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fix(form): correctly differentiate between nil / present-but-empty slices #4498

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fix(form): correctly differentiate between nil / present-but-empty slices #4498

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7b7f800
correctly differentiate between nil / present-but-empty slices
NyaaaWhatsUpDoc Jan 12, 2026
47a2602
fix broken tests
NyaaaWhatsUpDoc Feb 5, 2026
cafe8d9
Merge branch 'master' into bugfix/form-array-binding-regression
appleboy Feb 13, 2026
e7abd0b
Merge branch 'master' into bugfix/form-array-binding-regression
appleboy Feb 28, 2026
c406e6d
Merge branch 'master' into bugfix/form-array-binding-regression
appleboy Mar 3, 2026
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12 changes: 9 additions & 3 deletions binding/form_mapping.go
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Original file line number Diff line number Diff line change
Expand Up @@ -250,19 +250,24 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][

switch value.Kind() {
case reflect.Slice:
if len(vs) == 0 {
if vs == nil {
if !opt.isDefaultExists {
return false, nil
}

vs = []string{opt.defaultValue}
// pre-process the default value for multi if present
cfTag := field.Tag.Get("collection_format")
if cfTag == "" || cfTag == "multi" {
vs = strings.Split(opt.defaultValue, ",")
} else {
vs = []string{opt.defaultValue}
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@NyaaaWhatsUpDoc NyaaaWhatsUpDoc Jan 13, 2026

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this isn't part of the fix (nor is it in the array part below), but it just means vs only needs to get written to once instead of twice in the case of cfTag == "" | cfTag == "multi"

}
}

if len(vs) == 0 {
return true, setSlice(vs, value, field, opt)
}

if ok, err = trySetUsingParser(vs[0], value, opt.parser); ok {
return ok, err
} else if ok, err = trySetCustom(vs[0], value); ok {
Expand All @@ -280,11 +285,12 @@ func setByForm(value reflect.Value, field reflect.StructField, form map[string][
return false, nil
}

vs = []string{opt.defaultValue}
// pre-process the default value for multi if present
cfTag := field.Tag.Get("collection_format")
if cfTag == "" || cfTag == "multi" {
vs = strings.Split(opt.defaultValue, ",")
} else {
vs = []string{opt.defaultValue}
}
}

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19 changes: 12 additions & 7 deletions binding/form_mapping_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -915,7 +915,7 @@ func TestMappingCustomSliceOfSliceUnmarshalTextDefault(t *testing.T) {
var s struct {
FileData []customPathUnmarshalText `form:"path,default=bar/foo;bar/foo/spam,parser=encoding.TextUnmarshaler" collection_format:"csv"`
}
err := mappingByPtr(&s, formSource{"path": {}}, "form")
err := mappingByPtr(&s, formSource{"path": nil}, "form")
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@NyaaaWhatsUpDoc NyaaaWhatsUpDoc Feb 5, 2026

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previously this was relying on an empty (but not nil!) value slice to rely on default, but an empty slice isn't the zero value. a nil slice is.

require.NoError(t, err)
assert.Equal(t, []customPathUnmarshalText{{"bar", "foo"}, {"bar", "foo", "spam"}}, s.FileData)
}
Expand Down Expand Up @@ -994,7 +994,7 @@ func TestMappingCustomArrayOfArrayUnmarshalTextDefault(t *testing.T) {
var s struct {
FileData []objectIDUnmarshalText `form:"ids,default=664a062ac74a8ad104e0e80e;664a062ac74a8ad104e0e80f,parser=encoding.TextUnmarshaler" collection_format:"csv"`
}
err := mappingByPtr(&s, formSource{"ids": {}}, "form")
err := mappingByPtr(&s, formSource{"ids": nil}, "form")
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@NyaaaWhatsUpDoc NyaaaWhatsUpDoc Feb 5, 2026

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previously this was relying on an empty (but not nil!) value slice to rely on default, but an empty slice isn't the zero value. a nil slice is.

require.NoError(t, err)
assert.Equal(t, []objectIDUnmarshalText{id1, id2}, s.FileData)
}
Expand Down Expand Up @@ -1079,7 +1079,7 @@ func TestMappingEmptyValues(t *testing.T) {
// field present but empty
err = mappingByPtr(&s, formSource{"slice": {}}, "form")
require.NoError(t, err)
assert.Equal(t, []int{5}, s.Slice)
assert.Equal(t, []int{}, s.Slice)

// field present with values
err = mappingByPtr(&s, formSource{"slice": {"1", "2", "3"}}, "form")
Expand Down Expand Up @@ -1108,10 +1108,15 @@ func TestMappingEmptyValues(t *testing.T) {
Slice []int `form:"slice"`
}

// field present but empty
err := mappingByPtr(&s, formSource{"slice": {}}, "form")
// field not present
err := mappingByPtr(&s, formSource{}, "form")
require.NoError(t, err)
assert.Equal(t, []int(nil), s.Slice)

// field present but empty
err = mappingByPtr(&s, formSource{"slice": {}}, "form")
require.NoError(t, err)
assert.Equal(t, []int{}, s.Slice)
})

t.Run("array without default", func(t *testing.T) {
Expand Down Expand Up @@ -1140,7 +1145,7 @@ func TestMappingEmptyValues(t *testing.T) {
// field present but empty
err = mappingByPtr(&s, formSource{"slice_multi": {}, "slice_csv": {}}, "form")
require.NoError(t, err)
assert.Equal(t, []int{1, 2, 3}, s.SliceMulti)
assert.Equal(t, []int{1, 2, 3}, s.SliceCsv)
assert.Equal(t, []int{}, s.SliceMulti)
assert.Equal(t, []int{}, s.SliceCsv)
})
}