reflect: add Value.CanConvert

For #395
For #46746

Change-Id: I4bfc094cf1cecd27ce48e31f92384cf470f371a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/334669
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

api/go1.17.txt

@@ -80,6 +80,7 @@ pkg net/url, method (Values) Has(string) bool
 pkg reflect, func VisibleFields(Type) []StructField
 pkg reflect, method (Method) IsExported() bool
 pkg reflect, method (StructField) IsExported() bool
+pkg reflect, method (Value) CanConvert(Type) bool
 pkg runtime/cgo (darwin-amd64-cgo), func NewHandle(interface{}) Handle
 pkg runtime/cgo (darwin-amd64-cgo), method (Handle) Delete()
 pkg runtime/cgo (darwin-amd64-cgo), method (Handle) Value() interface{}

doc/go1.17.html

@@ -989,6 +989,18 @@ func Foo() bool {
 
 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
   <dd>
+    <p><!-- CL 334669 -->
+      The new
+      <a href="/pkg/reflect/#Value.CanConvert"><code>Value.CanConvert</code></a>
+      method reports whether a value can be converted to a type.
+      This may be used to avoid a panic when converting a slice to an
+      array pointer type if the slice is too short.
+      Previously it was sufficient to use
+      <a href="/pkg/reflect/#Type.ConvertibleTo"><code>Type.ConvertibleTo</code></a>
+      for this, but the newly permitted conversion from slice to array
+      pointer type can panic even if the types are convertible.
+    </p>
+
     <p><!-- CL 266197 -->
       The new
       <a href="/pkg/reflect/#StructField.IsExported"><code>StructField.IsExported</code></a>

src/reflect/all_test.go

@@ -4304,6 +4304,9 @@ func TestConvert(t *testing.T) {
 
 		// vout1 represents the in value converted to the in type.
 		v1 := tt.in
+		if !v1.CanConvert(t1) {
+			t.Errorf("ValueOf(%T(%[1]v)).CanConvert(%s) = false, want true", tt.in.Interface(), t1)
+		}
 		vout1 := v1.Convert(t1)
 		out1 := vout1.Interface()
 		if vout1.Type() != tt.in.Type() || !DeepEqual(out1, tt.in.Interface()) {
@@ -4311,6 +4314,9 @@ func TestConvert(t *testing.T) {
 		}
 
 		// vout2 represents the in value converted to the out type.
+		if !v1.CanConvert(t2) {
+			t.Errorf("ValueOf(%T(%[1]v)).CanConvert(%s) = false, want true", tt.in.Interface(), t2)
+		}
 		vout2 := v1.Convert(t2)
 		out2 := vout2.Interface()
 		if vout2.Type() != tt.out.Type() || !DeepEqual(out2, tt.out.Interface()) {
@@ -4371,6 +4377,9 @@ func TestConvertPanic(t *testing.T) {
 	if !v.Type().ConvertibleTo(pt) {
 		t.Errorf("[]byte should be convertible to *[8]byte")
 	}
+	if v.CanConvert(pt) {
+		t.Errorf("slice with length 4 should not be convertible to *[8]byte")
+	}
 	shouldPanic("reflect: cannot convert slice with length 4 to pointer to array with length 8", func() {
 		_ = v.Convert(pt)
 	})

src/reflect/value.go

@@ -2811,6 +2811,26 @@ func (v Value) Convert(t Type) Value {
 	return op(v, t)
 }
 
+// CanConvert reports whether the value v can be converted to type t.
+// If v.CanConvert(t) returns true then v.Convert(t) will not panic.
+func (v Value) CanConvert(t Type) bool {
+	vt := v.Type()
+	if !vt.ConvertibleTo(t) {
+		return false
+	}
+	// Currently the only conversion that is OK in terms of type
+	// but that can panic depending on the value is converting
+	// from slice to pointer-to-array.
+	if vt.Kind() == Slice && t.Kind() == Ptr && t.Elem().Kind() == Array {
+		n := t.Elem().Len()
+		h := (*unsafeheader.Slice)(v.ptr)
+		if n > h.Len {
+			return false
+		}
+	}
+	return true
+}
+
 // convertOp returns the function to convert a value of type src
 // to a value of type dst. If the conversion is illegal, convertOp returns nil.
 func convertOp(dst, src *rtype) func(Value, Type) Value {