From a38d19288fcaba4557cb80284abf14fe96520b58 Mon Sep 17 00:00:00 2001
From: Marc Vertes <mvertes@free.fr>
Date: Fri, 30 Oct 2020 16:20:04 +0100
Subject: [PATCH] interp: fix testing for nil interface values

Fixes #924.
---
 _test/nil3.go | 28 ++++++++++++++++++++++++++++
 interp/run.go | 26 ++++++++++++++++++--------
 2 files changed, 46 insertions(+), 8 deletions(-)
 create mode 100644 _test/nil3.go

diff --git a/_test/nil3.go b/_test/nil3.go
new file mode 100644
index 000000000..fc884e72c
--- /dev/null
+++ b/_test/nil3.go
@@ -0,0 +1,28 @@
+package main
+
+type I interface {
+	Hello()
+}
+
+type T struct {
+	h I
+}
+
+func (t *T) Hello() { println("Hello") }
+
+func main() {
+	t := &T{}
+	println(t.h != nil)
+	println(t.h == nil)
+	t.h = t
+	println(t.h != nil)
+	println(t.h == nil)
+	t.h.Hello()
+}
+
+// Output:
+// false
+// true
+// true
+// false
+// Hello
diff --git a/interp/run.go b/interp/run.go
index 03c736387..9dadbee16 100644
--- a/interp/run.go
+++ b/interp/run.go
@@ -3091,9 +3091,9 @@ func isNil(n *node) {
 		fnext := getExec(n.fnext)
 		if c0.typ.cat == interfaceT {
 			n.exec = func(f *frame) bltn {
-				vi := value(f).Interface().(valueInterface)
-				if (vi == valueInterface{} ||
-					vi.node.kind == basicLit && vi.node.typ.cat == nilT) {
+				v := value(f)
+				vi, ok := v.Interface().(valueInterface)
+				if ok && (vi == valueInterface{} || vi.node.kind == basicLit && vi.node.typ.cat == nilT) || v.IsNil() {
 					dest(f).SetBool(true)
 					return tnext
 				}
@@ -3113,7 +3113,12 @@ func isNil(n *node) {
 	} else {
 		if c0.typ.cat == interfaceT {
 			n.exec = func(f *frame) bltn {
-				dest(f).SetBool(value(f).Interface().(valueInterface) == valueInterface{})
+				v := value(f)
+				if vi, ok := v.Interface().(valueInterface); ok {
+					dest(f).SetBool(vi == valueInterface{} || vi.node.kind == basicLit && vi.node.typ.cat == nilT)
+				} else {
+					dest(f).SetBool(v.IsNil())
+				}
 				return tnext
 			}
 		} else {
@@ -3140,9 +3145,9 @@ func isNotNil(n *node) {
 		fnext := getExec(n.fnext)
 		if c0.typ.cat == interfaceT {
 			n.exec = func(f *frame) bltn {
-				vi := value(f).Interface().(valueInterface)
-				if (vi == valueInterface{} ||
-					vi.node.kind == basicLit && vi.node.typ.cat == nilT) {
+				v := value(f)
+				vi, ok := v.Interface().(valueInterface)
+				if ok && (vi == valueInterface{} || vi.node.kind == basicLit && vi.node.typ.cat == nilT) || v.IsNil() {
 					dest(f).SetBool(false)
 					return fnext
 				}
@@ -3162,7 +3167,12 @@ func isNotNil(n *node) {
 	} else {
 		if c0.typ.cat == interfaceT {
 			n.exec = func(f *frame) bltn {
-				dest(f).SetBool(!(value(f).Interface().(valueInterface) == valueInterface{}))
+				v := value(f)
+				if vi, ok := v.Interface().(valueInterface); ok {
+					dest(f).SetBool(!(vi == valueInterface{} || vi.node.kind == basicLit && vi.node.typ.cat == nilT))
+				} else {
+					dest(f).SetBool(!v.IsNil())
+				}
 				return tnext
 			}
 		} else {