[cfe] Implement least upper bound for inline types

Closes #51556

Change-Id: I44f2ae947b5316d21d23b39b88f1515702525955
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/294983
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

pkg/front_end/test/spell_checking_list_tests.txt

@@ -41,6 +41,7 @@ approximation
 arbitrarily
 area
 arglebargle
+arr
 asdf
 asserter
 assure

pkg/front_end/testcases/inline_class/issue51556.dart

@@ -0,0 +1,44 @@
+// 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.
+
+inline class JSAny {
+  final Object value;
+  JSAny(this.value);
+}
+
+inline class JSObject implements JSAny {
+  final Object value;
+  JSObject(this.value);
+}
+
+inline class JSArray implements JSObject {
+  final List<JSAny?> value;
+  JSArray(this.value);
+}
+
+inline class JSExportedDartObject implements JSObject {
+  final Object value;
+  JSExportedDartObject(this.value);
+}
+
+inline class JSNumber implements JSAny {
+  final double value;
+  JSNumber(this.value);
+}
+
+extension ObjectToJSExportedDartObject on Object {
+  JSExportedDartObject get toJS => JSExportedDartObject(this);
+}
+
+extension ListToJSArray on List<JSAny?> {
+  JSArray get toJS => JSArray(this);
+}
+
+extension DoubleToJSNumber on double {
+  JSNumber get toJS => JSNumber(this);
+}
+
+void main() {
+  JSArray arr = [1.0.toJS, 'foo'.toJS].toJS;
+}

pkg/front_end/testcases/inline_class/issue51556.dart.strong.expect

@@ -0,0 +1,72 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension ObjectToJSExportedDartObject on core::Object {
+  get toJS = self::ObjectToJSExportedDartObject|get#toJS;
+}
+extension ListToJSArray on core::List<self::JSAny?> {
+  get toJS = self::ListToJSArray|get#toJS;
+}
+extension DoubleToJSNumber on core::double {
+  get toJS = self::DoubleToJSNumber|get#toJS;
+}
+inline class JSAny /* declaredRepresentationType = core::Object */ {
+  constructor • = self::JSAny|;
+  tearoff • = self::JSAny|get#;
+}
+inline class JSObject /* declaredRepresentationType = core::Object */ implements self::JSAny {
+  constructor • = self::JSObject|;
+  tearoff • = self::JSObject|get#;
+}
+inline class JSArray /* declaredRepresentationType = core::List<self::JSAny?> */ implements self::JSObject {
+  constructor • = self::JSArray|;
+  tearoff • = self::JSArray|get#;
+}
+inline class JSExportedDartObject /* declaredRepresentationType = core::Object */ implements self::JSObject {
+  constructor • = self::JSExportedDartObject|;
+  tearoff • = self::JSExportedDartObject|get#;
+}
+inline class JSNumber /* declaredRepresentationType = core::double */ implements self::JSAny {
+  constructor • = self::JSNumber|;
+  tearoff • = self::JSNumber|get#;
+}
+static method JSAny|(core::Object value) → self::JSAny {
+  final self::JSAny #this = value;
+  return #this;
+}
+static method JSAny|get#(core::Object value) → self::JSAny
+  return self::JSAny|(value);
+static method JSObject|(core::Object value) → self::JSObject {
+  final self::JSObject #this = value;
+  return #this;
+}
+static method JSObject|get#(core::Object value) → self::JSObject
+  return self::JSObject|(value);
+static method JSArray|(core::List<self::JSAny?> value) → self::JSArray {
+  final self::JSArray #this = value;
+  return #this;
+}
+static method JSArray|get#(core::List<self::JSAny?> value) → self::JSArray
+  return self::JSArray|(value);
+static method JSExportedDartObject|(core::Object value) → self::JSExportedDartObject {
+  final self::JSExportedDartObject #this = value;
+  return #this;
+}
+static method JSExportedDartObject|get#(core::Object value) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(value);
+static method JSNumber|(core::double value) → self::JSNumber {
+  final self::JSNumber #this = value;
+  return #this;
+}
+static method JSNumber|get#(core::double value) → self::JSNumber
+  return self::JSNumber|(value);
+static method ObjectToJSExportedDartObject|get#toJS(lowered final core::Object #this) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(#this);
+static method ListToJSArray|get#toJS(lowered final core::List<self::JSAny?> #this) → self::JSArray
+  return self::JSArray|(#this);
+static method DoubleToJSNumber|get#toJS(lowered final core::double #this) → self::JSNumber
+  return self::JSNumber|(#this);
+static method main() → void {
+  self::JSArray arr = self::ListToJSArray|get#toJS(<self::JSAny>[self::DoubleToJSNumber|get#toJS(1.0), self::ObjectToJSExportedDartObject|get#toJS("foo")]);
+}

