interp: fix handling of empty interfaces in map index expressions

There should be no need now to wrap empty interfaces in order to
retrieve its value.

Fixes #1344.

_test/issue-1344.go

@@ -0,0 +1,13 @@
+package main
+
+import "fmt"
+
+func main() {
+	var m = map[string]interface{}{"a": "a"}
+	a, _ := m["a"]
+	b, ok := a.(string)
+	fmt.Println("a:", a, ", b:", b, ", ok:", ok)
+}
+
+// Output:
+// a: a , b: a , ok: true

interp/run.go

@@ -1766,9 +1766,6 @@ func getIndexArray(n *node) {
 	}
 }
 
-// valueInterfaceType is the reflection type of valueInterface.
-var valueInterfaceType = reflect.TypeOf((*valueInterface)(nil)).Elem()
-
 // getIndexMap retrieves map value from index.
 func getIndexMap(n *node) {
 	dest := genValue(n)
@@ -1833,7 +1830,6 @@ func getIndexMap2(n *node) {
 	value0 := genValue(n.child[0])     // map
 	value2 := genValue(n.anc.child[1]) // status
 	next := getExec(n.tnext)
-	typ := n.anc.child[0].typ
 	doValue := n.anc.child[0].ident != "_"
 	doStatus := n.anc.child[1].ident != "_"
 
@@ -1850,21 +1846,6 @@ func getIndexMap2(n *node) {
 				value2(f).SetBool(v.IsValid())
 				return next
 			}
-		case isInterfaceSrc(typ):
-			n.exec = func(f *frame) bltn {
-				v := value0(f).MapIndex(mi)
-				if v.IsValid() {
-					if e := v.Elem(); e.Type().AssignableTo(valueInterfaceType) {
-						dest(f).Set(e)
-					} else {
-						dest(f).Set(reflect.ValueOf(valueInterface{n, e}))
-					}
-				}
-				if doStatus {
-					value2(f).SetBool(v.IsValid())
-				}
-				return next
-			}
 		default:
 			n.exec = func(f *frame) bltn {
 				v := value0(f).MapIndex(mi)
@@ -1886,21 +1867,6 @@ func getIndexMap2(n *node) {
 				value2(f).SetBool(v.IsValid())
 				return next
 			}
-		case isInterfaceSrc(typ):
-			n.exec = func(f *frame) bltn {
-				v := value0(f).MapIndex(value1(f))
-				if v.IsValid() {
-					if e := v.Elem(); e.Type().AssignableTo(valueInterfaceType) {
-						dest(f).Set(e)
-					} else {
-						dest(f).Set(reflect.ValueOf(valueInterface{n, e}))
-					}
-				}
-				if doStatus {
-					value2(f).SetBool(v.IsValid())
-				}
-				return next
-			}
 		default:
 			n.exec = func(f *frame) bltn {
 				v := value0(f).MapIndex(value1(f))