evaluator: cycle error incorrectly reported for non-concrete field

[myitcv]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.0.0-20220825081001-d3c14a684375

       -compiler gc
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
             vcs git
    vcs.revision d3c14a68437588b9dbdec9cb3919f7aafec58111
        vcs.time 2022-08-25T08:10:01Z
    vcs.modified false

Does this issue reproduce with the latest release?

Yes

What did you do?

# This should fail with an error about a and b being
# incomplete
! exec cue export x.cue
stderr 'a: incomplete value'
stderr 'b: incomplete value'

-- x.cue --
x: 200
x: y + 100
y: x - 100

a: b + 100
b: a - 100

What did you expect to see?

Passing test.

What did you see instead?

# This should fail with an error about a and b being
# incomplete (0.014s)
> ! exec cue export x.cue
[stderr]
cycle error:
    ./x.cue:5:4
[exit status 1]
> stderr 'a: incomplete value'
FAIL: /tmp/testscript4050841429/repro.txtar/script.txt:4: no match for `a: incomplete value` found in stderr

Related to #1884.

cc @mpvl

[mpvl]

Cycle errors (not to be confused with structural cycles) are a kind of incomplete error. So this is correct. The spec mentions that cycles can be broken.

So this is mostly a documentation / presentation thing.

[myitcv]

Also porting the example from #1778 which I totally forgot I raised with @rogpeppe before:

What did you do?

# Below, a.cue and b.cue are identical, but for the order
# of _t and r.

# With _t before r
! exec cue export a.cue
stderr 'non-concrete value string'

# With r before _t
! exec cue export b.cue
stderr 'non-concrete value string'

-- a.cue --
_t: {
	#p: string
	"""
		hello \(#p)
		"""
}

r: """
	\(_t)
	"""
-- b.cue --
r: """
	\(_t)
	"""

_t: {
	#p: string
	"""
		hello \(#p)
		"""
}

What did you expect?

Passing test

What did you see instead?

# Below, a.cue and b.cue are identical, but for the order
# of _t and r.
# With _t before r (0.013s)
# With r before _t (0.011s)
> ! exec cue export b.cue
[stderr]
invalid interpolation: cycle error:
    ./b.cue:1:4
[exit status 1]
> stderr 'non-concrete value string'
FAIL: /tmp/testscript937876582/repro.txtar/script.txtar:10: no match for `non-concrete value string` found in stderr

The error message here is also not ideal because it doesn't say exactly what was non-concrete.