pkg/front_end/testcases/inline_class/issue51556.dart.strong.transformed.expect

@@ -0,0 +1,72 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension ObjectToJSExportedDartObject on core::Object {
+  get toJS = self::ObjectToJSExportedDartObject|get#toJS;
+}
+extension ListToJSArray on core::List<self::JSAny?> {
+  get toJS = self::ListToJSArray|get#toJS;
+}
+extension DoubleToJSNumber on core::double {
+  get toJS = self::DoubleToJSNumber|get#toJS;
+}
+inline class JSAny /* declaredRepresentationType = core::Object */ {
+  constructor • = self::JSAny|;
+  tearoff • = self::JSAny|get#;
+}
+inline class JSObject /* declaredRepresentationType = core::Object */ implements self::JSAny {
+  constructor • = self::JSObject|;
+  tearoff • = self::JSObject|get#;
+}
+inline class JSArray /* declaredRepresentationType = core::List<self::JSAny?> */ implements self::JSObject {
+  constructor • = self::JSArray|;
+  tearoff • = self::JSArray|get#;
+}
+inline class JSExportedDartObject /* declaredRepresentationType = core::Object */ implements self::JSObject {
+  constructor • = self::JSExportedDartObject|;
+  tearoff • = self::JSExportedDartObject|get#;
+}
+inline class JSNumber /* declaredRepresentationType = core::double */ implements self::JSAny {
+  constructor • = self::JSNumber|;
+  tearoff • = self::JSNumber|get#;
+}
+static method JSAny|(core::Object value) → self::JSAny {
+  final self::JSAny #this = value;
+  return #this;
+}
+static method JSAny|get#(core::Object value) → self::JSAny
+  return self::JSAny|(value);
+static method JSObject|(core::Object value) → self::JSObject {
+  final self::JSObject #this = value;
+  return #this;
+}
+static method JSObject|get#(core::Object value) → self::JSObject
+  return self::JSObject|(value);
+static method JSArray|(core::List<self::JSAny?> value) → self::JSArray {
+  final self::JSArray #this = value;
+  return #this;
+}
+static method JSArray|get#(core::List<self::JSAny?> value) → self::JSArray
+  return self::JSArray|(value);
+static method JSExportedDartObject|(core::Object value) → self::JSExportedDartObject {
+  final self::JSExportedDartObject #this = value;
+  return #this;
+}
+static method JSExportedDartObject|get#(core::Object value) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(value);
+static method JSNumber|(core::double value) → self::JSNumber {
+  final self::JSNumber #this = value;
+  return #this;
+}
+static method JSNumber|get#(core::double value) → self::JSNumber
+  return self::JSNumber|(value);
+static method ObjectToJSExportedDartObject|get#toJS(lowered final core::Object #this) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(#this);
+static method ListToJSArray|get#toJS(lowered final core::List<self::JSAny?> #this) → self::JSArray
+  return self::JSArray|(#this);
+static method DoubleToJSNumber|get#toJS(lowered final core::double #this) → self::JSNumber
+  return self::JSNumber|(#this);
+static method main() → void {
+  self::JSArray arr = self::ListToJSArray|get#toJS(core::_GrowableList::_literal2<self::JSAny>(self::DoubleToJSNumber|get#toJS(1.0), self::ObjectToJSExportedDartObject|get#toJS("foo")));
+}

