internal/core/adt: add test for issue 3819

This was fixed with the new closedness
implementation.

Closes #3819

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I66d344841d31865054a5a1a9eb45aa70dc036cd0
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211765
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: mpvl

cmd/cue/cmd/testdata/script/issue3819.txtar

@@ -0,0 +1,60 @@
+# -- evalv2 --
+env CUE_EXPERIMENT=evalv3=0
+exec cue eval -e output main.cue
+cmp stdout stdout.golden
+
+# -- evalv3 --
+env CUE_EXPERIMENT=evalv3=1
+env CUE_DEBUG=openinline=0
+exec cue eval -e output main.cue
+cmp stdout stdout.golden
+
+-- cue.mod/module.cue --
+module: "example.org/module@v0"
+language: {
+        version: "v0.9.0"
+}
+-- main.cue --
+package main
+
+import (
+        "example.org/module/a_package"
+)
+
+output: a_package.level2
+-- a_package/accountconfig.cue --
+package a_package
+
+// must be a structure to trigger the panic
+#structure: {
+        f:   string
+}
+#Level0: {
+        structure?: #structure
+}
+#Level1: {
+        #Level0
+}
+#Level2: {
+        #Level1
+        structure?: #structure
+        if structure != _|_ {
+                if settings.child != _|_ {
+                        x: "x"
+                }
+        }
+        settings: {
+                child?: string
+        }
+}
+
+level2: #Level2 & {
+        structure: {
+                f:   "a"
+        }
+}
+-- stdout.golden --
+structure: {
+    f: "a"
+}
+settings: {}