internal/core/adt: do not require closeContext.group to be non-nil

In fields.go CloseInfo.spawnCloseContext creates a closeContext with a
nil group field. That explodes closeContext.assignConjunct.

closeContext.assignConject has been reworked so it can cope with a nil
group field. I did also try changing CloseInfo.spawnCloseContext to make
it always populate the group field, but that caused other tests to break
via a "group misaligned" panic in overlayContext.allocCC.

Added a test and verified it fails without this fix.

Fixes #3301.

Change-Id: Ifcfb4b8d38d6ccc84e47f2a9a5de36a1721548ba
Signed-off-by: Matthew Sackman <matthew@cue.works>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198356
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/testdata/eval/issue3301.txtar

@@ -0,0 +1,77 @@
+-- in.cue --
+object: #Leaf & {}
+
+#Base: {
+	extra?: {...}
+}
+
+#Mid: {
+	#Base
+}
+
+#Leaf: {
+	#Mid
+	extra?:  {...}
+	more?: int
+	if extra.foo != _|_ {
+		if more != _|_ {
+			foo: "bar"
+		}
+	}
+}
+-- out/eval --
+(struct){
+  object: (#struct){
+    extra?: (#struct){
+    }
+    more?: (int){ int }
+  }
+  #Base: (#struct){
+    extra?: (#struct){
+    }
+  }
+  #Mid: (#struct){
+    extra?: (#struct){
+    }
+  }
+  #Leaf: (#struct){
+    extra?: (#struct){
+    }
+    more?: (int){ int }
+  }
+}
+-- out/eval/stats --
+Leaks:  0
+Freed:  11
+Reused: 5
+Allocs: 6
+Retain: 7
+
+Unifications: 11
+Conjuncts:    25
+Disjuncts:    18
+-- out/compile --
+--- in.cue
+{
+  object: (〈0;#Leaf〉 & {})
+  #Base: {
+    extra?: {
+      ...
+    }
+  }
+  #Mid: {
+    〈1;#Base〉
+  }
+  #Leaf: {
+    〈1;#Mid〉
+    extra?: {
+      ...
+    }
+    more?: int
+    if (〈0;extra〉.foo != _|_(explicit error (_|_ literal) in source)) {
+      if (〈1;more〉 != _|_(explicit error (_|_ literal) in source)) {
+        foo: "bar"
+      }
+    }
+  }
+}

internal/core/adt/fields.go

@@ -433,17 +433,28 @@ func (cc *closeContext) linkNotify(ctx *OpContext, dst *Vertex, key *closeContex
 func (cc *closeContext) assignConjunct(ctx *OpContext, root *closeContext, c Conjunct, mode ArcType, check, checkClosed bool) (arc *closeContext, pos int, added bool) {
 	arc = cc.getKeyedCC(ctx, root, c.CloseInfo.CycleInfo, mode, checkClosed)
 
-	pos = len(*arc.group)
-
 	c.CloseInfo.cc = nil
-	added = !check || !hasConjunct(*arc.group, c)
+
+	var group ConjunctGroup
+	if arc.group != nil {
+		group = *arc.group
+	}
+	pos = len(group)
+
+	added = !check || !hasConjunct(group, c)
 	if added {
 		c.CloseInfo.cc = arc
 
 		if c.CloseInfo.cc.src != arc.src {
 			panic("Inconsistent src")
 		}
-		*arc.group = append(*arc.group, c)
+
+		group = append(group, c)
+		if arc.group == nil {
+			arc.group = &group
+		} else {
+			*arc.group = group
+		}
 	}
 
 	return arc, pos, added