pkg/front_end/testcases/inline_class/issue51556.dart.textual_outline.expect

@@ -0,0 +1,38 @@
+inline class JSAny {
+  final Object value;
+  JSAny(this.value);
+}
+
+inline class JSObject implements JSAny {
+  final Object value;
+  JSObject(this.value);
+}
+
+inline class JSArray implements JSObject {
+  final List<JSAny?> value;
+  JSArray(this.value);
+}
+
+inline class JSExportedDartObject implements JSObject {
+  final Object value;
+  JSExportedDartObject(this.value);
+}
+
+inline class JSNumber implements JSAny {
+  final double value;
+  JSNumber(this.value);
+}
+
+extension ObjectToJSExportedDartObject on Object {
+  JSExportedDartObject get toJS => JSExportedDartObject(this);
+}
+
+extension ListToJSArray on List<JSAny?> {
+  JSArray get toJS => JSArray(this);
+}
+
+extension DoubleToJSNumber on double {
+  JSNumber get toJS => JSNumber(this);
+}
+
+void main() {}

pkg/front_end/testcases/inline_class/issue51556.dart.textual_outline_modelled.expect

@@ -0,0 +1,38 @@
+extension DoubleToJSNumber on double {
+  JSNumber get toJS => JSNumber(this);
+}
+
+extension ListToJSArray on List<JSAny?> {
+  JSArray get toJS => JSArray(this);
+}
+
+extension ObjectToJSExportedDartObject on Object {
+  JSExportedDartObject get toJS => JSExportedDartObject(this);
+}
+
+inline class JSAny {
+  JSAny(this.value);
+  final Object value;
+}
+
+inline class JSArray implements JSObject {
+  JSArray(this.value);
+  final List<JSAny?> value;
+}
+
+inline class JSExportedDartObject implements JSObject {
+  JSExportedDartObject(this.value);
+  final Object value;
+}
+
+inline class JSNumber implements JSAny {
+  JSNumber(this.value);
+  final double value;
+}
+
+inline class JSObject implements JSAny {
+  JSObject(this.value);
+  final Object value;
+}
+
+void main() {}

pkg/front_end/testcases/inline_class/issue51556.dart.weak.expect

@@ -0,0 +1,72 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension ObjectToJSExportedDartObject on core::Object {
+  get toJS = self::ObjectToJSExportedDartObject|get#toJS;
+}
+extension ListToJSArray on core::List<self::JSAny?> {
+  get toJS = self::ListToJSArray|get#toJS;
+}
+extension DoubleToJSNumber on core::double {
+  get toJS = self::DoubleToJSNumber|get#toJS;
+}
+inline class JSAny /* declaredRepresentationType = core::Object */ {
+  constructor • = self::JSAny|;
+  tearoff • = self::JSAny|get#;
+}
+inline class JSObject /* declaredRepresentationType = core::Object */ implements self::JSAny {
+  constructor • = self::JSObject|;
+  tearoff • = self::JSObject|get#;
+}
+inline class JSArray /* declaredRepresentationType = core::List<self::JSAny?> */ implements self::JSObject {
+  constructor • = self::JSArray|;
+  tearoff • = self::JSArray|get#;
+}
+inline class JSExportedDartObject /* declaredRepresentationType = core::Object */ implements self::JSObject {
+  constructor • = self::JSExportedDartObject|;
+  tearoff • = self::JSExportedDartObject|get#;
+}
+inline class JSNumber /* declaredRepresentationType = core::double */ implements self::JSAny {
+  constructor • = self::JSNumber|;
+  tearoff • = self::JSNumber|get#;
+}
+static method JSAny|(core::Object value) → self::JSAny {
+  final self::JSAny #this = value;
+  return #this;
+}
+static method JSAny|get#(core::Object value) → self::JSAny
+  return self::JSAny|(value);
+static method JSObject|(core::Object value) → self::JSObject {
+  final self::JSObject #this = value;
+  return #this;
+}
+static method JSObject|get#(core::Object value) → self::JSObject
+  return self::JSObject|(value);
+static method JSArray|(core::List<self::JSAny?> value) → self::JSArray {
+  final self::JSArray #this = value;
+  return #this;
+}
+static method JSArray|get#(core::List<self::JSAny?> value) → self::JSArray
+  return self::JSArray|(value);
+static method JSExportedDartObject|(core::Object value) → self::JSExportedDartObject {
+  final self::JSExportedDartObject #this = value;
+  return #this;
+}
+static method JSExportedDartObject|get#(core::Object value) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(value);
+static method JSNumber|(core::double value) → self::JSNumber {
+  final self::JSNumber #this = value;
+  return #this;
+}
+static method JSNumber|get#(core::double value) → self::JSNumber
+  return self::JSNumber|(value);
+static method ObjectToJSExportedDartObject|get#toJS(lowered final core::Object #this) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(#this);
+static method ListToJSArray|get#toJS(lowered final core::List<self::JSAny?> #this) → self::JSArray
+  return self::JSArray|(#this);
+static method DoubleToJSNumber|get#toJS(lowered final core::double #this) → self::JSNumber
+  return self::JSNumber|(#this);
+static method main() → void {
+  self::JSArray arr = self::ListToJSArray|get#toJS(<self::JSAny>[self::DoubleToJSNumber|get#toJS(1.0), self::ObjectToJSExportedDartObject|get#toJS("foo")]);
+}

