[cfe] Adjust nullability of FutureOr in ReplacementVisitor

This fix is analogous to that made in _TypeSubstitutor in
https://dart-review.googlesource.com/c/sdk/+/240049, but made for
ReplacementVisitor.

Closes https://github.com/dart-lang/sdk/issues/48768

Change-Id: Ie3ac8fe834210d1249cbd5bd3d68b416f9efc034
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240722
Auto-Submit: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/testcases/nnbd/issue48768.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2022, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+foo(dynamic y) async {
+  var a = await (<X>(List<X> Function() x, X x2) => x2)(() => y, throw 0);
+}
+
+main() {}

pkg/front_end/testcases/nnbd/issue48768.dart.strong.expect

@@ -0,0 +1,8 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+static method foo(dynamic y) → dynamic async /* futureValueType= dynamic */ {
+  dynamic a = await(<X extends core::Object? = dynamic>(() → core::List<X%> x, X% x2) → X% => x2)<FutureOr<dynamic>>(() → core::List<FutureOr<dynamic>> => y as{TypeError,ForDynamic,ForNonNullableByDefault} core::List<FutureOr<dynamic>>, throw 0){(() → core::List<FutureOr<dynamic>?>, FutureOr<dynamic>?) → FutureOr<dynamic>?};
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue48768.dart.strong.transformed.expect

@@ -0,0 +1,34 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:async" as asy;
+import "dart:core" as core;
+
+static method foo(dynamic y) → dynamic /* futureValueType= dynamic */ /* originally async */ {
+  final asy::_Future<dynamic> :async_future = new asy::_Future::•<dynamic>();
+  core::bool* :is_sync = false;
+  dynamic :return_value;
+  (dynamic) → dynamic :async_op_then;
+  (core::Object, core::StackTrace) → dynamic :async_op_error;
+  core::int :await_jump_var = 0;
+  dynamic :await_ctx_var;
+  dynamic :saved_try_context_var0;
+  function :async_op(dynamic :result_or_exception, dynamic :stack_trace) → dynamic yielding 
+    try {
+      #L1:
+      {
+        [yield] let dynamic #t1 = asy::_awaitHelper((<X extends core::Object? = dynamic>(() → core::List<X%> x, X% x2) → X% => x2)<FutureOr<dynamic>>(() → core::List<FutureOr<dynamic>> => y as{TypeError,ForDynamic,ForNonNullableByDefault} core::List<FutureOr<dynamic>>, throw 0){(() → core::List<FutureOr<dynamic>?>, FutureOr<dynamic>?) → FutureOr<dynamic>?}, :async_op_then, :async_op_error) in null;
+        dynamic a = :result_or_exception;
+      }
+      asy::_completeWithNoFutureOnAsyncReturn(:async_future, :return_value, :is_sync);
+      return;
+    }
+    on dynamic catch(dynamic exception, core::StackTrace stack_trace) {
+      asy::_completeOnAsyncError(:async_future, exception, stack_trace, :is_sync);
+    }
+  :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+  :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+  :async_op(null, null){() → dynamic};
+  :is_sync = true;
+  return :async_future;
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue48768.dart.textual_outline.expect

@@ -0,0 +1,2 @@
+foo(dynamic y) async {}
+main() {}

pkg/front_end/testcases/nnbd/issue48768.dart.textual_outline_modelled.expect

@@ -0,0 +1,2 @@
+foo(dynamic y) async {}
+main() {}

pkg/front_end/testcases/nnbd/issue48768.dart.weak.expect

@@ -0,0 +1,8 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+static method foo(dynamic y) → dynamic async /* futureValueType= dynamic */ {
+  dynamic a = await(<X extends core::Object? = dynamic>(() → core::List<X%> x, X% x2) → X% => x2)<FutureOr<dynamic>>(() → core::List<FutureOr<dynamic>> => y as{TypeError,ForDynamic,ForNonNullableByDefault} core::List<FutureOr<dynamic>>, throw 0){(() → core::List<FutureOr<dynamic>?>, FutureOr<dynamic>?) → FutureOr<dynamic>?};
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue48768.dart.weak.modular.expect

@@ -0,0 +1,8 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+static method foo(dynamic y) → dynamic async /* futureValueType= dynamic */ {
+  dynamic a = await(<X extends core::Object? = dynamic>(() → core::List<X%> x, X% x2) → X% => x2)<FutureOr<dynamic>>(() → core::List<FutureOr<dynamic>> => y as{TypeError,ForDynamic,ForNonNullableByDefault} core::List<FutureOr<dynamic>>, throw 0){(() → core::List<FutureOr<dynamic>?>, FutureOr<dynamic>?) → FutureOr<dynamic>?};
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/issue48768.dart.weak.outline.expect

@@ -0,0 +1,7 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+
+static method foo(dynamic y) → dynamic async 
+  ;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/nnbd/issue48768.dart.weak.transformed.expect

@@ -0,0 +1,34 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:async" as asy;
+import "dart:core" as core;
+
+static method foo(dynamic y) → dynamic /* futureValueType= dynamic */ /* originally async */ {
+  final asy::_Future<dynamic> :async_future = new asy::_Future::•<dynamic>();
+  core::bool* :is_sync = false;
+  dynamic :return_value;
+  (dynamic) → dynamic :async_op_then;
+  (core::Object, core::StackTrace) → dynamic :async_op_error;
+  core::int :await_jump_var = 0;
+  dynamic :await_ctx_var;
+  dynamic :saved_try_context_var0;
+  function :async_op(dynamic :result_or_exception, dynamic :stack_trace) → dynamic yielding 
+    try {
+      #L1:
+      {
+        [yield] let dynamic #t1 = asy::_awaitHelper((<X extends core::Object? = dynamic>(() → core::List<X%> x, X% x2) → X% => x2)<FutureOr<dynamic>>(() → core::List<FutureOr<dynamic>> => y as{TypeError,ForDynamic,ForNonNullableByDefault} core::List<FutureOr<dynamic>>, throw 0){(() → core::List<FutureOr<dynamic>?>, FutureOr<dynamic>?) → FutureOr<dynamic>?}, :async_op_then, :async_op_error) in null;
+        dynamic a = :result_or_exception;
+      }
+      asy::_completeWithNoFutureOnAsyncReturn(:async_future, :return_value, :is_sync);
+      return;
+    }
+    on dynamic catch(dynamic exception, core::StackTrace stack_trace) {
+      asy::_completeOnAsyncError(:async_future, exception, stack_trace, :is_sync);
+    }
+  :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+  :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+  :async_op(null, null){() → dynamic};
+  :is_sync = true;
+  return :async_future;
+}
+static method main() → dynamic {}

pkg/kernel/lib/src/replacement_visitor.dart

@@ -171,8 +171,26 @@ class ReplacementVisitor implements DartTypeVisitor1<DartType?, int> {
       // No nullability or type arguments needed to be substituted.
       return null;
     } else {
-      return new FutureOrType(newTypeArgument ?? node.typeArgument,
-          newNullability ?? node.declaredNullability);
+      newTypeArgument ??= node.typeArgument;
+      newNullability ??= node.declaredNullability;
+
+      // The top-level nullability of a FutureOr should remain the same, with
+      // the exception of the case of [Nullability.undetermined].  In that case
+      // it remains undetermined if the nullability of [typeArgument] is
+      // undetermined, and otherwise it should become
+      // [Nullability.nonNullable].
+      Nullability adjustedNullability;
+      if (newNullability == Nullability.undetermined) {
+        if (newTypeArgument.nullability == Nullability.undetermined) {
+          adjustedNullability = Nullability.undetermined;
+        } else {
+          adjustedNullability = Nullability.nonNullable;
+        }
+      } else {
+        adjustedNullability = newNullability;
+      }
+
+      return new FutureOrType(newTypeArgument, adjustedNullability);
     }
   }