fix: handle conversion of nil to an interface type

_test/type15.go

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	err := error(nil)
+	println(err == nil)
+}
+
+// Output:
+// true

interp/cfg.go

@@ -743,7 +743,7 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 				if isInt(n.child[0].typ.TypeOf()) && n.child[1].kind == basicLit && isFloat(n.child[1].typ.TypeOf()) {
 					err = n.cfgErrorf("truncated to integer")
 				}
-				if isInterface(n.child[0].typ) {
+				if isInterface(n.child[0].typ) && !n.child[1].isNil() {
 					// Convert to interface: just check that all required methods are defined by concrete type.
 					c0, c1 := n.child[0], n.child[1]
 					if !c1.typ.implements(c0.typ) {
@@ -1695,6 +1695,9 @@ func (n *node) isNatural() bool {
 	return false
 }
 
+// isNil returns true if node is a literal nil value, false otherwise
+func (n *node) isNil() bool { return n.kind == basicLit && !n.rval.IsValid() }
+
 // fieldType returns the nth parameter field node (type) of a fieldList node
 func (n *node) fieldType(m int) *node {
 	k := 0

interp/run.go

@@ -216,7 +216,7 @@ func convert(n *node) {
 	typ := n.child[0].typ.TypeOf()
 	next := getExec(n.tnext)
 
-	if c.kind == basicLit && !c.rval.IsValid() { // convert nil to type
+	if c.isNil() { // convert nil to type
 		n.exec = func(f *frame) bltn {
 			dest(f).Set(reflect.New(typ).Elem())
 			return next