helper/schema: reading a diff with computed should return zero value

helper/schema/field_reader_diff.go

@@ -76,17 +76,14 @@ func (r *DiffFieldReader) readPrimitive(
 	if !ok {
 		return FieldReadResult{}, nil
 	}
-	if attrD.NewComputed {
-		return FieldReadResult{
-			Exists:   true,
-			Computed: true,
-		}, nil
-	}
 
-	result := attrD.New
-	if attrD.NewExtra != nil {
-		if err := mapstructure.WeakDecode(attrD.NewExtra, &result); err != nil {
-			return FieldReadResult{}, err
+	var result string
+	if !attrD.NewComputed {
+		result = attrD.New
+		if attrD.NewExtra != nil {
+			if err := mapstructure.WeakDecode(attrD.NewExtra, &result); err != nil {
+				return FieldReadResult{}, err
+			}
 		}
 	}
 
@@ -96,8 +93,9 @@ func (r *DiffFieldReader) readPrimitive(
 	}
 
 	return FieldReadResult{
-		Value:  returnVal,
-		Exists: true,
+		Value:    returnVal,
+		Exists:   true,
+		Computed: attrD.NewComputed,
 	}, nil
 }
 

helper/schema/field_reader_diff_test.go

@@ -30,6 +30,12 @@ func TestDiffFieldReader(t *testing.T) {
 					New: "string",
 				},
 
+				"stringComputed": &terraform.ResourceAttrDiff{
+					Old:         "foo",
+					New:         "bar",
+					NewComputed: true,
+				},
+
 				"list.#": &terraform.ResourceAttrDiff{
 					Old: "0",
 					New: "2",
@@ -172,6 +178,17 @@ func TestDiffFieldReader(t *testing.T) {
 			false,
 		},
 
+		"stringComputed": {
+			[]string{"stringComputed"},
+			&Schema{Type: TypeString},
+			FieldReadResult{
+				Value:    "",
+				Exists:   true,
+				Computed: true,
+			},
+			false,
+		},
+
 		"list": {
 			[]string{"list"},
 			&Schema{