reflect.Value.Elem is unaddressable for interfaces

[dmage]

What version of Go are you using (go version)?

tip, go version devel +27eebba

What did you do?

I tried to change the value inside interface{} variable.

i := interface{}("hello")
v := reflect.ValueOf(&i).Elem()
v.Elem().SetString("world")
fmt.Println(i)

https://play.golang.org/p/U8WaCBA4o4

What did you expect to see?

"world" stored in i.

What did you see instead?

Runtime error.

Proposed change

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 780799c..4bb408b 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -5752,3 +5752,10 @@ func BenchmarkNew(b *testing.B) {
        New(v)
    }
 }
+
+func TestInterfaceElemSet(t *testing.T) {
+   i := interface{}("hello")
+   v := ValueOf(&i).Elem()
+   v.Elem().SetString("world")
+   assert(t, i.(string), "world")
+}
diff --git a/src/reflect/value.go b/src/reflect/value.go
index e6b846e..0340d25 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -714,6 +714,9 @@ func (v Value) Elem() Value {
        x := unpackEface(eface)
        if x.flag != 0 {
            x.flag |= v.flag & flagRO
+           if x.flag&flagIndir != 0 {
+               x.flag |= flagAddr
+           }
        }
        return x
    case Ptr:

[ianlancetaylor]

That does not correspond to an operation permitted by the language. In the language, a value stored in an interface variable is not addressable. The interface variable is addressable, but not the value contained within. It's also inconsistent: your code permits changing the value if it is something like a string, but not if it is something like a pointer. We're not going to make this change.