internal/cue/adt: remove use of Default in node

It is not necessary to do this. This is mainly done to
close open lists. But this only matters for
comprehensions, which will handle open lists anyway.
So all this does is extra work and potentially introduce
a bug if the Vertex pointer is used for dereferencing
or caching.

Change-Id: I45102393f53673e1bb8a77c6a4709af242d4e976
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7781
Reviewed-by: Marcel van Lohuizen <[email protected]>
Reviewed-by: CUE cueckoo <[email protected]>

Author: mpvl

internal/core/adt/context.go

@@ -709,7 +709,8 @@ func (c *OpContext) node(x Expr, scalar bool) *Vertex {
 			return emptyNode
 		}
 	}
-	return node.Default()
+
+	return node
 }
 
 // Elems returns the elements of a list.

internal/core/adt/default.go

@@ -42,6 +42,8 @@ func (d *Disjunction) Default() Value {
 }
 
 // Default returns the default value or itself if there is no default.
+//
+// It also closes a list, representing its default value.
 func (v *Vertex) Default() *Vertex {
 	switch d := v.BaseValue.(type) {
 	default: