reflect: handle types with unexported methods before exported ones

The method Method expects index to be an index of exported fields,
but, before this change, the index used by MethodByName could
take into account unexported fields if those happened sort
before the exported one.

Fixes #21177

Change-Id: I90bb64a47b23e2e43fdd2b8a1e0a2c9a8a63ded2
Reviewed-on: https://go-review.googlesource.com/51810Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/all_test.go

@@ -5521,6 +5521,25 @@ func TestKeepFuncLive(t *testing.T) {
 	MakeFunc(typ, f).Call([]Value{ValueOf(10)})
 }
 
+type UnExportedFirst int
+
+func (i UnExportedFirst) ΦExported()  {}
+func (i UnExportedFirst) unexported() {}
+
+// Issue 21177
+func TestMethodByNameUnExportedFirst(t *testing.T) {
+	defer func() {
+		if recover() != nil {
+			t.Errorf("should not panic")
+		}
+	}()
+	typ := TypeOf(UnExportedFirst(0))
+	m, _ := typ.MethodByName("ΦExported")
+	if m.Name != "ΦExported" {
+		t.Errorf("got %s, expected ΦExported", m.Name)
+	}
+}
+
 // Issue 18635 (method version).
 type KeepMethodLive struct{}
 

src/reflect/type.go

@@ -871,11 +871,15 @@ func (t *rtype) MethodByName(name string) (m Method, ok bool) {
 		return Method{}, false
 	}
 	utmethods := ut.methods()
+	var eidx int
 	for i := 0; i < int(ut.mcount); i++ {
 		p := utmethods[i]
 		pname := t.nameOff(p.name)
-		if pname.isExported() && pname.name() == name {
-			return t.Method(i), true
+		if pname.isExported() {
+			if pname.name() == name {
+				return t.Method(eidx), true
+			}
+			eidx++
 		}
 	}
 	return Method{}, false