pkg/front_end/testcases/inline_class/issue51556.dart.weak.modular.expect

@@ -0,0 +1,72 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension ObjectToJSExportedDartObject on core::Object {
+  get toJS = self::ObjectToJSExportedDartObject|get#toJS;
+}
+extension ListToJSArray on core::List<self::JSAny?> {
+  get toJS = self::ListToJSArray|get#toJS;
+}
+extension DoubleToJSNumber on core::double {
+  get toJS = self::DoubleToJSNumber|get#toJS;
+}
+inline class JSAny /* declaredRepresentationType = core::Object */ {
+  constructor • = self::JSAny|;
+  tearoff • = self::JSAny|get#;
+}
+inline class JSObject /* declaredRepresentationType = core::Object */ implements self::JSAny {
+  constructor • = self::JSObject|;
+  tearoff • = self::JSObject|get#;
+}
+inline class JSArray /* declaredRepresentationType = core::List<self::JSAny?> */ implements self::JSObject {
+  constructor • = self::JSArray|;
+  tearoff • = self::JSArray|get#;
+}
+inline class JSExportedDartObject /* declaredRepresentationType = core::Object */ implements self::JSObject {
+  constructor • = self::JSExportedDartObject|;
+  tearoff • = self::JSExportedDartObject|get#;
+}
+inline class JSNumber /* declaredRepresentationType = core::double */ implements self::JSAny {
+  constructor • = self::JSNumber|;
+  tearoff • = self::JSNumber|get#;
+}
+static method JSAny|(core::Object value) → self::JSAny {
+  final self::JSAny #this = value;
+  return #this;
+}
+static method JSAny|get#(core::Object value) → self::JSAny
+  return self::JSAny|(value);
+static method JSObject|(core::Object value) → self::JSObject {
+  final self::JSObject #this = value;
+  return #this;
+}
+static method JSObject|get#(core::Object value) → self::JSObject
+  return self::JSObject|(value);
+static method JSArray|(core::List<self::JSAny?> value) → self::JSArray {
+  final self::JSArray #this = value;
+  return #this;
+}
+static method JSArray|get#(core::List<self::JSAny?> value) → self::JSArray
+  return self::JSArray|(value);
+static method JSExportedDartObject|(core::Object value) → self::JSExportedDartObject {
+  final self::JSExportedDartObject #this = value;
+  return #this;
+}
+static method JSExportedDartObject|get#(core::Object value) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(value);
+static method JSNumber|(core::double value) → self::JSNumber {
+  final self::JSNumber #this = value;
+  return #this;
+}
+static method JSNumber|get#(core::double value) → self::JSNumber
+  return self::JSNumber|(value);
+static method ObjectToJSExportedDartObject|get#toJS(lowered final core::Object #this) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(#this);
+static method ListToJSArray|get#toJS(lowered final core::List<self::JSAny?> #this) → self::JSArray
+  return self::JSArray|(#this);
+static method DoubleToJSNumber|get#toJS(lowered final core::double #this) → self::JSNumber
+  return self::JSNumber|(#this);
+static method main() → void {
+  self::JSArray arr = self::ListToJSArray|get#toJS(<self::JSAny>[self::DoubleToJSNumber|get#toJS(1.0), self::ObjectToJSExportedDartObject|get#toJS("foo")]);
+}

