From ef1528aa78c646f786717ad6174b521699ece1a2 Mon Sep 17 00:00:00 2001
From: Stephen Adams <sra@google.com>
Date: Thu, 30 Apr 2020 01:41:02 +0000
Subject: [PATCH] [js_runtime] Never is a bottom in legacy mode

Opt-in code can have Never types even with legacy subtyping.

Bug: 41675

Change-Id: I6dbdc064dceeedf0774d10faaca336561b41b7d5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/145561
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>
---
 sdk/lib/_internal/js_runtime/lib/rti.dart          | 9 ++++-----
 sdk_nnbd/lib/_internal/js_runtime/lib/rti.dart     | 9 ++++-----
 tests/compiler/dart2js_extra/rti/subtype_test.dart | 6 ++++++
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/sdk/lib/_internal/js_runtime/lib/rti.dart b/sdk/lib/_internal/js_runtime/lib/rti.dart
index 06ddaff5197..d89ededc408 100644
--- a/sdk/lib/_internal/js_runtime/lib/rti.dart
+++ b/sdk/lib/_internal/js_runtime/lib/rti.dart
@@ -2559,11 +2559,7 @@ bool _isSubtype(universe, Rti s, sEnv, Rti t, tEnv) {
   if (isStrongTopType(s)) return false;
 
   // Left Bottom:
-  if (isLegacy) {
-    if (isNullType(s)) return true;
-  } else {
-    if (sKind == Rti.kindNever) return true;
-  }
+  if (isBottomType(s)) return true;
 
   // Left Type Variable Bound 1:
   bool leftTypeVariable = sKind == Rti.kindGenericFunctionParameter;
@@ -2897,6 +2893,9 @@ bool isStrongTopType(Rti t) {
       isNullableObjectType(t);
 }
 
+bool isBottomType(Rti t) =>
+    Rti._getKind(t) == Rti.kindNever || JS_GET_FLAG('LEGACY') && isNullType(t);
+
 bool isObjectType(Rti t) => _Utils.isIdentical(t, TYPE_REF<Object>());
 bool isLegacyObjectType(Rti t) =>
     _Utils.isIdentical(t, LEGACY_TYPE_REF<Object>());
diff --git a/sdk_nnbd/lib/_internal/js_runtime/lib/rti.dart b/sdk_nnbd/lib/_internal/js_runtime/lib/rti.dart
index 685e8f245dd..05e37102ca4 100644
--- a/sdk_nnbd/lib/_internal/js_runtime/lib/rti.dart
+++ b/sdk_nnbd/lib/_internal/js_runtime/lib/rti.dart
@@ -2653,11 +2653,7 @@ bool _isSubtype(Object? universe, Rti s, Object? sEnv, Rti t, Object? tEnv) {
   if (isStrongTopType(s)) return false;
 
   // Left Bottom:
-  if (isLegacy) {
-    if (isNullType(s)) return true;
-  } else {
-    if (sKind == Rti.kindNever) return true;
-  }
+  if (isBottomType(s)) return true;
 
   // Left Type Variable Bound 1:
   bool leftTypeVariable = sKind == Rti.kindGenericFunctionParameter;
@@ -2993,6 +2989,9 @@ bool isStrongTopType(Rti t) {
       isNullableObjectType(t);
 }
 
+bool isBottomType(Rti t) =>
+    Rti._getKind(t) == Rti.kindNever || JS_GET_FLAG('LEGACY') && isNullType(t);
+
 bool isObjectType(Rti t) => _Utils.isIdentical(t, TYPE_REF<Object>());
 bool isLegacyObjectType(Rti t) =>
     _Utils.isIdentical(t, LEGACY_TYPE_REF<Object>());
diff --git a/tests/compiler/dart2js_extra/rti/subtype_test.dart b/tests/compiler/dart2js_extra/rti/subtype_test.dart
index 0d34f3f84ae..3ffea44656c 100644
--- a/tests/compiler/dart2js_extra/rti/subtype_test.dart
+++ b/tests/compiler/dart2js_extra/rti/subtype_test.dart
@@ -39,6 +39,7 @@ void runTests() {
   testInterfaces();
   testTopTypes();
   testNull();
+  testBottom();
   testFutureOr();
   testFunctions();
   testGenericFunctions();
@@ -80,6 +81,11 @@ void testNull() {
   equivalent(nullName, nullName);
 }
 
+void testBottom() {
+  String never = '0&';
+  equivalent(nullName, never); // This test is run with legacy subtyping
+}
+
 void testFutureOr() {
   strictSubtype('$futureName<int>', '$futureName<num>');
   strictSubtype('int', 'int/');