diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.kt
new file mode 100644
index 00000000000..b2a46d6ba21
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.kt
@@ -0,0 +1,38 @@
+// FILE: AnnotatedTypeArguments.java
+
+import org.jetbrains.annotations.*;
+
+interface P<X, Y> {}
+interface L<T> {}
+interface S<T> {}
+
+class AnnotatedTypeArguments {
+
+    class A {
+        L<P<L<String>, S<?>>> foo(L<P<L<String>, S<?>>> x) {return null;}
+    }
+
+    class B extends A {
+        // some complicated type tree
+        // return type and argument's type differ only by nullability of outermost type
+        @Nullable
+        L<P<@NotNull L<@Nullable String>, @NotNull S<?>>> foo(@NotNull L<P<@NotNull L<@Nullable String>, @NotNull S<?>>> x) {return null;}
+    }
+
+    class C extends B {
+        // signature should be the same as in A
+        L<P<L<String>, S<?>>> foo(L<P<L<String>, S<?>>> x) {return null;}
+    }
+
+    class D1 extends C {
+        // signature should be the same as in A, but annotated String-type should be platform
+        L<P<L<@NotNull String>, S<?>>> foo(L<P<L<@NotNull String>, S<?>>> x) {return null;}
+    }
+
+    class D2 extends C {
+        // return type refined to not-nullable
+        // argument type here same as in A except outermost type (it becomes flexible because of conflict)
+        @NotNull
+        L<P<L<String>, S<?>>> foo(@Nullable L<P<L<String>, S<?>>> x) {return null;}
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.txt
new file mode 100644
index 00000000000..e5833bf45e6
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.txt
@@ -0,0 +1,48 @@
+package 
+
+public/*package*/ open class AnnotatedTypeArguments {
+    public/*package*/ constructor AnnotatedTypeArguments()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public/*package*/ open inner class A {
+        public/*package*/ constructor A()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public/*package*/ open fun foo(/*0*/ x: L<P<L<kotlin.String!>!, S<*>!>!>!): L<P<L<kotlin.String!>!, S<*>!>!>!
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class B : AnnotatedTypeArguments.A {
+        public/*package*/ constructor B()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        org.jetbrains.annotations.Nullable() public/*package*/ open override /*1*/ fun foo(/*0*/ org.jetbrains.annotations.NotNull() x: L<P<L<kotlin.String?>, S<*>>!>): L<P<L<kotlin.String?>, S<*>>!>?
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class C : AnnotatedTypeArguments.B {
+        public/*package*/ constructor C()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public/*package*/ open override /*1*/ fun foo(/*0*/ x: L<P<L<kotlin.String?>, S<*>>!>): L<P<L<kotlin.String?>, S<*>>!>?
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class D1 : AnnotatedTypeArguments.C {
+        public/*package*/ constructor D1()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public/*package*/ open override /*1*/ fun foo(/*0*/ x: L<P<L<kotlin.String!>, S<*>>!>): L<P<L<kotlin.String!>, S<*>>!>?
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class D2 : AnnotatedTypeArguments.C {
+        public/*package*/ constructor D2()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        org.jetbrains.annotations.NotNull() public/*package*/ open override /*1*/ fun foo(/*0*/ org.jetbrains.annotations.Nullable() x: L<P<L<kotlin.String?>, S<*>>!>!): L<P<L<kotlin.String?>, S<*>>!>
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.kt
new file mode 100644
index 00000000000..b676019d8fb
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.kt
@@ -0,0 +1,51 @@
+// FILE: Outer.java
+
+import org.jetbrains.annotations.*;
+
+interface X<T> {}
+interface Y<T> {}
+
+class Outer {
+    class A {
+        <K, V> V foo(K x) { return null; }
+
+        <T> X<T> bar(Y<T> x) { return null; }
+    }
+
+    class B extends A {
+        // OK, non-platform types
+        @Override
+        <T1, T2> @NotNull T2 foo(@Nullable T1 x) { return null; }
+
+        // Parameter type is fully non-flexible (OK)
+        // Return type is `X<R!>?`.
+        // The reason is that we do not treat it as equal to return type of A.bar because they are base on different type parameters,
+        // so type enhancing happens only for outermost type.
