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

This reverts commit ab46b8b.
The upstream change in Go1.16 has been rolled back.
See golang/go#42123

Author: dsnet

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:

cmp/internal/value/iface.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