pkg/front_end/testcases/inline_class/issue51556.dart.weak.outline.expect

@@ -0,0 +1,61 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension ObjectToJSExportedDartObject on core::Object {
+  get toJS = self::ObjectToJSExportedDartObject|get#toJS;
+}
+extension ListToJSArray on core::List<self::JSAny?> {
+  get toJS = self::ListToJSArray|get#toJS;
+}
+extension DoubleToJSNumber on core::double {
+  get toJS = self::DoubleToJSNumber|get#toJS;
+}
+inline class JSAny /* declaredRepresentationType = core::Object */ {
+  constructor • = self::JSAny|;
+  tearoff • = self::JSAny|get#;
+}
+inline class JSObject /* declaredRepresentationType = core::Object */ implements self::JSAny {
+  constructor • = self::JSObject|;
+  tearoff • = self::JSObject|get#;
+}
+inline class JSArray /* declaredRepresentationType = core::List<self::JSAny?> */ implements self::JSObject {
+  constructor • = self::JSArray|;
+  tearoff • = self::JSArray|get#;
+}
+inline class JSExportedDartObject /* declaredRepresentationType = core::Object */ implements self::JSObject {
+  constructor • = self::JSExportedDartObject|;
+  tearoff • = self::JSExportedDartObject|get#;
+}
+inline class JSNumber /* declaredRepresentationType = core::double */ implements self::JSAny {
+  constructor • = self::JSNumber|;
+  tearoff • = self::JSNumber|get#;
+}
+static method JSAny|(core::Object value) → self::JSAny
+  ;
+static method JSAny|get#(core::Object value) → self::JSAny
+  return self::JSAny|(value);
+static method JSObject|(core::Object value) → self::JSObject
+  ;
+static method JSObject|get#(core::Object value) → self::JSObject
+  return self::JSObject|(value);
+static method JSArray|(core::List<self::JSAny?> value) → self::JSArray
+  ;
+static method JSArray|get#(core::List<self::JSAny?> value) → self::JSArray
+  return self::JSArray|(value);
+static method JSExportedDartObject|(core::Object value) → self::JSExportedDartObject
+  ;
+static method JSExportedDartObject|get#(core::Object value) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(value);
+static method JSNumber|(core::double value) → self::JSNumber
+  ;
+static method JSNumber|get#(core::double value) → self::JSNumber
+  return self::JSNumber|(value);
+static method ObjectToJSExportedDartObject|get#toJS(lowered final core::Object #this) → self::JSExportedDartObject
+  ;
+static method ListToJSArray|get#toJS(lowered final core::List<self::JSAny?> #this) → self::JSArray
+  ;
+static method DoubleToJSNumber|get#toJS(lowered final core::double #this) → self::JSNumber
+  ;
+static method main() → void
+  ;