+        // TODO: We should properly compare equality with specific local equality axioms (as when calculating overriden descriptors)
+        @Override
+        <R> @Nullable X<@Nullable R> bar(@NotNull Y<@NotNull R> x) { return null; }
+    }
+
+    class C extends B {
+        // OK, non-platform types
+        @Override
+        <I, J> J foo(I x) { return null; }
+
+        // Parameter type is fully non-flexible (OK)
+        // Return type is `X<R!>?`, same is in B
+        @Override
+        <E> X<E> bar(Y<E> x) { return null; }
+    }
+
+    class D extends C {
+        // Return type is not-nullable, covariantly overridden, OK
+        // Parameter type is flexible, because of conflict with supertype, OK
+        @Override
+        @NotNull
+        <U, W> W foo(@Nullable U x) { return null; }
+
+
+        @Override
+        <F> @NotNull X<@NotNull F> bar(@Nullable Y<@Nullable F> x) { return null; }
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.txt
new file mode 100644
index 00000000000..1621e611eb6
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.txt
@@ -0,0 +1,44 @@
+package
+
+public/*package*/ open class Outer {
+    public/*package*/ constructor Outer()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public/*package*/ open inner class A {
+        public/*package*/ constructor A()
+        public/*package*/ open fun </*0*/ T> bar(/*0*/ x: Y<T!>!): X<T!>!
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public/*package*/ open fun </*0*/ K, /*1*/ V> foo(/*0*/ x: K!): V!
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class B : Outer.A {
+        public/*package*/ constructor B()
+        java.lang.Override() public/*package*/ open override /*1*/ fun </*0*/ R> bar(/*0*/ org.jetbrains.annotations.NotNull() x: Y<R>): X<R!>?
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() public/*package*/ open override /*1*/ fun </*0*/ T1, /*1*/ T2> foo(/*0*/ org.jetbrains.annotations.Nullable() x: T1?): T2
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class C : Outer.B {
+        public/*package*/ constructor C()
+        java.lang.Override() public/*package*/ open override /*1*/ fun </*0*/ E> bar(/*0*/ x: Y<E>): X<E!>?
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() public/*package*/ open override /*1*/ fun </*0*/ I, /*1*/ J> foo(/*0*/ x: I?): J
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class D : Outer.C {
+        public/*package*/ constructor D()
+        java.lang.Override() public/*package*/ open override /*1*/ fun </*0*/ F> bar(/*0*/ org.jetbrains.annotations.Nullable() x: Y<F!>!): X<F!>
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() org.jetbrains.annotations.NotNull() public/*package*/ open override /*1*/ fun </*0*/ U, /*1*/ W> foo(/*0*/ org.jetbrains.annotations.Nullable() x: U?): W
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.kt
new file mode 100644
index 00000000000..1cff51ea18f
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.kt
@@ -0,0 +1,63 @@
+// FILE: A.kt
+open class A {
+    open fun String.foo(y: String?): Int = 1
+    open fun String?.bar(y: String): Int = 1
+}
+
+class E : B1() {
+    fun baz() {
+        val x: String? = ""
+
+        x<!UNSAFE_CALL!>.<!>foo(x)
+        x<!UNSAFE_CALL!>.<!>foo("")
+        x.bar(<!TYPE_MISMATCH!>x<!>)
+        x.bar("")
+    }
+
+    override fun String.foo(y: String?): Int = 1
+    override fun String?.bar(y: String): Int = 1
+}
+
+// FILE: B.java
+import org.jetbrains.annotations.*;
+
+// Just inherit enhanced types
+class B extends A {
+    @Override
+    int foo(String x, String y);
+    @Override
+    int bar(String x, String y);
+}
+
+// FILE: B1.java
+import org.jetbrains.annotations.*;
+
+// Just inherit enhanced types (annotations without conflicts)
+class B1 extends A {
+    @Override
+    int foo(@NotNull String x, String y);
+    @Override
+    int bar(@Nullable String x, String y);
+}
+
+// FILE: C.java
+import org.jetbrains.annotations.*;
+
+// Conflicting annotations. Everything is flexible
+class C extends A {
+    @Override
+    int foo(@Nullable String x, @NotNull String y);
+    @Override
+    int bar(@NotNull String x, @Nullable String y);
+}
+
+// FILE: D.java
+import org.jetbrains.annotations.*;
+
+// Just inherit enhanced types (annotations without conflicts)
+class D extends B {
+    @Override
