fix: added defaultVariant type validation for Resolve<T> and ResolveAll

core/pkg/evaluator/json.go

@@ -52,7 +52,7 @@ type flagdProperties struct {
 	Timestamp int64  `json:"timestamp"`
 }
 
-type variantEvaluator func(context.Context, string, string, map[string]any) (
+type variantEvaluator func(context.Context, string, string, map[string]any, string) (
 	variant string, variants map[string]interface{}, reason string, metadata map[string]interface{}, error error)
 
 // Deprecated - this will be remove in the next release
@@ -283,7 +283,8 @@ func (je *Resolver) ResolveAsAnyValue(
 func resolve[T constraints](ctx context.Context, reqID string, key string, context map[string]any,
 	variantEval variantEvaluator) (value T, variant string, reason string, metadata map[string]interface{}, err error,
 ) {
-	variant, variants, reason, metadata, err := variantEval(ctx, reqID, key, context)
+	resolveType := getTypeName(value)
+	variant, variants, reason, metadata, err := variantEval(ctx, reqID, key, context, resolveType)
 	if err != nil {
 		return value, variant, reason, metadata, err
 	}
@@ -298,7 +299,13 @@ func resolve[T constraints](ctx context.Context, reqID string, key string, conte
 }
 
 // nolint: funlen
-func (je *Resolver) evaluateVariant(ctx context.Context, reqID string, flagKey string, evalCtx map[string]any) (
+func (je *Resolver) evaluateVariant(
+	ctx context.Context,
+	reqID string,
+	flagKey string,
+	evalCtx map[string]any,
+	resolveType string,
+) (
 	variant string, variants map[string]interface{}, reason string, metadata map[string]interface{}, err error,
 ) {
 	flag, metadata, ok := je.store.Get(ctx, flagKey)
@@ -326,6 +333,13 @@ func (je *Resolver) evaluateVariant(ctx context.Context, reqID string, flagKey s
 		return "", flag.Variants, model.ErrorReason, metadata, errors.New(model.FlagDisabledErrorCode)
 	}
 
+	defaultValue := flag.Variants[flag.DefaultVariant]
+	defaultValueType := getTypeName(defaultValue)
+
+	if resolveType != "" && defaultValueType != resolveType {
+		return "", flag.Variants, model.ErrorReason, metadata, errors.New(model.TypeMismatchErrorCode)
+	}
+
 	// get the targeting logic, if any
 	targeting := flag.Targeting
 
@@ -386,6 +400,20 @@ func (je *Resolver) evaluateVariant(ctx context.Context, reqID string, flagKey s
 	return flag.DefaultVariant, flag.Variants, model.StaticReason, metadata, nil
 }
 
+func getTypeName(value any) string {
+	switch value.(type) {
+	case bool:
+		return "bool"
+	case string:
+		return "string"
+	case float64:
+		return "double"
+	case map[string]any:
+		return "object"
+	}
+	return ""
+}
+
 func setFlagdProperties(
 	log *logger.Logger,
 	context map[string]any,