From b4d09053db1ba73dc0a5184caf162befd2e91373 Mon Sep 17 00:00:00 2001
From: Pratham Gadkari <prathamgadkari@gmail.com>
Date: Fri, 14 Nov 2025 23:44:50 +0530
Subject: [PATCH 1/2] fix(binding): support custom unmarshaler for slices &
 arrays

---
 binding/form_mapping.go | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/binding/form_mapping.go b/binding/form_mapping.go
index 1244b522..6a07330a 100644
--- a/binding/form_mapping.go
+++ b/binding/form_mapping.go
@@ -186,7 +186,10 @@ type BindUnmarshaler interface {
 func trySetCustom(val string, value reflect.Value) (isSet bool, err error) {
 	switch v := value.Addr().Interface().(type) {
 	case BindUnmarshaler:
-		return true, v.UnmarshalParam(val)
+		if err := v.UnmarshalParam(val); err != nil {
+			return true, fmt.Errorf("invalid value %q: %w", val, err)
+		}
+		return true, nil
 	}
 	return false, nil
 }
@@ -245,7 +248,10 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 		}
 
 		if ok, err = trySetCustom(vs[0], value); ok {
-			return ok, err
+			if err != nil {
+				return true, fmt.Errorf("field %q: %w", field.Name, err)
+			}
+			return true, nil
 		}
 
 		if vs, err = trySplit(vs, field); err != nil {
@@ -268,7 +274,10 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 		}
 
 		if ok, err = trySetCustom(vs[0], value); ok {
-			return ok, err
+			if err != nil {
+				return true, fmt.Errorf("field %q: %w", field.Name, err)
+			}
+			return true, nil
 		}
 
 		if vs, err = trySplit(vs, field); err != nil {
@@ -293,7 +302,10 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
 			}
 		}
 		if ok, err := trySetCustom(val, value); ok {
-			return ok, err
+			if err != nil {
+				return true, fmt.Errorf("field %q: %w", field.Name, err)
+			}
+			return true, nil
 		}
 		return true, setWithProperType(val, value, field)
 	}

From 513a86e17fa3ffed325c7f4f41fa3a919ce32afd Mon Sep 17 00:00:00 2001
From: Pratham Gadkari <prathamgadkari@gmail.com>
Date: Sat, 15 Nov 2025 22:28:32 +0530
Subject: [PATCH 2/2] added tests for feature and coverage

---
 binding/form_mapping_test.go | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/binding/form_mapping_test.go b/binding/form_mapping_test.go
index 006eddf1..d0086d31 100644
--- a/binding/form_mapping_test.go
+++ b/binding/form_mapping_test.go
@@ -715,3 +715,22 @@ func TestMappingEmptyValues(t *testing.T) {
 		assert.Equal(t, []int{1, 2, 3}, s.SliceCsv)
 	})
 }
+
+type testCustom struct {
+	Value string
+}
+
+func (t *testCustom) UnmarshalParam(param string) error {
+	t.Value = "prefix_" + param
+	return nil
+}
+
+func TestTrySetCustomIntegration(t *testing.T) {
+	var s struct {
+		F testCustom `form:"f"`
+	}
+
+	err := mappingByPtr(&s, formSource{"f": {"hello"}}, "form")
+	require.NoError(t, err)
+	assert.Equal(t, "prefix_hello", s.F.Value)
+}