cmd/cue: add a regression test for issue 2704

Trying to export or vet the cue and json files together, where the first
contained a schema with a disjunction and a regular expression,
results in an incorrect error with the old evaluator.

Thankfully, the new evaluator already handles this correctly.
Add a regression test to ensure that this edge case remains fixed.

Note that I was unable to reproduce this bug with cmd/cue
with the old evaluator using a single CUE file, so it was hard
to add this reproducer as a unit test case elsewhere.

Closes #2704.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I13e4180dba7486909a50fa0fe1300ac05b183ca0
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194741
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

cmd/cue/cmd/testdata/script/vet_issue2704.txtar

@@ -0,0 +1,29 @@
+# https://cuelang.org/issues/2704
+
+! exec cue export schema.cue data.json
+cmp stderr evalv2-stderr.golden
+! exec cue vet schema.cue data.json
+cmp stderr evalv2-stderr.golden
+
+# This issue is already fixed in the new evaluator.
+env CUE_EXPERIMENT=evalv3
+
+exec cue export schema.cue data.json
+cmp stdout evalv3-stdout.golden
+exec cue vet schema.cue data.json
+! stdout .
+
+-- evalv2-stderr.golden --
+1: invalid value "value" (out of bound =~"^[0-9]+$"):
+    ./schema.cue:1:14
+    ./data.json:1:7
+-- evalv3-stdout.golden --
+[
+    "5",
+    "value"
+]
+-- schema.cue --
+#schema: [...=~"^[0-9]+$" | string]
+#schema
+-- data.json --
+["5", "value"]