+    int foo(@Nullable String x, @Nullable String y);
+    @Override
+    int bar(@NotNull String x, @NotNull String y);
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.txt
new file mode 100644
index 00000000000..1813968a443
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.txt
@@ -0,0 +1,56 @@
+package
+
+internal open class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    internal open fun kotlin.String?.bar(/*0*/ y: kotlin.String): kotlin.Int
+    internal open fun kotlin.String.foo(/*0*/ y: kotlin.String?): kotlin.Int
+}
+
+public/*package*/ open class B : A {
+    public/*package*/ constructor B()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String?.bar(/*0*/ y: kotlin.String): kotlin.Int
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String.foo(/*0*/ y: kotlin.String?): kotlin.Int
+}
+
+public/*package*/ open class B1 : A {
+    public/*package*/ constructor B1()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String?.bar(/*0*/ y: kotlin.String): kotlin.Int
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String.foo(/*0*/ y: kotlin.String?): kotlin.Int
+}
+
+public/*package*/ open class C : A {
+    public/*package*/ constructor C()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String!.bar(/*0*/ org.jetbrains.annotations.Nullable() y: kotlin.String!): kotlin.Int
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String!.foo(/*0*/ org.jetbrains.annotations.NotNull() y: kotlin.String!): kotlin.Int
+}
+
+public/*package*/ open class D : B {
+    public/*package*/ constructor D()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String!.bar(/*0*/ org.jetbrains.annotations.NotNull() y: kotlin.String): kotlin.Int
+    java.lang.Override() public/*package*/ open override /*1*/ fun kotlin.String!.foo(/*0*/ org.jetbrains.annotations.Nullable() y: kotlin.String?): kotlin.Int
+}
+
+internal final class E : B1 {
+    public constructor E()
+    internal final fun baz(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    internal open override /*1*/ fun kotlin.String?.bar(/*0*/ y: kotlin.String): kotlin.Int
+    internal open override /*1*/ fun kotlin.String.foo(/*0*/ y: kotlin.String?): kotlin.Int
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.kt
new file mode 100644
index 00000000000..c96a266a34f
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.kt
@@ -0,0 +1,37 @@
+// FILE: Outer.java
+
+import org.jetbrains.annotations.*;
+
+interface Base<T> {}
+interface Derived<E> extends Base<E> {}
+
+class Outer {
+    class A {
+        @Nullable Base<@NotNull String> foo() { return null; }
+    }
+
+    class B extends A {
+        @Override
+        Base<String> foo() {}
+    }
+
+    class C extends A {
+        @Override
+        @NotNull Base<String> foo() {}
+    }
+
+    class D extends A {
+        @Override
+        Derived<String> foo() {}
+    }
+
+    class E extends A {
+        @Override
+        @NotNull Derived<String> foo() {}
+    }
+
+    class F extends A {
+        @Override
+        @NotNull Derived<@NotNull String> foo() {}
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.txt
new file mode 100644
index 00000000000..3de4b6bfdcd
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.txt
@@ -0,0 +1,56 @@
+package
+
+public/*package*/ open class Outer {
+    public/*package*/ constructor Outer()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public/*package*/ open inner class A {
+        public/*package*/ constructor A()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        org.jetbrains.annotations.Nullable() public/*package*/ open fun foo(): Base<kotlin.String>?
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class B : Outer.A {
+        public/*package*/ constructor B()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() public/*package*/ open override /*1*/ fun foo(): Base<kotlin.String>?
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class C : Outer.A {
+        public/*package*/ constructor C()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() org.jetbrains.annotations.NotNull() public/*package*/ open override /*1*/ fun foo(): Base<kotlin.String>
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class D : Outer.A {
+        public/*package*/ constructor D()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() public/*package*/ open override /*1*/ fun foo(): Derived<kotlin.String!>?