pkg/front_end/testcases/inline_class/issue51556.dart.weak.transformed.expect

@@ -0,0 +1,72 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+extension ObjectToJSExportedDartObject on core::Object {
+  get toJS = self::ObjectToJSExportedDartObject|get#toJS;
+}
+extension ListToJSArray on core::List<self::JSAny?> {
+  get toJS = self::ListToJSArray|get#toJS;
+}
+extension DoubleToJSNumber on core::double {
+  get toJS = self::DoubleToJSNumber|get#toJS;
+}
+inline class JSAny /* declaredRepresentationType = core::Object */ {
+  constructor • = self::JSAny|;
+  tearoff • = self::JSAny|get#;
+}
+inline class JSObject /* declaredRepresentationType = core::Object */ implements self::JSAny {
+  constructor • = self::JSObject|;
+  tearoff • = self::JSObject|get#;
+}
+inline class JSArray /* declaredRepresentationType = core::List<self::JSAny?> */ implements self::JSObject {
+  constructor • = self::JSArray|;
+  tearoff • = self::JSArray|get#;
+}
+inline class JSExportedDartObject /* declaredRepresentationType = core::Object */ implements self::JSObject {
+  constructor • = self::JSExportedDartObject|;
+  tearoff • = self::JSExportedDartObject|get#;
+}
+inline class JSNumber /* declaredRepresentationType = core::double */ implements self::JSAny {
+  constructor • = self::JSNumber|;
+  tearoff • = self::JSNumber|get#;
+}
+static method JSAny|(core::Object value) → self::JSAny {
+  final self::JSAny #this = value;
+  return #this;
+}
+static method JSAny|get#(core::Object value) → self::JSAny
+  return self::JSAny|(value);
+static method JSObject|(core::Object value) → self::JSObject {
+  final self::JSObject #this = value;
+  return #this;
+}
+static method JSObject|get#(core::Object value) → self::JSObject
+  return self::JSObject|(value);
+static method JSArray|(core::List<self::JSAny?> value) → self::JSArray {
+  final self::JSArray #this = value;
+  return #this;
+}
+static method JSArray|get#(core::List<self::JSAny?> value) → self::JSArray
+  return self::JSArray|(value);
+static method JSExportedDartObject|(core::Object value) → self::JSExportedDartObject {
+  final self::JSExportedDartObject #this = value;
+  return #this;
+}
+static method JSExportedDartObject|get#(core::Object value) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(value);
+static method JSNumber|(core::double value) → self::JSNumber {
+  final self::JSNumber #this = value;
+  return #this;
+}
+static method JSNumber|get#(core::double value) → self::JSNumber
+  return self::JSNumber|(value);
+static method ObjectToJSExportedDartObject|get#toJS(lowered final core::Object #this) → self::JSExportedDartObject
+  return self::JSExportedDartObject|(#this);
+static method ListToJSArray|get#toJS(lowered final core::List<self::JSAny?> #this) → self::JSArray
+  return self::JSArray|(#this);
+static method DoubleToJSNumber|get#toJS(lowered final core::double #this) → self::JSNumber
+  return self::JSNumber|(#this);
+static method main() → void {
+  self::JSArray arr = self::ListToJSArray|get#toJS(core::_GrowableList::_literal2<self::JSAny>(self::DoubleToJSNumber|get#toJS(1.0), self::ObjectToJSExportedDartObject|get#toJS("foo")));
+}

pkg/kernel/lib/src/standard_bounds.dart

@@ -865,22 +865,22 @@ mixin StandardBounds {
     }
 
     // UP(T1, T2) = T2 if T1 <: T2
-    //   Note that both types must be class types at this point.
-    assert(type1 is InterfaceType,
+    //   Note that both types must be interface or inline types at this point.
+    assert(type1 is InterfaceType || type1 is InlineType,
         "Expected type1 to be an interface type, got '${type1.runtimeType}'.");
-    assert(type2 is InterfaceType,
+    assert(type2 is InterfaceType || type2 is InlineType,
         "Expected type2 to be an interface type, got '${type2.runtimeType}'.");
 
