Adjust for reflect.Type.NumMethod change in Go1.16 (#240)

In Go1.16, the reflect.Type.NumMethod method will no longer report
unexported fields, matching the documented behavior on the method.
This means that t.NumMethod() == 0 is no longer a reliable means
to detect whether an interface type is the empty interface or not.
Fix the code to check whether the empty interface itself implements
the target type.

cmp/cmpopts/ignore.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/internal/function"
+	"github.com/google/go-cmp/cmp/internal/value"
 )
 
 // IgnoreFields returns an Option that ignores fields of the
@@ -82,7 +83,7 @@ func newIfaceFilter(ifaces interface{}) (tf ifaceFilter) {
 			panic("struct cannot have named fields")
 		case fi.Type.Kind() != reflect.Interface:
 			panic("embedded field must be an interface type")
-		case fi.Type.NumMethod() == 0:
+		case value.IsEmptyInterface(fi.Type):
 			// This matches everything; why would you ever want this?
 			panic("cannot ignore empty interface")
 		default:

cmp/internal/value/iface.go

@@ -0,0 +1,14 @@
+// Copyright 2020, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+package value
+
+import "reflect"
+
+var emptyIfaceType = reflect.TypeOf((*interface{})(nil)).Elem()
+
+// IsEmptyInterface reports whether t is an interface type with no methods.
+func IsEmptyInterface(t reflect.Type) bool {
+	return t.Kind() == reflect.Interface && emptyIfaceType.Implements(t)
+}

cmp/internal/value/iface_test.go

@@ -0,0 +1,35 @@
+// Copyright 2020, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+package value
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestIsEmptyInterface(t *testing.T) {
+	type (
+		Empty      interface{}
+		Exported   interface{ X() }
+		Unexported interface{ x() }
+	)
+	tests := []struct {
+		in   reflect.Type
+		want bool
+	}{
+		{reflect.TypeOf((*interface{})(nil)).Elem(), true},
+		{reflect.TypeOf((*Empty)(nil)).Elem(), true},
+		{reflect.TypeOf((*Exported)(nil)).Elem(), false},
+		{reflect.TypeOf((*Unexported)(nil)).Elem(), false},
+		{reflect.TypeOf(5), false},
+		{reflect.TypeOf(struct{}{}), false},
+	}
+	for _, tt := range tests {
+		got := IsEmptyInterface(tt.in)
+		if got != tt.want {
+			t.Errorf("IsEmptyInterface(%v) = %v, want %v", tt.in, got, tt.want)
+		}
+	}
+}

cmp/options.go

@@ -11,6 +11,7 @@ import (
 	"strings"
 
 	"github.com/google/go-cmp/cmp/internal/function"
+	"github.com/google/go-cmp/cmp/internal/value"
 )
 
 // Option configures for specific behavior of Equal and Diff. In particular,
@@ -161,7 +162,7 @@ func FilterValues(f interface{}, opt Option) Option {
 	}
 	if opt := normalizeOption(opt); opt != nil {
 		vf := &valuesFilter{fnc: v, opt: opt}
-		if ti := v.Type().In(0); ti.Kind() != reflect.Interface || ti.NumMethod() > 0 {
+		if ti := v.Type().In(0); !value.IsEmptyInterface(ti) {
 			vf.typ = ti
 		}
 		return vf
@@ -286,7 +287,7 @@ func Transformer(name string, f interface{}) Option {
 		panic(fmt.Sprintf("invalid name: %q", name))
 	}
 	tr := &transformer{name: name, fnc: reflect.ValueOf(f)}
-	if ti := v.Type().In(0); ti.Kind() != reflect.Interface || ti.NumMethod() > 0 {
+	if ti := v.Type().In(0); !value.IsEmptyInterface(ti) {
 		tr.typ = ti
 	}
 	return tr
@@ -345,7 +346,7 @@ func Comparer(f interface{}) Option {
 		panic(fmt.Sprintf("invalid comparer function: %T", f))
 	}
 	cm := &comparer{fnc: v}
-	if ti := v.Type().In(0); ti.Kind() != reflect.Interface || ti.NumMethod() > 0 {
+	if ti := v.Type().In(0); !value.IsEmptyInterface(ti) {
 		cm.typ = ti
 	}
 	return cm