fix: define a correct zero value for an not initialized interface{}

_test/interface19.go

@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+var I interface{}
+
+func main() {
+	fmt.Printf("%T %v\n", I, I)
+}
+
+// Output:
+// <nil> <nil>

interp/cfg.go

@@ -1443,8 +1443,14 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 				}
 			}
 			for _, c := range n.child[:l] {
-				index := sc.add(n.typ)
-				sc.sym[c.ident] = &symbol{index: index, kind: varSym, typ: n.typ}
+				var index int
+				if sc.global {
+					// Global object allocation is already performed in GTA.
+					index = sc.sym[c.ident].index
+				} else {
+					index = sc.add(n.typ)
+					sc.sym[c.ident] = &symbol{index: index, kind: varSym, typ: n.typ}
+				}
 				c.typ = n.typ
 				c.findex = index
 			}

interp/value.go

@@ -180,11 +180,23 @@ func genValueInterface(n *node) func(*frame) reflect.Value {
 	}
 }
 
+func zeroInterfaceValue() reflect.Value {
+	n := &node{kind: basicLit, typ: &itype{cat: nilT, untyped: true}}
+	v := reflect.New(reflect.TypeOf((*interface{})(nil)).Elem()).Elem()
+	return reflect.ValueOf(valueInterface{n, v})
+}
+
 func genValueInterfaceValue(n *node) func(*frame) reflect.Value {
 	value := genValue(n)
 
 	return func(f *frame) reflect.Value {
-		return value(f).Interface().(valueInterface).value
+		v := value(f)
+		if v.Interface().(valueInterface).node == nil {
+			// Uninitialized interface value, set it to a correct zero value.
+			v.Set(zeroInterfaceValue())
+			v = value(f)
+		}
+		return v.Interface().(valueInterface).value
 	}
 }