diff --git a/pkg/front_end/lib/src/fasta/type_inference/inference_visitor_base.dart b/pkg/front_end/lib/src/fasta/type_inference/inference_visitor_base.dart
index b3f31131e38..298a707f8ac 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/inference_visitor_base.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/inference_visitor_base.dart
@@ -1025,10 +1025,9 @@ abstract class InferenceVisitorBase implements InferenceVisitor {
     } else {
       for (DartType implement
           in extensionType.extensionTypeDeclaration.implements) {
-        // TODO(johnniwinther): Handle non-extension type supertypes.
         if (implement is ExtensionType) {
-          ExtensionType supertype =
-              hierarchyBuilder.getExtensionTypeAsInstanceOf(
+          ExtensionType supertype = hierarchyBuilder
+              .getExtensionTypeAsInstanceOfExtensionTypeDeclaration(
                   extensionType, implement.extensionTypeDeclaration,
                   isNonNullableByDefault: isNonNullableByDefault)!;
           ObjectAccessTarget? target = _findDirectExtensionTypeMember(
@@ -1039,6 +1038,23 @@ abstract class InferenceVisitorBase implements InferenceVisitor {
           if (target != null) {
             return target;
           }
+        } else if (implement is InterfaceType) {
+          InterfaceType supertype =
+              hierarchyBuilder.getExtensionTypeAsInstanceOfClass(
+                  extensionType, implement.classNode,
+                  isNonNullableByDefault: isNonNullableByDefault)!;
+          Member? interfaceMember = _getInterfaceMember(
+              supertype.classNode, name, isSetter, fileOffset);
+          if (interfaceMember != null) {
+            return new ObjectAccessTarget.interfaceMember(
+                receiverType, interfaceMember,
+                isPotentiallyNullable: isReceiverTypePotentiallyNullable);
+          }
+        } else {
+          assert(
+              false,
+              "Unexpected supertype $implement extension type declaration of "
+              "$extensionType.");
         }
       }
       return null;
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart
new file mode 100644
index 00000000000..e8715b4d2cc
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart
@@ -0,0 +1,62 @@
+// 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 b
+// BSD-style license that can be found in the LICENSE file.
+
+class A {
+  int field = 42;
+  int method() => field;
+  int get getter => field;
+  void set setter(int value) {
+    field = value;
+  }
+}
+
+class B extends A {}
+
+extension type E(B it) implements B {}
+
+extension type F(B it) implements E {}
+
+main() {
+  B b = B();
+  E e = E(b);
+  F f = F(b);
+
+  expect(42, b.field);
+  expect(42, e.field);
+  expect(42, f.field);
+
+  b.field = 87;
+  expect(87, b.method());
+  expect(87, e.method());
+  expect(87, f.method());
+
+  b.setter = 123;
+  expect(123, b.getter);
+  expect(123, e.getter);
+  expect(123, f.getter);
+
+  e.setter = 87;
+  expect(87, b.field);
+  expect(87, e.field);
+  expect(87, f.field);
+
+  e.field = 42;
+  expect(42, b.getter);
+  expect(42, e.getter);
+  expect(42, f.getter);
+
+  f.field = 87;
+  expect(87, b.field);
+  expect(87, e.field);
+  expect(87, f.field);
+
+  f.setter = 123;
+  expect(123, b.method());
+  expect(123, e.method());
+  expect(123, f.method());
+}
+
+expect(expected, actual) {
+  if (expected != actual) throw 'Expected $expected, actual $actual';
+}
\ No newline at end of file
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.strong.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.strong.expect
new file mode 100644
index 00000000000..359b7646332
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.strong.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field core::int field = 42;
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method method() → core::int
+    return this.{self::A::field}{core::int};
+  get getter() → core::int
+    return this.{self::A::field}{core::int};
+  set setter(core::int value) → void {
+    this.{self::A::field} = value;
