internal/core/eval: handle embedded top correctly

Other basic types were already correctly handled,
but top slipped through the maze.

Uncovered while debugging Issue #758

Change-Id: Iee99e0a0c5e8c1a7b2faa02b484c64f0e549391f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8707
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>

cue/testdata/eval/basictypes.txtar

@@ -0,0 +1,49 @@
+-- in.cue --
+top: {
+    a: {_}
+    b: _
+}
+
+scalar: {
+    a: {string}
+    b: string
+    c: {{{number}}}
+    d: number
+}
+-- out/eval --
+(struct){
+  top: (struct){
+    a: (_){ _ }
+    b: (_){ _ }
+  }
+  scalar: (struct){
+    a: (string){ string }
+    b: (string){ string }
+    c: (number){ number }
+    d: (number){ number }
+  }
+}
+-- out/compile --
+--- in.cue
+{
+  top: {
+    a: {
+      _
+    }
+    b: _
+  }
+  scalar: {
+    a: {
+      string
+    }
+    b: string
+    c: {
+      {
+        {
+          number
+        }
+      }
+    }
+    d: number
+  }
+}

internal/core/adt/eval.go

@@ -1082,11 +1082,6 @@ func (n *nodeContext) getValidators() BaseValue {
 		// Src is the combined input.
 		v = &BasicType{K: n.kind}
 
-		if len(n.node.Structs) > 0 {
-			v = structSentinel
-
-		}
-
 	case 1:
 		v = a[0].(Value) // remove cast
 

internal/core/export/testdata/merge.txtar

@@ -54,11 +54,12 @@
 == Raw
 {
 	#A: {
+		_
 		#a: string
 	}
-	#E: {}
-	#F: {}
-	#G: {}
+	#E: _
+	#F: _
+	#G: _
 	#B: {
 		2
 		#a:   string
@@ -72,11 +73,12 @@
 == All
 {
 	#A: {
+		_
 		#a: string
 	}
-	#E: {}
-	#F: {}
-	#G: {}
+	#E: _
+	#F: _
+	#G: _
 	#B: {
 		2
 		#a:   string
@@ -88,11 +90,12 @@
 == Eval
 {
 	#A: {
+		_
 		#a: string
 	}
-	#E: {}
-	#F: {}
-	#G: {}
+	#E: _
+	#F: _
+	#G: _
 	#B: {
 		2
 		#a:   string