proc/eval: fix interface equality with nil (#914)

Fixes #904

_fixtures/testvariables2.go

@@ -163,6 +163,8 @@ func main() {
 	var err1 error = c1.sa[0]
 	var err2 error = c1.pb
 	var errnil error = nil
+	var psa *astruct = nil
+	var errtypednil error = psa
 	var iface1 interface{} = c1.sa[0]
 	var iface2 interface{} = "test"
 	var iface3 interface{} = map[string]constant.Value{}
@@ -237,5 +239,5 @@ func main() {
 	}
 
 	runtime.Breakpoint()
-	fmt.Println(i1, i2, i3, p1, amb1, s1, s3, a1, p2, p3, s2, as1, str1, f1, fn1, fn2, nilslice, nilptr, ch1, chnil, m1, mnil, m2, m3, up1, i4, i5, i6, err1, err2, errnil, iface1, iface2, ifacenil, arr1, parr, cpx1, const1, iface3, iface4, recursive1, recursive1.x, iface5, iface2fn1, iface2fn2, bencharr, benchparr, mapinf, mainMenu, b, b2, sd, anonstruct1, anonstruct2, anoniface1, anonfunc, mapanonstruct1, ifacearr, efacearr, ni8, ni16, ni32, pinf, ninf, nan, zsvmap, zsslice, zsvar, tm)
+	fmt.Println(i1, i2, i3, p1, amb1, s1, s3, a1, p2, p3, s2, as1, str1, f1, fn1, fn2, nilslice, nilptr, ch1, chnil, m1, mnil, m2, m3, up1, i4, i5, i6, err1, err2, errnil, iface1, iface2, ifacenil, arr1, parr, cpx1, const1, iface3, iface4, recursive1, recursive1.x, iface5, iface2fn1, iface2fn2, bencharr, benchparr, mapinf, mainMenu, b, b2, sd, anonstruct1, anonstruct2, anoniface1, anonfunc, mapanonstruct1, ifacearr, efacearr, ni8, ni16, ni32, pinf, ninf, nan, zsvmap, zsslice, zsvar, tm, errtypednil)
 }

pkg/proc/eval.go

@@ -947,7 +947,7 @@ func (v *Variable) isNil() bool {
 	case reflect.Ptr:
 		return v.Children[0].Addr == 0
 	case reflect.Interface:
-		return false
+		return v.Children[0].Addr == 0 && v.Children[0].Kind == reflect.Invalid
 	case reflect.Slice, reflect.Map, reflect.Func, reflect.Chan:
 		return v.Base == 0
 	}

service/test/variables_test.go

@@ -546,8 +546,9 @@ func TestEvalExpression(t *testing.T) {
 		{"ifacenil", true, "interface {} nil", "interface {} nil", "interface {}", nil},
 		{"err1 == err2", false, "false", "false", "", nil},
 		{"err1 == iface1", false, "", "", "", fmt.Errorf("mismatched types \"error\" and \"interface {}\"")},
-		{"errnil == nil", false, "false", "false", "", nil},
-		{"nil == errnil", false, "false", "false", "", nil},
+		{"errnil == nil", false, "true", "true", "", nil},
+		{"errtypednil == nil", false, "false", "false", "", nil},
+		{"nil == errnil", false, "true", "true", "", nil},
 		{"err1.(*main.astruct)", false, "*main.astruct {A: 1, B: 2}", "(*main.astruct)(0x…", "*main.astruct", nil},
 		{"err1.(*main.bstruct)", false, "", "", "", fmt.Errorf("interface conversion: error is *main.astruct, not *main.bstruct")},
 		{"errnil.(*main.astruct)", false, "", "", "", fmt.Errorf("interface conversion: error is nil, not *main.astruct")},