internal/core/export: retain top in embedded scalars

Fixes #718

Change-Id: I9070a8621c513c7adc3d57d2ef7e650e26a308e6
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8566
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

internal/core/export/expr.go

@@ -290,7 +290,6 @@ func (e *conjuncts) addExpr(env *adt.Environment, x adt.Expr, isEmbed bool) {
 	case adt.Value: // other values.
 		switch v := x.(type) {
 		case nil:
-		case *adt.Top:
 		default:
 			e.values.AddConjunct(adt.MakeRootConjunct(env, x)) // GOBBLE TOP
 

internal/core/export/testdata/embedscalar.txtar

@@ -12,6 +12,15 @@ sub: {
         #baz: "foo"
     }
 }
+
+issue718: {
+	#fn: {
+		#a + #b
+		#a: int
+		#b: int
+	}
+	#fn & {_, #a: 1, #b: 2}
+}
 -- out/definition --
 #top: 4
 sub: {
@@ -26,6 +35,18 @@ sub: {
 		#baz: "foo"
 	}
 }
+issue718: {
+	#fn & {
+		_
+		#a: 1
+		#b: 2
+	}
+	#fn: {
+		#a + #b
+		#a: int
+		#b: int
+	}
+}
 -- out/doc --
 []
 [#top]
@@ -38,13 +59,20 @@ sub: {
 [sub b #baz]
 [sub b 0]
 [sub b 1]
+[issue718]
+[issue718 #a]
+[issue718 #b]
+[issue718 #fn]
+[issue718 #fn #a]
+[issue718 #fn #b]
 -- out/value --
 == Simplified
 {
 	sub: {
 		a: 3
 		b: [1, 2]
 	}
+	issue718: 3
 }
 == Raw
 {
@@ -61,13 +89,24 @@ sub: {
 			[1, 2]
 		}
 	}
+	issue718: {
+		3
+		#fn: {
+			#a + #b
+			#a: int
+			#b: int
+		}
+		#a: 1
+		#b: 2
+	}
 }
 == Final
 {
 	sub: {
 		a: 3
 		b: [1, 2]
 	}
+	issue718: 3
 }
 == All
 {
@@ -84,6 +123,16 @@ sub: {
 			[1, 2]
 		}
 	}
+	issue718: {
+		3
+		#fn: {
+			#a + #b
+			#a: int
+			#b: int
+		}
+		#a: 1
+		#b: 2
+	}
 }
 == Eval
 {
@@ -100,4 +149,14 @@ sub: {
 			[1, 2]
 		}
 	}
+	issue718: {
+		3
+		#fn: {
+			#a + #b
+			#a: int
+			#b: int
+		}
+		#a: 1
+		#b: 2
+	}
 }

internal/core/export/testdata/merge.txtar

@@ -22,11 +22,12 @@
 }
 -- out/definition --
 #A: {
+	_
 	#a: string
 }
-#E: {}
-#F: {}
-#G: {}
+#E: _
+#F: _
+#G: _
 #B: #A & {
 	#E & #F
 	#G