reflect: implement Value.Elem() for interface values

src/reflect/value.go

@@ -400,7 +400,14 @@ func (v Value) Elem() Value {
 			value:    ptr,
 			flags:    v.flags | valueFlagIndirect,
 		}
-	default: // not implemented: Interface
+	case Interface:
+		typecode, value := decomposeInterface(*(*interface{})(v.value))
+		return Value{
+			typecode: typecode,
+			value:    value,
+			flags:    v.flags &^ valueFlagIndirect,
+		}
+	default:
 		panic(&ValueError{"Elem"})
 	}
 }

testdata/reflect.go

@@ -120,6 +120,8 @@ func main() {
 		&linkedList{
 			foo: 42,
 		},
+		// interfaces
+		[]interface{}{3, "str", -4 + 2.5i},
 	} {
 		showValue(reflect.ValueOf(v), "")
 	}
@@ -377,6 +379,9 @@ func showValue(rv reflect.Value, indent string) {
 	case reflect.Interface:
 		println(indent + "  interface")
 		println(indent+"  nil:", rv.IsNil())
+		if !rv.IsNil() {
+			showValue(rv.Elem(), indent+"  ")
+		}
 	case reflect.Map:
 		println(indent + "  map")
 		println(indent+"  nil:", rv.IsNil())

testdata/reflect.txt

@@ -334,6 +334,34 @@ reflect type: ptr
     exported: false
     reflect type: int addrable=true
       int: 42
+reflect type: slice comparable=false
+  slice: interface 3 3
+  pointer: true
+  nil: false
+  indexing: 0
+  reflect type: interface settable=true addrable=true
+    interface
+    nil: false
+    reflect type: int
+      int: 3
+  indexing: 1
+  reflect type: interface settable=true addrable=true
+    interface
+    nil: false
+    reflect type: string
+      string: str 3
+      reflect type: uint8
+        uint: 115
+      reflect type: uint8
+        uint: 116
+      reflect type: uint8
+        uint: 114
+  indexing: 2
+  reflect type: interface settable=true addrable=true
+    interface
+    nil: false
+    reflect type: complex128
+      complex: (-4.000000e+000+2.500000e+000i)
 reflect type: ptr
   pointer: true int8
   nil: false