     // We use the non-nullable variants of the two interfaces types to determine
     // T1 <: T2 without using the nullability of the outermost type. The result
     // uses [uniteNullabilities] to compute the resulting type if the subtype
     // relation is established.
-    InterfaceType typeWithoutNullabilityMarker1 =
+    DartType typeWithoutNullabilityMarker1 =
         computeTypeWithoutNullabilityMarker(type1,
-            isNonNullableByDefault: isNonNullableByDefault) as InterfaceType;
-    InterfaceType typeWithoutNullabilityMarker2 =
+            isNonNullableByDefault: isNonNullableByDefault);
+    DartType typeWithoutNullabilityMarker2 =
         computeTypeWithoutNullabilityMarker(type2,
-            isNonNullableByDefault: isNonNullableByDefault) as InterfaceType;
+            isNonNullableByDefault: isNonNullableByDefault);
 
     if (isSubtypeOf(typeWithoutNullabilityMarker1,
         typeWithoutNullabilityMarker2, SubtypeCheckMode.withNullabilities)) {
@@ -889,7 +889,7 @@ mixin StandardBounds {
     }
 
     // UP(T1, T2) = T1 if T2 <: T1
-    //   Note that both types must be class types at this point.
+    //   Note that both types must be interface or inline types at this point.
     if (isSubtypeOf(typeWithoutNullabilityMarker2,
         typeWithoutNullabilityMarker1, SubtypeCheckMode.withNullabilities)) {
       return type1.withDeclaredNullability(uniteNullabilities(
@@ -897,33 +897,59 @@ mixin StandardBounds {
     }
 
     // UP(C<T0, ..., Tn>, C<S0, ..., Sn>) = C<R0,..., Rn> where Ri is UP(Ti, Si)
+    Class? cls;
+    InlineClass? inlineClass;
+    List<TypeParameter>? typeParameters;
+    List<DartType>? leftArguments;
+    List<DartType>? rightArguments;
     if (type1 is InterfaceType && type2 is InterfaceType) {
-      Class klass = type1.classNode;
-      if (type2.classNode == klass) {
-        int n = klass.typeParameters.length;
-        List<DartType> leftArguments = type1.typeArguments;
-        List<DartType> rightArguments = type2.typeArguments;
-        List<DartType> typeArguments = new List<DartType>.of(leftArguments);
-        for (int i = 0; i < n; ++i) {
-          int variance = klass.typeParameters[i].variance;
-          if (variance == Variance.contravariant) {
-            typeArguments[i] = _getNullabilityAwareStandardLowerBound(
-                leftArguments[i], rightArguments[i],
-                isNonNullableByDefault: isNonNullableByDefault);
-          } else if (variance == Variance.invariant) {
-            if (!areMutualSubtypes(leftArguments[i], rightArguments[i],
-                SubtypeCheckMode.withNullabilities)) {
-              return hierarchy.getLegacyLeastUpperBound(type1, type2,
-                  isNonNullableByDefault: isNonNullableByDefault);
-            }
-          } else {
-            typeArguments[i] = _getNullabilityAwareStandardUpperBound(
-                leftArguments[i], rightArguments[i],
+      if (type1.classNode == type2.classNode) {
+        cls = type1.classNode;
+        typeParameters = cls.typeParameters;
+        leftArguments = type1.typeArguments;
+        rightArguments = type2.typeArguments;
+      }
+    }
+    if (type1 is InlineType && type2 is InlineType) {
+      if (type1.inlineClass == type2.inlineClass) {
+        inlineClass = type1.inlineClass;
+        leftArguments = type1.typeArguments;
+        rightArguments = type2.typeArguments;
+      }
+    }
+
+    if (typeParameters != null &&
+        leftArguments != null &&
+        rightArguments != null) {
+      int n = typeParameters.length;
+      List<DartType> typeArguments = new List<DartType>.of(leftArguments);
+      for (int i = 0; i < n; ++i) {
+        int variance = typeParameters[i].variance;
+        if (variance == Variance.contravariant) {
+          typeArguments[i] = _getNullabilityAwareStandardLowerBound(
+              leftArguments[i], rightArguments[i],
+              isNonNullableByDefault: isNonNullableByDefault);
+        } else if (variance == Variance.invariant) {
+          if (!areMutualSubtypes(leftArguments[i], rightArguments[i],
+              SubtypeCheckMode.withNullabilities)) {
+            return _getLegacyLeastUpperBound(type1, type2,
                 isNonNullableByDefault: isNonNullableByDefault);
           }
+        } else {
+          typeArguments[i] = _getNullabilityAwareStandardUpperBound(
+              leftArguments[i], rightArguments[i],
+              isNonNullableByDefault: isNonNullableByDefault);
         }
+      }
+      if (cls != null) {
         return new InterfaceType(
-            klass,
+            cls,
+            uniteNullabilities(
+                type1.declaredNullability, type2.declaredNullability),
+            typeArguments);
+      } else {
+        return new InlineType(
+            inlineClass!,
             uniteNullabilities(
                 type1.declaredNullability, type2.declaredNullability),
             typeArguments);
@@ -932,11 +958,79 @@ mixin StandardBounds {
 
