helper/schema: Get on computed diff should be empty

helper/schema/resource_data.go

@@ -558,20 +558,25 @@ func (d *ResourceData) getPrimitive(
 
 	if d.diff != nil && source >= getSourceDiff {
 		attrD, ok := d.diff.Attributes[k]
-		if ok && !attrD.NewComputed {
-			result = attrD.New
-			if attrD.NewExtra != nil {
-				// If NewExtra != nil, then we have processed data as the New,
-				// so we store that but decode the unprocessed data into result
-				resultProcessed = result
-
-				err := mapstructure.WeakDecode(attrD.NewExtra, &result)
-				if err != nil {
-					panic(err)
+		if ok {
+			if !attrD.NewComputed {
+				result = attrD.New
+				if attrD.NewExtra != nil {
+					// If NewExtra != nil, then we have processed data as the New,
+					// so we store that but decode the unprocessed data into result
+					resultProcessed = result
+
+					err := mapstructure.WeakDecode(attrD.NewExtra, &result)
+					if err != nil {
+						panic(err)
+					}
 				}
-			}
 
-			resultSet = true
+				resultSet = true
+			} else {
+				result = ""
+				resultSet = false
+			}
 		}
 	}
 

helper/schema/resource_data_test.go

@@ -30,8 +30,8 @@ func TestResourceDataGet(t *testing.T) {
 			Diff: &terraform.ResourceDiff{
 				Attributes: map[string]*terraform.ResourceAttrDiff{
 					"availability_zone": &terraform.ResourceAttrDiff{
-						Old:         "",
-						New:         "",
+						Old:         "foo",
+						New:         "bar",
 						NewComputed: true,
 					},
 				},
@@ -117,6 +117,36 @@ func TestResourceDataGet(t *testing.T) {
 			Value: "bar",
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"availability_zone": &Schema{
+					Type:     TypeString,
+					Optional: true,
+					Computed: true,
+					ForceNew: true,
+				},
+			},
+
+			State: &terraform.ResourceState{
+				Attributes: map[string]string{
+					"availability_zone": "foo",
+				},
+			},
+
+			Diff: &terraform.ResourceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"availability_zone": &terraform.ResourceAttrDiff{
+						Old:         "foo",
+						New:         "bar",
+						NewComputed: true,
+					},
+				},
+			},
+
+			Key:   "availability_zone",
+			Value: "",
+		},
+
 		{
 			Schema: map[string]*Schema{
 				"port": &Schema{