fix: do not attempt to store data in _ var

_test/map21.go

@@ -0,0 +1,15 @@
+package main
+
+var m = map[int]string{
+	1: "foo",
+}
+
+func main() {
+	var ok bool
+	if _, ok = m[1]; ok {
+		println("ok")
+	}
+}
+
+// Output:
+// ok

_test/map22.go

@@ -0,0 +1,14 @@
+package main
+
+var m = map[int]string{
+	1: "foo",
+}
+
+func main() {
+	var s string
+	s, _ = m[1]
+	println(s)
+}
+
+// Output:
+// foo

_test/map23.go

@@ -0,0 +1,13 @@
+package main
+
+var m = map[int]string{
+	1: "foo",
+}
+
+func main() {
+	_, _ = m[1]
+	println("ok")
+}
+
+// Output:
+// ok

interp/run.go

@@ -1096,46 +1096,74 @@ func getIndexMap2(n *node) {
 	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 != "_"
 
+	if !doValue && !doStatus {
+		nop(n)
+		return
+	}
 	if n.child[1].rval.IsValid() { // constant map index
 		mi := n.child[1].rval
-		if typ.cat == interfaceT {
+		switch {
+		case !doValue:
+			n.exec = func(f *frame) bltn {
+				v := value0(f).MapIndex(mi)
+				value2(f).SetBool(v.IsValid())
+				return next
+			}
+		case typ.cat == interfaceT:
 			n.exec = func(f *frame) bltn {
 				v := value0(f).MapIndex(mi)
 				if v.IsValid() {
 					dest(f).Set(v.Elem())
 				}
-				value2(f).SetBool(v.IsValid())
+				if doStatus {
+					value2(f).SetBool(v.IsValid())
+				}
 				return next
 			}
-		} else {
+		default:
 			n.exec = func(f *frame) bltn {
 				v := value0(f).MapIndex(mi)
 				if v.IsValid() {
 					dest(f).Set(v)
 				}
-				value2(f).SetBool(v.IsValid())
+				if doStatus {
+					value2(f).SetBool(v.IsValid())
+				}
 				return next
 			}
 		}
 	} else {
 		value1 := genValue(n.child[1]) // map index
-		if typ.cat == interfaceT {
+		switch {
+		case !doValue:
+			n.exec = func(f *frame) bltn {
+				v := value0(f).MapIndex(value1(f))
+				value2(f).SetBool(v.IsValid())
+				return next
+			}
+		case typ.cat == interfaceT:
 			n.exec = func(f *frame) bltn {
 				v := value0(f).MapIndex(value1(f))
 				if v.IsValid() {
 					dest(f).Set(v.Elem())
 				}
-				value2(f).SetBool(v.IsValid())
+				if doStatus {
+					value2(f).SetBool(v.IsValid())
+				}
 				return next
 			}
-		} else {
+		default:
 			n.exec = func(f *frame) bltn {
 				v := value0(f).MapIndex(value1(f))
 				if v.IsValid() {
 					dest(f).Set(v)
 				}
-				value2(f).SetBool(v.IsValid())
+				if doStatus {
+					value2(f).SetBool(v.IsValid())
+				}
 				return next
 			}
 		}

interp/value.go

@@ -164,7 +164,7 @@ func genValueInterface(n *node) func(*frame) reflect.Value {
 	return func(f *frame) reflect.Value {
 		v := value(f)
 		nod := n
-		for {
+		for v.IsValid() {
 			// traverse interface indirections to find out concrete type
 			vi, ok := v.Interface().(valueInterface)
 			if !ok {