From 07e292128754cfb35cdcf9ebd3f9db6c86143eb3 Mon Sep 17 00:00:00 2001
From: Nicholas Shahan <nshahan@google.com>
Date: Tue, 16 Jun 2020 21:20:01 +0000
Subject: [PATCH] [ddc] Cleanup false assumption from subtype check

FutureOr<A> <: FutureOr<B> iff A <: B was proven to not always be true.
Changing this to simply an optimization and no longer "if and only if".

Change-Id: I46f228933df65a86c2c525a48f11b18397b7c48c
Fixes: https://github.com/dart-lang/sdk/issues/38818
Fixes: https://github.com/dart-lang/sdk/issues/42326
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151325
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>
---
 .../js_dev_runtime/private/ddc_runtime/types.dart         | 8 ++++----
 .../js_dev_runtime/private/ddc_runtime/types.dart         | 8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart b/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
index 48d54adcb91..625e44f9cb3 100644
--- a/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
+++ b/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
@@ -1415,10 +1415,10 @@ bool _isSubtype(t1, t2, @notNull bool strictMode) => JS<bool>('!', '''(() => {
     let t1TypeArg = ${getGenericArgs(t1)}[0];
     if (${_isFutureOr(t2)}) {
       let t2TypeArg = ${getGenericArgs(t2)}[0];
-      // FutureOr<A> <: FutureOr<B> iff A <: B
-      // TODO(nshahan): Proven to not actually be true and needs cleanup.
-      // https://github.com/dart-lang/sdk/issues/38818
-      return $_isSubtype(t1TypeArg, t2TypeArg, $strictMode);
+      // FutureOr<A> <: FutureOr<B> if A <: B
+      if ($_isSubtype(t1TypeArg, t2TypeArg, $strictMode)) {
+        return true;
+      }
     }
 
     // given t1 is Future<A> | A, then:
diff --git a/sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart b/sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
index 41cc31d77c9..f5f25a0cc46 100644
--- a/sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
+++ b/sdk_nnbd/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
@@ -1413,10 +1413,10 @@ bool _isSubtype(t1, t2, @notNull bool strictMode) => JS<bool>('!', '''(() => {
     let t1TypeArg = ${getGenericArgs(t1)}[0];
     if (${_isFutureOr(t2)}) {
       let t2TypeArg = ${getGenericArgs(t2)}[0];
-      // FutureOr<A> <: FutureOr<B> iff A <: B
-      // TODO(nshahan): Proven to not actually be true and needs cleanup.
-      // https://github.com/dart-lang/sdk/issues/38818
-      return $_isSubtype(t1TypeArg, t2TypeArg, $strictMode);
+      // FutureOr<A> <: FutureOr<B> if A <: B
+      if ($_isSubtype(t1TypeArg, t2TypeArg, $strictMode)) {
+        return true;
+      }
     }
 
     // given t1 is Future<A> | A, then: