[cfe] Adjust the nullability of UP w.r.t. undetermined RHS

Closes #52726

Change-Id: I80c3c219738bb999a1a5939d95d3119d8b63332c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/334441
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

pkg/front_end/test/spell_checking_list_common.txt

@@ -851,6 +851,7 @@ desugar
 desugared
 desugaring
 desugars
+detached
 detail
 detailed
 details

pkg/front_end/testcases/general/issue52726.dart

@@ -0,0 +1,13 @@
+// Copyright (c) 2023, 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.
+
+class WeakMap<K extends Object, V> {
+  WeakMap(List<(K, V)> entries) {
+    var mapped = entries.map((i) => [i.$1, i.$2]);
+    var first = mapped.first;
+    first.add(null);
+    Object o = first.first; // Error.
+  }
+}
+

pkg/front_end/testcases/general/issue52726.dart.strong.expect

@@ -0,0 +1,24 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+//  - 'Object' is from 'dart:core'.
+//     Object o = first.first; // Error.
+//                      ^
+//
+import self as self;
+import "dart:core" as core;
+
+class WeakMap<K extends core::Object, V extends core::Object? = dynamic> extends core::Object {
+  constructor •(core::List<(self::WeakMap::K, self::WeakMap::V%)> entries) → self::WeakMap<self::WeakMap::K, self::WeakMap::V%>
+    : super core::Object::•() {
+    core::Iterable<core::List<core::Object?>> mapped = entries.{core::Iterable::map}<core::List<core::Object?>>(((self::WeakMap::K, self::WeakMap::V%) i) → core::List<core::Object?> => <core::Object?>[i.$1{self::WeakMap::K}, i.$2{self::WeakMap::V%}]){(((self::WeakMap::K, self::WeakMap::V%)) → core::List<core::Object?>) → core::Iterable<core::List<core::Object?>>};
+    core::List<core::Object?> first = mapped.{core::Iterable::first}{core::List<core::Object?>};
+    first.{core::List::add}(null){(core::Object?) → void};
+    core::Object o = invalid-expression "pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+ - 'Object' is from 'dart:core'.
+    Object o = first.first; // Error.
+                     ^" in first.{core::Iterable::first}{core::Object?} as{TypeError} core::Object;
+  }
+}

pkg/front_end/testcases/general/issue52726.dart.strong.transformed.expect

@@ -0,0 +1,24 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+//  - 'Object' is from 'dart:core'.
+//     Object o = first.first; // Error.
+//                      ^
+//
+import self as self;
+import "dart:core" as core;
+
+class WeakMap<K extends core::Object, V extends core::Object? = dynamic> extends core::Object {
+  constructor •(core::List<(self::WeakMap::K, self::WeakMap::V%)> entries) → self::WeakMap<self::WeakMap::K, self::WeakMap::V%>
+    : super core::Object::•() {
+    core::Iterable<core::List<core::Object?>> mapped = entries.{core::Iterable::map}<core::List<core::Object?>>(((self::WeakMap::K, self::WeakMap::V%) i) → core::List<core::Object?> => core::_GrowableList::_literal2<core::Object?>(i.$1{self::WeakMap::K}, i.$2{self::WeakMap::V%})){(((self::WeakMap::K, self::WeakMap::V%)) → core::List<core::Object?>) → core::Iterable<core::List<core::Object?>>};
+    core::List<core::Object?> first = mapped.{core::Iterable::first}{core::List<core::Object?>};
+    first.{core::List::add}(null){(core::Object?) → void};
+    core::Object o = invalid-expression "pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+ - 'Object' is from 'dart:core'.
+    Object o = first.first; // Error.
+                     ^" in let core::Object? #t1 = first.{core::Iterable::first}{core::Object?} in #t1 == null ?{core::Object} #t1 as{TypeError} core::Object : #t1{core::Object};
+  }
+}

pkg/front_end/testcases/general/issue52726.dart.textual_outline.expect

@@ -0,0 +1,3 @@
+class WeakMap<K extends Object, V> {
+  WeakMap(List<(K, V)> entries) {}
+}

pkg/front_end/testcases/general/issue52726.dart.textual_outline_modelled.expect

@@ -0,0 +1,3 @@
+class WeakMap<K extends Object, V> {
+  WeakMap(List<(K, V)> entries) {}
+}

pkg/front_end/testcases/general/issue52726.dart.weak.expect

@@ -0,0 +1,24 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+//  - 'Object' is from 'dart:core'.
+//     Object o = first.first; // Error.
+//                      ^
+//
+import self as self;
+import "dart:core" as core;
+
+class WeakMap<K extends core::Object, V extends core::Object? = dynamic> extends core::Object {
+  constructor •(core::List<(self::WeakMap::K, self::WeakMap::V%)> entries) → self::WeakMap<self::WeakMap::K, self::WeakMap::V%>
+    : super core::Object::•() {
+    core::Iterable<core::List<core::Object?>> mapped = entries.{core::Iterable::map}<core::List<core::Object?>>(((self::WeakMap::K, self::WeakMap::V%) i) → core::List<core::Object?> => <core::Object?>[i.$1{self::WeakMap::K}, i.$2{self::WeakMap::V%}]){(((self::WeakMap::K, self::WeakMap::V%)) → core::List<core::Object?>) → core::Iterable<core::List<core::Object?>>};
+    core::List<core::Object?> first = mapped.{core::Iterable::first}{core::List<core::Object?>};
+    first.{core::List::add}(null){(core::Object?) → void};
+    core::Object o = invalid-expression "pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+ - 'Object' is from 'dart:core'.
+    Object o = first.first; // Error.
+                     ^" in first.{core::Iterable::first}{core::Object?} as{TypeError} core::Object;
+  }
+}

pkg/front_end/testcases/general/issue52726.dart.weak.modular.expect

@@ -0,0 +1,24 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+//  - 'Object' is from 'dart:core'.
+//     Object o = first.first; // Error.
+//                      ^
+//
+import self as self;
+import "dart:core" as core;
+
+class WeakMap<K extends core::Object, V extends core::Object? = dynamic> extends core::Object {
+  constructor •(core::List<(self::WeakMap::K, self::WeakMap::V%)> entries) → self::WeakMap<self::WeakMap::K, self::WeakMap::V%>
+    : super core::Object::•() {
+    core::Iterable<core::List<core::Object?>> mapped = entries.{core::Iterable::map}<core::List<core::Object?>>(((self::WeakMap::K, self::WeakMap::V%) i) → core::List<core::Object?> => <core::Object?>[i.$1{self::WeakMap::K}, i.$2{self::WeakMap::V%}]){(((self::WeakMap::K, self::WeakMap::V%)) → core::List<core::Object?>) → core::Iterable<core::List<core::Object?>>};
+    core::List<core::Object?> first = mapped.{core::Iterable::first}{core::List<core::Object?>};
+    first.{core::List::add}(null){(core::Object?) → void};
+    core::Object o = invalid-expression "pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+ - 'Object' is from 'dart:core'.
+    Object o = first.first; // Error.
+                     ^" in first.{core::Iterable::first}{core::Object?} as{TypeError} core::Object;
+  }
+}

pkg/front_end/testcases/general/issue52726.dart.weak.outline.expect

@@ -0,0 +1,8 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class WeakMap<K extends core::Object, V extends core::Object? = dynamic> extends core::Object {
+  constructor •(core::List<(self::WeakMap::K, self::WeakMap::V%)> entries) → self::WeakMap<self::WeakMap::K, self::WeakMap::V%>
+    ;
+}

pkg/front_end/testcases/general/issue52726.dart.weak.transformed.expect

@@ -0,0 +1,24 @@
+library;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+//  - 'Object' is from 'dart:core'.
+//     Object o = first.first; // Error.
+//                      ^
+//
+import self as self;
+import "dart:core" as core;
+
+class WeakMap<K extends core::Object, V extends core::Object? = dynamic> extends core::Object {
+  constructor •(core::List<(self::WeakMap::K, self::WeakMap::V%)> entries) → self::WeakMap<self::WeakMap::K, self::WeakMap::V%>
+    : super core::Object::•() {
+    core::Iterable<core::List<core::Object?>> mapped = entries.{core::Iterable::map}<core::List<core::Object?>>(((self::WeakMap::K, self::WeakMap::V%) i) → core::List<core::Object?> => core::_GrowableList::_literal2<core::Object?>(i.$1{self::WeakMap::K}, i.$2{self::WeakMap::V%})){(((self::WeakMap::K, self::WeakMap::V%)) → core::List<core::Object?>) → core::Iterable<core::List<core::Object?>>};
+    core::List<core::Object?> first = mapped.{core::Iterable::first}{core::List<core::Object?>};
+    first.{core::List::add}(null){(core::Object?) → void};
+    core::Object o = invalid-expression "pkg/front_end/testcases/general/issue52726.dart:10:22: Error: A value of type 'Object?' can't be assigned to a variable of type 'Object' because 'Object?' is nullable and 'Object' isn't.
+ - 'Object' is from 'dart:core'.
+    Object o = first.first; // Error.
+                     ^" in first.{core::Iterable::first}{core::Object?};
+  }
+}

pkg/kernel/lib/src/standard_bounds.dart

@@ -1543,13 +1543,16 @@ mixin StandardBounds {
     //   otherwise UP(B1a, T2)
     //     where B1a is the greatest closure of B1 with respect to X1,
     //     as defined in [inference.md].
+
     if (isSubtypeOf(type1, type2, SubtypeCheckMode.withNullabilities)) {
-      return type2.withDeclaredNullability(
-          uniteNullabilities(type1.declaredNullability, type2.nullability));
+      return type2.withDeclaredNullability(combineNullabilitiesForSubstitution(
+          type2.nullability,
+          uniteNullabilities(type1.declaredNullability, type2.nullability)));
     }
     if (isSubtypeOf(type2, type1, SubtypeCheckMode.withNullabilities)) {
-      return type1.withDeclaredNullability(
-          uniteNullabilities(type1.declaredNullability, type2.nullability));
+      return type1.withDeclaredNullability(combineNullabilitiesForSubstitution(
+          type1.declaredNullability,
+          uniteNullabilities(type1.declaredNullability, type2.nullability)));
     }
     NullabilityAwareTypeVariableEliminator eliminator =
         new NullabilityAwareTypeVariableEliminator(
@@ -1558,13 +1561,13 @@ mixin StandardBounds {
             topType: coreTypes.objectNullableRawType,
             topFunctionType: coreTypes.functionNonNullableRawType,
             unhandledTypeHandler: (type, recursor) => false);
-    return _getNullabilityAwareStandardUpperBound(
-            eliminator.eliminateToGreatest(type1.parameter.bound), type2,
-            isNonNullableByDefault: isNonNullableByDefault)
-        .withDeclaredNullability(uniteNullabilities(
-            type1.declaredNullability,
-            uniteNullabilities(
-                type1.parameter.bound.declaredNullability, type2.nullability)));
+    DartType result = _getNullabilityAwareStandardUpperBound(
+        eliminator.eliminateToGreatest(type1.parameter.bound), type2,
+        isNonNullableByDefault: isNonNullableByDefault);
+    return result.withDeclaredNullability(combineNullabilitiesForSubstitution(
+        result.declaredNullability,
+        uniteNullabilities(
+            type1.parameter.bound.declaredNullability, type2.nullability)));
   }
 
   DartType _getNullabilityAwareIntersectionStandardUpperBound(