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Fix IsZero() panic #38

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Merged
alecthomas merged 2 commits into from
May 1, 2026
Merged

Fix IsZero() panic #38

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7721bea
chore: add failing test case for IsZero() panic
smlx Jan 5, 2026
60aa893
fix: avoid panic due to nil dereference in IsZero() call
smlx Jan 5, 2026
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14 changes: 12 additions & 2 deletions repr.go
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Original file line number Diff line number Diff line change
Expand Up @@ -299,8 +299,18 @@ func (p *Printer) reprValue(seen map[reflect.Value]bool, v reflect.Value, indent
continue
}

if p.omitZero && ((ft.Implements(isZeroerType) && f.CanInterface() && f.Interface().(isZeroer).IsZero()) || f.IsZero()) {
continue
if p.omitZero {
// check if this type is a nil pointer to a type implementing IsZero
// with a value receiver and, if so, avoid calling IsZero() on it as
// the method call will automatically dereference the nil pointer and
// panic.
var nilPtrValueReceiver bool
if ft.Kind() == reflect.Pointer && f.IsNil() {
_, nilPtrValueReceiver = ft.Elem().MethodByName("IsZero")
}
if (ft.Implements(isZeroerType) && !nilPtrValueReceiver && f.CanInterface() && f.Interface().(isZeroer).IsZero()) || f.IsZero() {
continue
}
}

if p.omitEmpty && (f.IsZero() ||
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5 changes: 3 additions & 2 deletions repr_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -81,8 +81,9 @@ func TestReprZeroSliceMapFields(t *testing.T) {
B bool
ZC zeroTest
C zeroTest
}{[]string{}, []string{"a", "b"}, map[string]string{}, map[string]string{"a": "b"}, 0, 1, "", "a", false, true, zeroTest{i: 100}, zeroTest{i: 10}}
equal(t, `struct { ZSl []string; Sl []string; ZM map[string]string; M map[string]string; ZI int; I int; ZS string; S string; ZB bool; B bool; ZC repr.zeroTest; C repr.zeroTest }{ZSl: []string{}, Sl: []string{"a", "b"}, ZM: map[string]string{}, M: map[string]string{"a": "b"}, I: 1, S: "a", B: true, C: repr.zeroTest{i: 10}}`, String(v, OmitEmpty(false), OmitZero(true)))
T *time.Time
}{[]string{}, []string{"a", "b"}, map[string]string{}, map[string]string{"a": "b"}, 0, 1, "", "a", false, true, zeroTest{i: 100}, zeroTest{i: 10}, nil}
equal(t, `struct { ZSl []string; Sl []string; ZM map[string]string; M map[string]string; ZI int; I int; ZS string; S string; ZB bool; B bool; ZC repr.zeroTest; C repr.zeroTest; T *time.Time }{ZSl: []string{}, Sl: []string{"a", "b"}, ZM: map[string]string{}, M: map[string]string{"a": "b"}, I: 1, S: "a", B: true, C: repr.zeroTest{i: 10}}`, String(v, OmitEmpty(false), OmitZero(true)))
}

func TestReprStringArray(t *testing.T) {
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