+  }
+}
+class B extends self::A {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+}
+extension type E(self::B it) implements self::B {
+  constructor • = self::E|;
+  tearoff • = self::E|_#new#tearOff;
+}
+extension type F(self::B it) implements self::E {
+  constructor • = self::F|;
+  tearoff • = self::F|_#new#tearOff;
+}
+static inline-class-member method E|(dynamic it) → self::E {
+  lowered final self::E #this = it;
+  return #this;
+}
+static inline-class-member method E|_#new#tearOff(dynamic it) → self::E
+  return self::E|(it);
+static inline-class-member method F|(dynamic it) → self::F {
+  lowered final self::F #this = it;
+  return #this;
+}
+static inline-class-member method F|_#new#tearOff(dynamic it) → self::F
+  return self::F|(it);
+static method main() → dynamic {
+  self::B b = new self::B::•();
+  self::E e = self::E|(b);
+  self::F f = self::F|(b);
+  self::expect(42, b.{self::A::field}{core::int});
+  self::expect(42, e.{self::A::field}{core::int});
+  self::expect(42, f.{self::A::field}{core::int});
+  b.{self::A::field} = 87;
+  self::expect(87, b.{self::A::method}(){() → core::int});
+  self::expect(87, e.{self::A::method}(){() → core::int});
+  self::expect(87, f.{self::A::method}(){() → core::int});
+  b.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::getter}{core::int});
+  self::expect(123, e.{self::A::getter}{core::int});
+  self::expect(123, f.{self::A::getter}{core::int});
+  e.{self::A::setter} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  e.{self::A::field} = 42;
+  self::expect(42, b.{self::A::getter}{core::int});
+  self::expect(42, e.{self::A::getter}{core::int});
+  self::expect(42, f.{self::A::getter}{core::int});
+  f.{self::A::field} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  f.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::method}(){() → core::int});
+  self::expect(123, e.{self::A::method}(){() → core::int});
+  self::expect(123, f.{self::A::method}(){() → core::int});
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.strong.transformed.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.strong.transformed.expect
new file mode 100644
index 00000000000..359b7646332
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.strong.transformed.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field core::int field = 42;
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method method() → core::int
+    return this.{self::A::field}{core::int};
+  get getter() → core::int
+    return this.{self::A::field}{core::int};
+  set setter(core::int value) → void {
+    this.{self::A::field} = value;
+  }
+}
+class B extends self::A {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+}
+extension type E(self::B it) implements self::B {
+  constructor • = self::E|;
+  tearoff • = self::E|_#new#tearOff;
+}
+extension type F(self::B it) implements self::E {
+  constructor • = self::F|;
+  tearoff • = self::F|_#new#tearOff;
+}
+static inline-class-member method E|(dynamic it) → self::E {
+  lowered final self::E #this = it;
+  return #this;
+}
+static inline-class-member method E|_#new#tearOff(dynamic it) → self::E
+  return self::E|(it);
+static inline-class-member method F|(dynamic it) → self::F {
+  lowered final self::F #this = it;
+  return #this;
+}
+static inline-class-member method F|_#new#tearOff(dynamic it) → self::F
+  return self::F|(it);
+static method main() → dynamic {
+  self::B b = new self::B::•();
+  self::E e = self::E|(b);
+  self::F f = self::F|(b);
+  self::expect(42, b.{self::A::field}{core::int});
+  self::expect(42, e.{self::A::field}{core::int});
+  self::expect(42, f.{self::A::field}{core::int});
+  b.{self::A::field} = 87;
+  self::expect(87, b.{self::A::method}(){() → core::int});
+  self::expect(87, e.{self::A::method}(){() → core::int});
+  self::expect(87, f.{self::A::method}(){() → core::int});
+  b.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::getter}{core::int});
+  self::expect(123, e.{self::A::getter}{core::int});
+  self::expect(123, f.{self::A::getter}{core::int});
