reflect: allow conversion from slice to array ptr

Note that this removes an invariant:

v.Type().ConvertibleTo(t) might return true,
yet v.Convert(t) might panic nevertheless.

This is a fairly unavoidable consequence of the decision
to add the first-ever conversion that can panic.

ConvertibleTo describes a relationship between types,
but whether the conversion panics now depends on the value,
not just the type.

If this turns out to be a problem, we can add v.ConvertibleTo(t),
or something similar, to allow callers to avoid the panic.

This is the last of the changes needed to complete the implementation.

Fixes #395

Change-Id: I79b7e4dd87a67a47723e00a65d0b1ac6090371b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/301652
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/reflect/all_test.go

@@ -3822,6 +3822,10 @@ type MyStruct2 struct {
 }
 type MyString string
 type MyBytes []byte
+type MyBytesArrayPtr0 *[0]byte
+type MyBytesArrayPtr *[4]byte
+type MyBytesArray0 [0]byte
+type MyBytesArray [4]byte
 type MyRunes []int32
 type MyFunc func()
 type MyByte byte
@@ -4128,6 +4132,30 @@ var convertTests = []struct {
 	{V(MyString("runes♝")), V(MyRunes("runes♝"))},
 	{V(MyRunes("runes♕")), V(MyString("runes♕"))},
 
+	// slice to array pointer
+	{V([]byte(nil)), V((*[0]byte)(nil))},
+	{V([]byte{}), V(new([0]byte))},
+	{V([]byte{7}), V(&[1]byte{7})},
+	{V(MyBytes([]byte(nil))), V((*[0]byte)(nil))},
+	{V(MyBytes([]byte{})), V(new([0]byte))},
+	{V(MyBytes([]byte{9})), V(&[1]byte{9})},
+	{V([]byte(nil)), V(MyBytesArrayPtr0(nil))},
+	{V([]byte{}), V(MyBytesArrayPtr0(new([0]byte)))},
+	{V([]byte{1, 2, 3, 4}), V(MyBytesArrayPtr(&[4]byte{1, 2, 3, 4}))},
+	{V(MyBytes([]byte{})), V(MyBytesArrayPtr0(new([0]byte)))},
+	{V(MyBytes([]byte{5, 6, 7, 8})), V(MyBytesArrayPtr(&[4]byte{5, 6, 7, 8}))},
+
+	{V([]byte(nil)), V((*MyBytesArray0)(nil))},
+	{V([]byte{}), V((*MyBytesArray0)(new([0]byte)))},
+	{V([]byte{1, 2, 3, 4}), V(&MyBytesArray{1, 2, 3, 4})},
+	{V(MyBytes([]byte(nil))), V((*MyBytesArray0)(nil))},
+	{V(MyBytes([]byte{})), V((*MyBytesArray0)(new([0]byte)))},
+	{V(MyBytes([]byte{5, 6, 7, 8})), V(&MyBytesArray{5, 6, 7, 8})},
+	{V(new([0]byte)), V(new(MyBytesArray0))},
+	{V(new(MyBytesArray0)), V(new([0]byte))},
+	{V(MyBytesArrayPtr0(nil)), V((*[0]byte)(nil))},
+	{V((*[0]byte)(nil)), V(MyBytesArrayPtr0(nil))},
+
 	// named types and equal underlying types
 	{V(new(int)), V(new(integer))},
 	{V(new(integer)), V(new(int))},
@@ -4288,6 +4316,9 @@ func TestConvert(t *testing.T) {
 		if vout2.Type() != tt.out.Type() || !DeepEqual(out2, tt.out.Interface()) {
 			t.Errorf("ValueOf(%T(%[1]v)).Convert(%s) = %T(%[3]v), want %T(%[4]v)", tt.in.Interface(), t2, out2, tt.out.Interface())
 		}
+		if got, want := vout2.Kind(), vout2.Type().Kind(); got != want {
+			t.Errorf("ValueOf(%T(%[1]v)).Convert(%s) has internal kind %v want %v", tt.in.Interface(), t1, got, want)
+		}
 
 		// vout3 represents a new value of the out type, set to vout2.  This makes
 		// sure the converted value vout2 is really usable as a regular value.
@@ -4332,6 +4363,19 @@ func TestConvert(t *testing.T) {
 	}
 }
 
+func TestConvertPanic(t *testing.T) {
+	s := make([]byte, 4)
+	p := new([8]byte)
+	v := ValueOf(s)
+	pt := TypeOf(p)
+	if !v.Type().ConvertibleTo(pt) {
+		t.Errorf("[]byte should be convertible to *[8]byte")
+	}
+	shouldPanic("reflect: cannot convert slice with length 4 to array pointer with length 8", func() {
+		_ = v.Convert(pt)
+	})
+}
+
 var gFloat32 float32
 
 func TestConvertNaNs(t *testing.T) {

src/reflect/type.go

@@ -106,9 +106,11 @@ type Type interface {
 	AssignableTo(u Type) bool
 
 	// ConvertibleTo reports whether a value of the type is convertible to type u.
+	// Even if ConvertibleTo returns true, the conversion may still panic.
 	ConvertibleTo(u Type) bool
 
 	// Comparable reports whether values of this type are comparable.
+	// Even if Comparable returns true, the comparison may still panic.
 	Comparable() bool
 
 	// Methods applicable only to some types, depending on Kind.

src/reflect/value.go

@@ -6,6 +6,7 @@ package reflect
 
 import (
 	"internal/abi"
+	"internal/itoa"
 	"internal/unsafeheader"
 	"math"
 	"runtime"
@@ -2770,7 +2771,7 @@ func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value
 
 // Convert returns the value v converted to type t.
 // If the usual Go conversion rules do not allow conversion
-// of the value v to type t, Convert panics.
+// of the value v to type t, or if converting v to type t panics, Convert panics.
 func (v Value) Convert(t Type) Value {
 	if v.flag&flagMethod != 0 {
 		v = makeMethodValue("Convert", v)
@@ -2841,6 +2842,11 @@ func convertOp(dst, src *rtype) func(Value, Type) Value {
 				return cvtRunesString
 			}
 		}
+		// "x is a slice, T is a pointer-to-array type,
+		// and the slice and array types have identical element types."
+		if dst.Kind() == Ptr && dst.Elem().Kind() == Array && src.Elem() == dst.Elem().Elem() {
+			return cvtSliceArrayPtr
+		}
 
 	case Chan:
 		if dst.Kind() == Chan && specialChannelAssignability(dst, src) {
@@ -3034,6 +3040,16 @@ func cvtStringRunes(v Value, t Type) Value {
 	return makeRunes(v.flag.ro(), []rune(v.String()), t)
 }
 
+// convertOp: []T -> *[N]T
+func cvtSliceArrayPtr(v Value, t Type) Value {
+	n := t.Elem().Len()
+	h := (*unsafeheader.Slice)(v.ptr)
+	if n > h.Len {
+		panic("reflect: cannot convert slice with length " + itoa.Itoa(h.Len) + " to array pointer with length " + itoa.Itoa(n))
+	}
+	return Value{t.common(), h.Data, v.flag&^(flagIndir|flagAddr|flagKindMask) | flag(Ptr)}
+}
+
 // convertOp: direct copy
 func cvtDirect(v Value, typ Type) Value {
 	f := v.flag