core/adt: correct copying of conjuncts in stripNonDefaults

If Conjunct.x is a ConjunctGroup, then Conjunct.Expr() may return an
expression which is a distant descendent. And then we have no way of
obtaining the correct Env for the expr which we need in
stripNonDefaults().

StripNonDefaults should duplicate the tree of conjuncts and conjunct
groups faithfully, so it is simpler to clone the current ConjunctGroup
and then update the x's with stripped versions of each conjunct's Elem
(which does not behave in the same way as the Conjunct.Expr()).

Fixes: #3718

Signed-off-by: Matthew Sackman <matthew@cue.works>
Change-Id: I7dd5c23cad4c9b4d7453c7c8d6de9cc9e07a7f92
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209119
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

encoding/openapi/openapi_test.go

@@ -176,6 +176,10 @@ func TestParseDefinitions(t *testing.T) {
 		in:     "omitvalue.cue",
 		out:    "omitvalue.json",
 		config: defaultConfig,
+	}, {
+		in:     "issue3718.cue",
+		out:    "issue3718.json",
+		config: defaultConfig,
 	}}
 	for _, tc := range testCases {
 		t.Run(tc.out+tc.variant, func(t *testing.T) {

encoding/openapi/testdata/issue3718.cue

@@ -0,0 +1,9 @@
+import (
+	"encoding/json"
+)
+
+let defaultPolicy = json.Marshal({ deny: "all" })
+
+#Spec: {
+    policy?: string | *defaultPolicy
+}

encoding/openapi/testdata/issue3718.json

@@ -0,0 +1,21 @@
+{
+   "openapi": "3.0.0",
+   "info": {
+      "title": "Generated by cue.",
+      "version": "no version"
+   },
+   "paths": {},
+   "components": {
+      "schemas": {
+         "Spec": {
+            "type": "object",
+            "properties": {
+               "policy": {
+                  "type": "string",
+                  "default": "{\"deny\":\"all\"}"
+               }
+            }
+         }
+      }
+   }
+}

internal/core/adt/default.go

@@ -14,6 +14,10 @@
 
 package adt
 
+import (
+	"slices"
+)
+
 // Default returns the default value or itself if there is no default.
 func Default(v Value) Value {
 	switch x := v.(type) {
@@ -131,9 +135,9 @@ func stripNonDefaults(elem Elem) (r Elem, stripped bool) {
 		// NOTE: this code requires allocations unconditional. This should be
 		// mitigated once we optimize conjunct groupings.
 		isNew := false
-		var a ConjunctGroup = make([]Conjunct, len(*x))
-		for i, c := range *x {
-			a[i].x, ok = stripNonDefaults(c.Expr())
+		a := slices.Clone(*x)
+		for i, c := range a {
+			a[i].x, ok = stripNonDefaults(c.Elem())
 			if ok {
 				isNew = true
 			}