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class E : Outer.A {
+        public/*package*/ constructor E()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() org.jetbrains.annotations.NotNull() public/*package*/ open override /*1*/ fun foo(): Derived<kotlin.String!>
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public/*package*/ open inner class F : Outer.A {
+        public/*package*/ constructor F()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        java.lang.Override() org.jetbrains.annotations.NotNull() public/*package*/ open override /*1*/ fun foo(): Derived<kotlin.String!>
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.kt
new file mode 100644
index 00000000000..a010f31db7c
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.kt
@@ -0,0 +1,31 @@
+// FILE: A.java
+
+import org.jetbrains.annotations.*;
+
+public class A {
+    @Nullable Base<@NotNull String> foo() { return null; }
+}
+
+// FILE: a.kt
+
+interface Base<T> {}
+interface Derived<E> : Base<E> {}
+
+fun bar1(): Derived<String> = null!!
+fun bar2(): Derived<String?> = null!!
+
+class B : A() {
+    override fun foo(): Base<String> { return bar1(); }
+}
+
+class C1 : A() {
+    override fun foo(): Derived<String> { return bar1(); }
+}
+
+class C2 : A() {
+    override fun foo(): Derived<String>? { return bar1(); }
+}
+
+class C3 : A() {
+    override fun foo(): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Derived<String?><!> { return bar2(); }
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.txt
new file mode 100644
index 00000000000..e5613b2f150
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.txt
@@ -0,0 +1,56 @@
+package
+
+internal fun bar1(): Derived<kotlin.String>
+internal fun bar2(): Derived<kotlin.String?>
+
+public open class A {
+    public constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    org.jetbrains.annotations.Nullable() public/*package*/ open fun foo(): Base<kotlin.String>?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class B : A {
+    public constructor B()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    internal open override /*1*/ fun foo(): Base<kotlin.String>
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal interface Base</*0*/ T> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C1 : A {
+    public constructor C1()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    internal open override /*1*/ fun foo(): Derived<kotlin.String>
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C2 : A {
+    public constructor C2()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    internal open override /*1*/ fun foo(): Derived<kotlin.String>?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C3 : A {
+    public constructor C3()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    internal open override /*1*/ fun foo(): Derived<kotlin.String?>
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal interface Derived</*0*/ E> : Base<E> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.kt
new file mode 100644
index 00000000000..1d50f6f48ce
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.kt
@@ -0,0 +1,33 @@
+// FILE: A.java
+import org.jetbrains.annotations.*;
+
+interface A {
+    void foo(@Nullable String x);
+}
+
+// FILE: B.java
+import org.jetbrains.annotations.*;
+
+interface B {
+    void foo(@NotNull String x);
+}
+
+// FILE: C.kt
+
+<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class C1<!> : A, B {
+    <!ACCIDENTAL_OVERRIDE!>override fun foo(x: String)<!> {}
+}
+
+<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class C2<!> : A, B {
+    <!ACCIDENTAL_OVERRIDE!>override fun foo(x: String?)<!> {}
+}
+
+interface <!ACCIDENTAL_OVERRIDE!>I<!> : A, B
+
+<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class C3<!> : I {
+    <!ACCIDENTAL_OVERRIDE!>override fun foo(x: String)<!> {}
+}
+
+<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class C4<!> : I {
+    <!ACCIDENTAL_OVERRIDE!>override fun foo(x: String?)<!> {}
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.txt
new file mode 100644
index 00000000000..2c6055689b2
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.txt
@@ -0,0 +1,62 @@
+package
+
+public/*package*/ /*synthesized*/ fun A(/*0*/ function: (kotlin.String!) -> kotlin.Unit): A
+public/*package*/ /*synthesized*/ fun B(/*0*/ function: (kotlin.String!) -> kotlin.Unit): B
+
+public/*package*/ interface A {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(/*0*/ org.jetbrains.annotations.Nullable() x: kotlin.String?): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public/*package*/ interface B {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(/*0*/ org.jetbrains.annotations.NotNull() x: kotlin.String): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C1 : A, B {
+    public constructor C1()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ fun foo(/*0*/ x: kotlin.String): kotlin.Unit
+    public abstract override /*1*/ /*fake_override*/ fun foo(/*0*/ org.jetbrains.annotations.Nullable() x: kotlin.String?): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C2 : A, B {
+    public constructor C2()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun foo(/*0*/ org.jetbrains.annotations.NotNull() x: kotlin.String): kotlin.Unit
+    public open override /*1*/ fun foo(/*0*/ x: kotlin.String?): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C3 : I {
+    public constructor C3()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ fun foo(/*0*/ x: kotlin.String): kotlin.Unit
+    public abstract override /*1*/ /*fake_override*/ fun foo(/*0*/ org.jetbrains.annotations.Nullable() x: kotlin.String?): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C4 : I {
