cue: try harder to find a position for Pos

Fixes #1459

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: I31ddee08e9db3d2c0e77a1ac91b9407994f2cd45
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/533468
Reviewed-by: Paul Jolly <paul@myitcv.io>

Author: mpvl

cue/types.go

@@ -1107,12 +1107,33 @@ func (v Value) Err() error {
 }
 
 // Pos returns position information.
+//
+// Use v.Expr to get positions for all conjuncts and disjuncts.
 func (v Value) Pos() token.Pos {
-	if v.v == nil || v.Source() == nil {
+	if v.v == nil {
 		return token.NoPos
 	}
-	pos := v.Source().Pos()
-	return pos
+
+	if src := v.Source(); src != nil {
+		if pos := src.Pos(); pos != token.NoPos {
+			return pos
+		}
+	}
+	// Pick the most-concrete field.
+	var p token.Pos
+	for _, c := range v.v.Conjuncts {
+		x := c.Elem()
+		pp := pos(x)
+		if pp == token.NoPos {
+			continue
+		}
+		p = pp
+		// Prefer struct conjuncts with actual fields.
+		if s, ok := x.(*adt.StructLit); ok && len(s.Fields) > 0 {
+			break
+		}
+	}
+	return p
 }
 
 // TODO: IsFinal: this value can never be changed.

cue/types_test.go

@@ -2918,6 +2918,51 @@ func TestReferencePath(t *testing.T) {
 	}
 }
 
+func TestPos(t *testing.T) {
+	testCases := []struct {
+		value string
+		pos   string
+	}{{
+		value: `
+a: string
+a: "foo"`,
+		pos: "3:4",
+	}, {
+		value: `
+a: x: string
+a: x: "x"`,
+		pos: "2:4",
+	}, {
+		// Prefer struct conjuncts with actual fields.
+		value: `
+a: [string]: string
+a: x: "x"`,
+		pos: "3:4",
+	}, {
+		value: `
+a: [string]: [string]: string
+a: x: y: "x"`,
+		pos: "3:4",
+	}, {
+		value: `
+a: [string]: [string]: [string]: string
+a: x: y: z: "x"`,
+		pos: "3:4",
+	}}
+	for _, tc := range testCases {
+		t.Run("", func(t *testing.T) {
+			var c Context
+			c.runtime().Init()
+			v := c.CompileString(tc.value)
+			v = v.LookupPath(ParsePath("a"))
+			pos := v.Pos().String()
+			if pos != tc.pos {
+				t.Errorf("got %v; want %v", pos, tc.pos)
+			}
+		})
+	}
+}
+
 func TestPathCorrection(t *testing.T) {
 	testCases := []struct {
 		input  string