+  e.{self::A::setter} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  e.{self::A::field} = 42;
+  self::expect(42, b.{self::A::getter}{core::int});
+  self::expect(42, e.{self::A::getter}{core::int});
+  self::expect(42, f.{self::A::getter}{core::int});
+  f.{self::A::field} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  f.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::method}(){() → core::int});
+  self::expect(123, e.{self::A::method}(){() → core::int});
+  self::expect(123, f.{self::A::method}(){() → core::int});
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.textual_outline.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.textual_outline.expect
new file mode 100644
index 00000000000..e06fa973dd3
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.textual_outline.expect
@@ -0,0 +1,10 @@
+class A {
+  int field = 42;
+  int method() => field;
+  int get getter => field;
+  void set setter(int value) {}
+}
+class B extends A {}
+extension type E(B it) implements B {} extension type F(B it) implements E {}
+main() {}
+expect(expected, actual) {}
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.expect
new file mode 100644
index 00000000000..359b7646332
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field core::int field = 42;
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method method() → core::int
+    return this.{self::A::field}{core::int};
+  get getter() → core::int
+    return this.{self::A::field}{core::int};
+  set setter(core::int value) → void {
+    this.{self::A::field} = value;
+  }
+}
+class B extends self::A {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+}
+extension type E(self::B it) implements self::B {
+  constructor • = self::E|;
+  tearoff • = self::E|_#new#tearOff;
+}
+extension type F(self::B it) implements self::E {
+  constructor • = self::F|;
+  tearoff • = self::F|_#new#tearOff;
+}
+static inline-class-member method E|(dynamic it) → self::E {
+  lowered final self::E #this = it;
+  return #this;
+}
+static inline-class-member method E|_#new#tearOff(dynamic it) → self::E
+  return self::E|(it);
+static inline-class-member method F|(dynamic it) → self::F {
+  lowered final self::F #this = it;
+  return #this;
+}
+static inline-class-member method F|_#new#tearOff(dynamic it) → self::F
+  return self::F|(it);
+static method main() → dynamic {
+  self::B b = new self::B::•();
+  self::E e = self::E|(b);
+  self::F f = self::F|(b);
+  self::expect(42, b.{self::A::field}{core::int});
+  self::expect(42, e.{self::A::field}{core::int});
+  self::expect(42, f.{self::A::field}{core::int});
+  b.{self::A::field} = 87;
+  self::expect(87, b.{self::A::method}(){() → core::int});
+  self::expect(87, e.{self::A::method}(){() → core::int});
+  self::expect(87, f.{self::A::method}(){() → core::int});
+  b.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::getter}{core::int});
+  self::expect(123, e.{self::A::getter}{core::int});
+  self::expect(123, f.{self::A::getter}{core::int});
+  e.{self::A::setter} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  e.{self::A::field} = 42;
+  self::expect(42, b.{self::A::getter}{core::int});
+  self::expect(42, e.{self::A::getter}{core::int});
+  self::expect(42, f.{self::A::getter}{core::int});
+  f.{self::A::field} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  f.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::method}(){() → core::int});
+  self::expect(123, e.{self::A::method}(){() → core::int});
+  self::expect(123, f.{self::A::method}(){() → core::int});
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.modular.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.modular.expect
new file mode 100644
index 00000000000..359b7646332
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.modular.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field core::int field = 42;
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method method() → core::int
+    return this.{self::A::field}{core::int};
+  get getter() → core::int
+    return this.{self::A::field}{core::int};
+  set setter(core::int value) → void {
+    this.{self::A::field} = value;
