Return error on unset variant.

candid/idl/encode_test.go

@@ -31,3 +31,20 @@ func TestEncode_issue7(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestVariantType_default(t *testing.T) {
+	type V = struct {
+		A *Null `ic:"A,variant"`
+		B *Null `ic:"B,variant"`
+	}
+	raw, err := Marshal([]any{
+		V{},
+	})
+	if err == nil {
+		t.Error("expected error")
+	}
+	var v V
+	if err := Unmarshal(raw, []any{&v}); err == nil {
+		t.Error("expected error")
+	}
+}

candid/idl/variant.go

@@ -136,7 +136,7 @@ func (variant VariantType) EncodeValue(value any) ([]byte, error) {
 			return concat(id, v_), nil
 		}
 	}
-	return nil, fmt.Errorf("unknown variant: %variant", value)
+	return nil, fmt.Errorf("unknown variant: %v", value)
 }
 
 func (variant VariantType) String() string {
@@ -220,10 +220,7 @@ func (variant VariantType) structToVariant(value any) (*Variant, error) {
 				}, nil
 			}
 			if i == v.NumField()-1 {
-				return &Variant{
-					Name:  tag.Name,
-					Value: nil,
-				}, nil
+				return nil, fmt.Errorf("invalid variant: no variant selected")
 			}
 		}
 		return nil, fmt.Errorf("invalid variant: %s", v.Type())