Sanity check for empty variables

query/query.go

@@ -1678,9 +1678,16 @@ func (sg *SubGraph) fillVars(mp map[string]varValue) error {
 			default:
 				// This var does not match any uids or vals but we are still trying to access it.
 				if v.Typ == gql.ValueVar {
+					// * * * * * * * * * * * * * * * * * * *
+					// Default value vars
+					// * * * * * * * * * * * * * * * * * * *
+					//
 					// Provide a default value for valueVarAggregation() to eval val().
 					// This is a noop for aggregation funcs that would fail.
 					// The zero aggs won't show because there are no uids matched.
+					//
+					// NOTE: If you need to make type assertions that might involve
+					// default value vars, use `Safe()` func and not val.Value directly.
 					mp[v.Name].Vals[0] = types.Val{}
 					sg.Params.uidToVal = mp[v.Name].Vals
 				}

types/conversion.go

@@ -564,7 +564,7 @@ func (v Val) MarshalJSON() ([]byte, error) {
 	case GeoID:
 		return geojson.Marshal(v.Value.(geom.T))
 	case StringID, DefaultID:
-		return json.Marshal(v.Value.(string))
+		return json.Marshal(v.Safe().(string))
 	case PasswordID:
 		return json.Marshal(v.Value.(string))
 	}

types/scalar_types.go

@@ -94,6 +94,20 @@ type Val struct {
 	Value interface{}
 }
 
+// Safe ensures that Val's Value is not nil. This is useful when doing type
+// assertions and default values might be involved.
+// This function won't change the original v.Value, may it be nil.
+// See: "Default value vars" in `fillVars()`
+// Returns a safe v.Value suitable for type assertions.
+func (v Val) Safe() interface{} {
+	if v.Value == nil {
+		// get zero value for this v.Tid
+		va := ValueForType(v.Tid)
+		return va.Value
+	}
+	return v.Value
+}
+
 // TypeForName returns the type corresponding to the given name.
 // If name is not recognized, it returns nil.
 func TypeForName(name string) (TypeID, bool) {

types/sort.go

@@ -133,7 +133,7 @@ func less(a, b Val) bool {
 	case UidID:
 		return (a.Value.(uint64) < b.Value.(uint64))
 	case StringID, DefaultID:
-		return (a.Value.(string)) < (b.Value.(string))
+		return (a.Safe().(string)) < (b.Safe().(string))
 	}
 	return false
 }