+    public constructor C4()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun foo(/*0*/ org.jetbrains.annotations.NotNull() x: kotlin.String): kotlin.Unit
+    public open override /*1*/ fun foo(/*0*/ x: kotlin.String?): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal interface I : A, B {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun foo(/*0*/ org.jetbrains.annotations.NotNull() x: kotlin.String): kotlin.Unit
+    public abstract override /*1*/ /*fake_override*/ fun foo(/*0*/ org.jetbrains.annotations.Nullable() x: kotlin.String?): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.kt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.kt
new file mode 100644
index 00000000000..fbc8c65470a
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.kt
@@ -0,0 +1,35 @@
+// FILE: A.java
+import org.jetbrains.annotations.*;
+
+interface A {
+    @Nullable
+    String foo();
+}
+
+// FILE: B.java
+import org.jetbrains.annotations.*;
+
+interface B {
+    @NotNull
+    String foo();
+}
+
+// FILE: C.kt
+
+class C1 : A, B {
+    override fun foo(): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>String?<!> = ""
+}
+
+class C2 : A, B {
+    override fun foo(): String = ""
+}
+
+interface I : A, B
+
+class C3 : I {
+    override fun foo(): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>String?<!> = ""
+}
+
+class C4 : I {
+    override fun foo(): String = ""
+}
diff --git a/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.txt b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.txt
new file mode 100644
index 00000000000..20e5b74578a
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.txt
@@ -0,0 +1,57 @@
+package
+
+public/*package*/ /*synthesized*/ fun A(/*0*/ function: () -> kotlin.String!): A
+public/*package*/ /*synthesized*/ fun B(/*0*/ function: () -> kotlin.String!): B
+
+public/*package*/ interface A {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    org.jetbrains.annotations.Nullable() public abstract fun foo(): kotlin.String?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public/*package*/ interface B {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    org.jetbrains.annotations.NotNull() public abstract fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C1 : A, B {
+    public constructor C1()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ fun foo(): kotlin.String?
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C2 : A, B {
+    public constructor C2()
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*2*/ fun foo(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C3 : I {
+    public constructor C3()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ fun foo(): kotlin.String?
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal final class C4 : I {
+    public constructor C4()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+internal interface I : A, B {
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    org.jetbrains.annotations.NotNull() public abstract override /*2*/ /*fake_override*/ fun foo(): kotlin.String
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java
index b5529a443fa..345a5ccaf38 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java
@@ -10115,6 +10115,57 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                     doTest(fileName);
                 }
             }
+
+            @TestMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement")
+            @TestDataPath("$PROJECT_ROOT")
+            @RunWith(JUnit3RunnerWithInners.class)
+            public static class TypeEnhancement extends AbstractJetDiagnosticsTest {
+                public void testAllFilesPresentInTypeEnhancement() throws Exception {
+                    JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement"), Pattern.compile("^(.+)\\.kt$"), true);
+                }
+
+                @TestMetadata("annotatedTypeArguments.kt")
+                public void testAnnotatedTypeArguments() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/annotatedTypeArguments.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("methodWithTypeParameter.kt")
+                public void testMethodWithTypeParameter() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/methodWithTypeParameter.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("overriddenExtensions.kt")
+                public void testOverriddenExtensions() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/overriddenExtensions.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("returnTypeDifferentConstructor.kt")
+                public void testReturnTypeDifferentConstructor() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeDifferentConstructor.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("returnTypeOverrideInKotlin.kt")
+                public void testReturnTypeOverrideInKotlin() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/returnTypeOverrideInKotlin.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("supertypeDifferentParameterNullability.kt")
+                public void testSupertypeDifferentParameterNullability() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentParameterNullability.kt");
+                    doTest(fileName);
+                }
+
+                @TestMetadata("supertypeDifferentReturnNullability.kt")
+                public void testSupertypeDifferentReturnNullability() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/typeEnhancement/supertypeDifferentReturnNullability.kt");
+                    doTest(fileName);
+                }
+            }
         }
 
         @TestMetadata("compiler/testData/diagnostics/tests/recovery")