+  }
+}
+class B extends self::A {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+}
+extension type E(self::B it) implements self::B {
+  constructor • = self::E|;
+  tearoff • = self::E|_#new#tearOff;
+}
+extension type F(self::B it) implements self::E {
+  constructor • = self::F|;
+  tearoff • = self::F|_#new#tearOff;
+}
+static inline-class-member method E|(dynamic it) → self::E {
+  lowered final self::E #this = it;
+  return #this;
+}
+static inline-class-member method E|_#new#tearOff(dynamic it) → self::E
+  return self::E|(it);
+static inline-class-member method F|(dynamic it) → self::F {
+  lowered final self::F #this = it;
+  return #this;
+}
+static inline-class-member method F|_#new#tearOff(dynamic it) → self::F
+  return self::F|(it);
+static method main() → dynamic {
+  self::B b = new self::B::•();
+  self::E e = self::E|(b);
+  self::F f = self::F|(b);
+  self::expect(42, b.{self::A::field}{core::int});
+  self::expect(42, e.{self::A::field}{core::int});
+  self::expect(42, f.{self::A::field}{core::int});
+  b.{self::A::field} = 87;
+  self::expect(87, b.{self::A::method}(){() → core::int});
+  self::expect(87, e.{self::A::method}(){() → core::int});
+  self::expect(87, f.{self::A::method}(){() → core::int});
+  b.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::getter}{core::int});
+  self::expect(123, e.{self::A::getter}{core::int});
+  self::expect(123, f.{self::A::getter}{core::int});
+  e.{self::A::setter} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  e.{self::A::field} = 42;
+  self::expect(42, b.{self::A::getter}{core::int});
+  self::expect(42, e.{self::A::getter}{core::int});
+  self::expect(42, f.{self::A::getter}{core::int});
+  f.{self::A::field} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  f.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::method}(){() → core::int});
+  self::expect(123, e.{self::A::method}(){() → core::int});
+  self::expect(123, f.{self::A::method}(){() → core::int});
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.outline.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.outline.expect
new file mode 100644
index 00000000000..d5b3e0ff33c
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.outline.expect
@@ -0,0 +1,39 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field core::int field;
+  synthetic constructor •() → self::A
+    ;
+  method method() → core::int
+    ;
+  get getter() → core::int
+    ;
+  set setter(core::int value) → void
+    ;
+}
+class B extends self::A {
+  synthetic constructor •() → self::B
+    ;
+}
+extension type E(self::B it) implements self::B {
+  constructor • = self::E|;
+  tearoff • = self::E|_#new#tearOff;
+}
+extension type F(self::B it) implements self::E {
+  constructor • = self::F|;
+  tearoff • = self::F|_#new#tearOff;
+}
+static inline-class-member method E|(dynamic it) → self::E
+  ;
+static inline-class-member method E|_#new#tearOff(dynamic it) → self::E
+  return self::E|(it);
+static inline-class-member method F|(dynamic it) → self::F
+  ;
+static inline-class-member method F|_#new#tearOff(dynamic it) → self::F
+  return self::F|(it);
+static method main() → dynamic
+  ;
+static method expect(dynamic expected, dynamic actual) → dynamic
+  ;
diff --git a/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.transformed.expect b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.transformed.expect
new file mode 100644
index 00000000000..359b7646332
--- /dev/null
+++ b/pkg/front_end/testcases/inline_class/extension_types/access_non_extension_type_members.dart.weak.transformed.expect
@@ -0,0 +1,78 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field core::int field = 42;
+  synthetic constructor •() → self::A
+    : super core::Object::•()
+    ;
+  method method() → core::int
+    return this.{self::A::field}{core::int};
+  get getter() → core::int
+    return this.{self::A::field}{core::int};
+  set setter(core::int value) → void {
+    this.{self::A::field} = value;
+  }
+}
+class B extends self::A {
+  synthetic constructor •() → self::B
+    : super self::A::•()
+    ;
+}
+extension type E(self::B it) implements self::B {
+  constructor • = self::E|;
+  tearoff • = self::E|_#new#tearOff;
+}
+extension type F(self::B it) implements self::E {
+  constructor • = self::F|;
+  tearoff • = self::F|_#new#tearOff;
+}
+static inline-class-member method E|(dynamic it) → self::E {
+  lowered final self::E #this = it;
+  return #this;
+}
+static inline-class-member method E|_#new#tearOff(dynamic it) → self::E
+  return self::E|(it);
+static inline-class-member method F|(dynamic it) → self::F {
+  lowered final self::F #this = it;
+  return #this;
+}
+static inline-class-member method F|_#new#tearOff(dynamic it) → self::F
+  return self::F|(it);
+static method main() → dynamic {
+  self::B b = new self::B::•();
+  self::E e = self::E|(b);
+  self::F f = self::F|(b);
+  self::expect(42, b.{self::A::field}{core::int});
+  self::expect(42, e.{self::A::field}{core::int});
+  self::expect(42, f.{self::A::field}{core::int});
+  b.{self::A::field} = 87;
+  self::expect(87, b.{self::A::method}(){() → core::int});
+  self::expect(87, e.{self::A::method}(){() → core::int});
+  self::expect(87, f.{self::A::method}(){() → core::int});
+  b.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::getter}{core::int});
+  self::expect(123, e.{self::A::getter}{core::int});
+  self::expect(123, f.{self::A::getter}{core::int});
+  e.{self::A::setter} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  e.{self::A::field} = 42;
+  self::expect(42, b.{self::A::getter}{core::int});
+  self::expect(42, e.{self::A::getter}{core::int});
+  self::expect(42, f.{self::A::getter}{core::int});
+  f.{self::A::field} = 87;
+  self::expect(87, b.{self::A::field}{core::int});
+  self::expect(87, e.{self::A::field}{core::int});
+  self::expect(87, f.{self::A::field}{core::int});
+  f.{self::A::setter} = 123;
+  self::expect(123, b.{self::A::method}(){() → core::int});
+  self::expect(123, e.{self::A::method}(){() → core::int});
+  self::expect(123, f.{self::A::method}(){() → core::int});
+}
+static method expect(dynamic expected, dynamic actual) → dynamic {
+  if(!(expected =={core::Object::==}{(core::Object) → core::bool} actual))
+    throw "Expected ${expected}, actual ${actual}";
+}
diff --git a/pkg/front_end/testcases/textual_outline.status b/pkg/front_end/testcases/textual_outline.status
index 06ab9a6606c..929a389e8b4 100644
--- a/pkg/front_end/testcases/textual_outline.status
+++ b/pkg/front_end/testcases/textual_outline.status
@@ -124,6 +124,7 @@ general/type_variable_in_static_context: FormatterCrash
 general/var_as_type_name: FormatterCrash
 inline_class/inline_class_declaration: FormatterCrash
 inline_class/issue52667: FormatterCrash
+inline_class/extension_types/access_non_extension_type_members: FormatterCrash
 inline_class/extension_types/annotations: FormatterCrash
 inline_class/extension_types/const_constructor: FormatterCrash
 inline_class/extension_types/constructor_access: FormatterCrash
diff --git a/pkg/kernel/lib/class_hierarchy.dart b/pkg/kernel/lib/class_hierarchy.dart
index 0c43b0f1bb3..be0090c8838 100644
--- a/pkg/kernel/lib/class_hierarchy.dart
+++ b/pkg/kernel/lib/class_hierarchy.dart
@@ -42,17 +42,23 @@ abstract class ClassHierarchyBase {
   List<DartType>? getTypeArgumentsAsInstanceOf(
       InterfaceType type, Class superclass);
 
-  /// Returns the instantiation of [superclass] that is implemented by [type],
-  /// or `null` if [type] does not implement [superclass] at all.
-  ExtensionType? getExtensionTypeAsInstanceOf(
-      ExtensionType type, ExtensionTypeDeclaration superclass,
+  /// Returns the instantiation of [superDeclaration] that is implemented by
+  /// [type], or `null` if [type] does not implement [superDeclaration] at all.
+  ExtensionType? getExtensionTypeAsInstanceOfExtensionTypeDeclaration(
+      ExtensionType type, ExtensionTypeDeclaration superDeclaration,
       {required bool isNonNullableByDefault});
 
-  /// Returns the type arguments of the instantiation of [superclass] that is
-  /// implemented by [type], or `null` if [type] does not implement [superclass]
-  /// at all.
+  /// Returns the instantiation of [superclass] that is implemented by [type],
+  /// or `null` if [type] does not implement [superclass] at all.
+  InterfaceType? getExtensionTypeAsInstanceOfClass(
+      ExtensionType type, Class superclass,
+      {required bool isNonNullableByDefault});
+
+  /// Returns the type arguments of the instantiation of [superDeclaration] that
+  /// is implemented by [type], or `null` if [type] does not implement
+  /// [superDeclaration] at all.
   List<DartType>? getExtensionTypeArgumentsAsInstanceOf(
-      ExtensionType type, ExtensionTypeDeclaration superclass);
+      ExtensionType type, ExtensionTypeDeclaration superDeclaration);
 
   /// True if [subtype] inherits from [superclass] though zero or more
   /// `extends`, `with`, and `implements` relationships.
@@ -78,26 +84,26 @@ abstract class ClassHierarchyBase {
       {required bool isNonNullableByDefault});
 }
 
-mixin ClassHierarchyExtensionTypeMixin {
-  CoreTypes get coreTypes;
-
-  ExtensionType? getExtensionTypeDeclarationAsInstanceOf(
-      ExtensionTypeDeclaration subclass, ExtensionTypeDeclaration superclass,
-      {required bool isNonNullableByDefault}) {
+mixin ClassHierarchyExtensionTypeMixin implements ClassHierarchyBase {
+  ExtensionType?
+      getExtensionTypeDeclarationAsInstanceOfExtensionTypeDeclaration(
+          ExtensionTypeDeclaration subDeclaration,
+          ExtensionTypeDeclaration superDeclaration,
+          {required bool isNonNullableByDefault}) {
     // TODO(johnniwinther): Improve lookup performance.
-    if (identical(subclass, superclass)) {
+    if (identical(subDeclaration, superDeclaration)) {
       return coreTypes.thisExtensionType(
-          subclass,
+          subDeclaration,
           isNonNullableByDefault
               ? Nullability.nonNullable
               : Nullability.legacy);
     }
-    for (DartType implement in subclass.implements) {
-      // TODO(johnniwinther): Handle non-extension type supertypes.
+    for (DartType implement in subDeclaration.implements) {
       if (implement is ExtensionType) {
-        ExtensionType? supertype = getExtensionTypeDeclarationAsInstanceOf(
-            implement.extensionTypeDeclaration, superclass,
-            isNonNullableByDefault: isNonNullableByDefault);
+        ExtensionType? supertype =
+            getExtensionTypeDeclarationAsInstanceOfExtensionTypeDeclaration(
+                implement.extensionTypeDeclaration, superDeclaration,
+                isNonNullableByDefault: isNonNullableByDefault);
         if (supertype != null) {
           if (implement.typeArguments.isNotEmpty) {
             supertype = Substitution.fromExtensionType(implement)
@@ -105,17 +111,63 @@ mixin ClassHierarchyExtensionTypeMixin {
           }
           return supertype;
         }
+      } else if (implement is InterfaceType) {
+        // Extension type declarations cannot be implemented through classes.
+      } else {
+        assert(
+            false,
+            "Unexpected supertype $implement extension type declaration of "
+            "$subDeclaration.");
       }
     }
     return null;
   }
 
-  ExtensionType? getExtensionTypeAsInstanceOf(
+  InterfaceType? getExtensionTypeDeclarationAsInstanceOfClass(
+      ExtensionTypeDeclaration subDeclaration, Class superclass,
+      {required bool isNonNullableByDefault}) {
+    // TODO(johnniwinther): Improve lookup performance.
+    for (DartType implement in subDeclaration.implements) {
+      if (implement is ExtensionType) {
+        InterfaceType? supertype = getExtensionTypeDeclarationAsInstanceOfClass(
+            implement.extensionTypeDeclaration, superclass,
+            isNonNullableByDefault: isNonNullableByDefault);
+        if (supertype != null) {
+          if (implement.typeArguments.isNotEmpty) {
+            supertype = Substitution.fromExtensionType(implement)
+                .substituteType(supertype) as InterfaceType;
+          }
+          return supertype;
+        }
+      } else if (implement is InterfaceType) {
+        Supertype? supertype =
+            getClassAsInstanceOf(implement.classNode, superclass);
+        if (supertype != null) {
+          if (implement.typeArguments.isNotEmpty) {
+            supertype = Substitution.fromInterfaceType(implement)
+                .substituteSupertype(supertype);
+          }
+          return new InterfaceType(supertype.classNode, Nullability.nonNullable,
+              supertype.typeArguments);
+        }
+      } else {
+        assert(
+            false,
+            "Unexpected supertype $implement extension type declaration of "
+            "$subDeclaration.");
+      }
+    }
+    return null;
+  }
+
+  @override
+  ExtensionType? getExtensionTypeAsInstanceOfExtensionTypeDeclaration(
       ExtensionType type, ExtensionTypeDeclaration superclass,
       {required bool isNonNullableByDefault}) {
-    ExtensionType? supertype = getExtensionTypeDeclarationAsInstanceOf(
-        type.extensionTypeDeclaration, superclass,
-        isNonNullableByDefault: isNonNullableByDefault);
+    ExtensionType? supertype =
+        getExtensionTypeDeclarationAsInstanceOfExtensionTypeDeclaration(
+            type.extensionTypeDeclaration, superclass,
+            isNonNullableByDefault: isNonNullableByDefault);
     if (supertype != null) {
       if (type.typeArguments.isNotEmpty) {
         supertype = Substitution.fromExtensionType(type)
@@ -126,9 +178,28 @@ mixin ClassHierarchyExtensionTypeMixin {
     return null;
   }
 
+  @override
+  InterfaceType? getExtensionTypeAsInstanceOfClass(
+      ExtensionType type, Class superclass,
+      {required bool isNonNullableByDefault}) {
+    InterfaceType? supertype = getExtensionTypeDeclarationAsInstanceOfClass(
+        type.extensionTypeDeclaration, superclass,
+        isNonNullableByDefault: isNonNullableByDefault);
+    if (supertype != null) {
+      if (type.typeArguments.isNotEmpty) {
+        supertype = Substitution.fromExtensionType(type)
+            .substituteType(supertype) as InterfaceType;
+      }
+      return supertype;
+    }
+    return null;
+  }
+
+  @override
   List<DartType>? getExtensionTypeArgumentsAsInstanceOf(
       ExtensionType type, ExtensionTypeDeclaration superclass) {
-    return getExtensionTypeAsInstanceOf(type, superclass,
+    return getExtensionTypeAsInstanceOfExtensionTypeDeclaration(
+            type, superclass,
             isNonNullableByDefault: true)
         ?.typeArguments;
   }
diff --git a/pkg/kernel/lib/src/standard_bounds.dart b/pkg/kernel/lib/src/standard_bounds.dart
index a157e97ebff..0292632d013 100644
--- a/pkg/kernel/lib/src/standard_bounds.dart
+++ b/pkg/kernel/lib/src/standard_bounds.dart
@@ -1032,9 +1032,10 @@ mixin StandardBounds {
         supertypes.add(type);
         for (DartType implemented in type.extensionTypeDeclaration.implements) {
           if (implemented is ExtensionType) {
-            ExtensionType supertype = hierarchy.getExtensionTypeAsInstanceOf(
-                type, implemented.extensionTypeDeclaration,
-                isNonNullableByDefault: isNonNullableByDefault)!;
+            ExtensionType supertype =
+                hierarchy.getExtensionTypeAsInstanceOfExtensionTypeDeclaration(
+                    type, implemented.extensionTypeDeclaration,
+                    isNonNullableByDefault: isNonNullableByDefault)!;
             computeSuperTypes(supertype, supertypes);
           }
         }