Revert "Adjust for reflect.Type.NumMethod change in Go1.16 (#240)" (#242)

This reverts commit ab46b8bd0abd4c4557cc4709ad7ae12d47570603.
The upstream change in Go1.16 has been rolled back.
See golang/go#42123

4 files changed, 4 insertions, 55 deletions

diff --git a/cmp/cmpopts/ignore.go b/cmp/cmpopts/ignore.go
index 3cdc211..80c6061 100644
--- a/cmp/cmpopts/ignore.go
+++ b/cmp/cmpopts/ignore.go
@@ -12,7 +12,6 @@ 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
@@ -83,7 +82,7 @@ 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 value.IsEmptyInterface(fi.Type):
+		case fi.Type.NumMethod() == 0:
 			// This matches everything; why would you ever want this?
 			panic("cannot ignore empty interface")
 		default:
diff --git a/cmp/internal/value/iface.go b/cmp/internal/value/iface.go
deleted file mode 100644
index d8d07a9..0000000
--- a/cmp/internal/value/iface.go
+++ /dev/null
@@ -1,14 +0,0 @@
-// 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)
-}
diff --git a/cmp/internal/value/iface_test.go b/cmp/internal/value/iface_test.go
deleted file mode 100644
index 817e1d6..0000000
--- a/cmp/internal/value/iface_test.go
+++ /dev/null
@@ -1,35 +0,0 @@
-// 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)
-		}
-	}
-}
diff --git a/cmp/options.go b/cmp/options.go
index 3b71f54..e57b9eb 100644
--- a/cmp/options.go
+++ b/cmp/options.go
@@ -11,7 +11,6 @@ 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,
@@ -162,7 +161,7 @@ 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); !value.IsEmptyInterface(ti) {
+		if ti := v.Type().In(0); ti.Kind() != reflect.Interface || ti.NumMethod() > 0 {
 			vf.typ = ti
 		}
 		return vf
@@ -287,7 +286,7 @@ 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); !value.IsEmptyInterface(ti) {
+	if ti := v.Type().In(0); ti.Kind() != reflect.Interface || ti.NumMethod() > 0 {
 		tr.typ = ti
 	}
 	return tr
@@ -346,7 +345,7 @@ func Comparer(f interface{}) Option {
 		panic(fmt.Sprintf("invalid comparer function: %T", f))
 	}
 	cm := &comparer{fnc: v}
-	if ti := v.Type().In(0); !value.IsEmptyInterface(ti) {
+	if ti := v.Type().In(0); ti.Kind() != reflect.Interface || ti.NumMethod() > 0 {
 		cm.typ = ti
 	}
 	return cm