     // UP(C0<T0, ..., Tn>, C1<S0, ..., Sk>)
     //   = least upper bound of two interfaces as in Dart 1.
-    return hierarchy.getLegacyLeastUpperBound(
-        type1 as InterfaceType, type2 as InterfaceType,
+    return _getLegacyLeastUpperBound(type1, type2,
         isNonNullableByDefault: isNonNullableByDefault);
   }
 
+  DartType _getLegacyLeastUpperBound(DartType type1, DartType type2,
+      {required bool isNonNullableByDefault}) {
+    if (type1 is InterfaceType && type2 is InterfaceType) {
+      return hierarchy.getLegacyLeastUpperBound(type1, type2,
+          isNonNullableByDefault: isNonNullableByDefault);
+    } else if (type1 is InlineType && type2 is InlineType) {
+      // This mimics the legacy least upper bound implementation for regular
+      // classes, where the least upper bound is found as the single common
+      // supertype with the highest class hierarchy depth.
+
+      // TODO(johnniwinther): Move this computation to [ClassHierarchyBase] and
+      // cache it there.
+      Map<InlineClass, int> inlineClassDepth = {};
+
+      int computeInlineClassDepth(InlineClass inlineClass) {
+        int? depth = inlineClassDepth[inlineClass];
+        if (depth == null) {
+          int maxDepth = 0;
+          for (InlineType implemented in inlineClass.implements) {
+            int supertypeDepth =
+                computeInlineClassDepth(implemented.inlineClass);
+            if (supertypeDepth >= maxDepth) {
+              maxDepth = supertypeDepth + 1;
+            }
+          }
+          depth = inlineClassDepth[inlineClass] = maxDepth;
+        }
+        return depth;
+      }
+
+      void computeSuperTypes(InlineType type, List<InlineType> supertypes) {
+        computeInlineClassDepth(type.inlineClass);
+        supertypes.add(type);
+        for (InlineType implemented in type.inlineClass.implements) {
+          InlineType supertype = hierarchy.getInlineTypeAsInstanceOf(
+              type, implemented.inlineClass,
+              isNonNullableByDefault: isNonNullableByDefault)!;
+          computeSuperTypes(supertype, supertypes);
+        }
+      }
+
+      List<InlineType> supertypes1 = [];
+      computeSuperTypes(type1, supertypes1);
+      List<InlineType> supertypes2 = [];
+      computeSuperTypes(type2, supertypes2);
+
+      Set<InlineType> set = supertypes1.toSet()..retainAll(supertypes2);
+      Map<int, List<InlineType>> commonSupertypesByDepth = {};
+      for (InlineType type in set) {
+        (commonSupertypesByDepth[inlineClassDepth[type.inlineClass]!] ??= [])
+            .add(type);
+      }
+      int maxDepth = -1;
+      InlineType? candidate;
+      for (MapEntry<int, List<InlineType>> entry
+          in commonSupertypesByDepth.entries) {
+        if (entry.key > maxDepth && entry.value.length == 1) {
+          maxDepth = entry.key;
+          candidate = entry.value.single;
+        }
+      }
+      if (candidate != null) {
+        return candidate;
+      }
+    }
+    return coreTypes.objectRawType(
+        uniteNullabilities(type1.nullability, type2.nullability));
+  }
+
   /// Computes the nullability-aware lower bound of two function types.
   ///
   /// The algorithm is defined as follows: