schema: _ | {
    _ok: true
    let Ok = _ok
    _ | {x: Ok}
}
// Order of the following two fields matters.
foo: bar & Schema
bar: Schema

[myitcv]

Noting that #3654 also triggers this under evalv3.

[mpvl]

A somewhat simplified reproducer:

ok: true
X: _ | {
    let Ok = ok
    _ | {x: Ok}
}
foo: X & {X}