reflect: make Field panic when out of bounds, as documented

Fixes #15046.

Change-Id: Iba7216297735be8e1ec550ce5336d17dcd3fd6b7
Reviewed-on: https://go-review.googlesource.com/22992
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/all_test.go

@@ -5221,6 +5221,41 @@ func TestLargeGCProg(t *testing.T) {
 	fv.Call([]Value{ValueOf([256]*byte{})})
 }
 
+func fieldIndexRecover(t Type, i int) (recovered interface{}) {
+	defer func() {
+		recovered = recover()
+	}()
+
+	t.Field(i)
+	return
+}
+
+// Issue 15046.
+func TestTypeFieldOutOfRangePanic(t *testing.T) {
+	typ := TypeOf(struct{ X int }{10})
+	testIndices := [...]struct {
+		i         int
+		mustPanic bool
+	}{
+		0: {-2, true},
+		1: {0, false},
+		2: {1, true},
+		3: {1 << 10, true},
+	}
+	for i, tt := range testIndices {
+		recoveredErr := fieldIndexRecover(typ, tt.i)
+		if tt.mustPanic {
+			if recoveredErr == nil {
+				t.Errorf("#%d: fieldIndex %d expected to panic", i, tt.i)
+			}
+		} else {
+			if recoveredErr != nil {
+				t.Errorf("#%d: got err=%v, expected no panic", i, recoveredErr)
+			}
+		}
+	}
+}
+
 // Issue 9179.
 func TestCallGC(t *testing.T) {
 	f := func(a, b, c, d, e string) {

src/reflect/type.go

@@ -1178,7 +1178,7 @@ func (tag StructTag) Lookup(key string) (value string, ok bool) {
 // Field returns the i'th struct field.
 func (t *structType) Field(i int) (f StructField) {
 	if i < 0 || i >= len(t.fields) {
-		return
+		panic("reflect: Field index out of bounds")
 	}
 	p := &t.fields[i]
 	f.Type = toType(p.typ)