Put fix in 9123c4f73baf77f8a50dede6c890c46f5ffafd6c under the inference compatibility flag

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/InferenceCompatibilityChecker.kt

@@ -0,0 +1,10 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.calls.inference
+
+interface InferenceCompatibilityChecker {
+    val isCompatibilityModeEnabled: Boolean
+}

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/AbstractTypeCheckerContextForConstraintSystem.kt

@@ -21,6 +21,8 @@ abstract class AbstractTypeCheckerContextForConstraintSystem : AbstractTypeCheck
     override val isStubTypeEqualsToAnything: Boolean
         get() = true
 
+    abstract val isInferenceCompatibilityEnabled: Boolean
+
     abstract fun isMyTypeVariable(type: SimpleTypeMarker): Boolean
 
     // super and sub type isSingleClassifierType
@@ -209,16 +211,20 @@ abstract class AbstractTypeCheckerContextForConstraintSystem : AbstractTypeCheck
                 if (typeVariable.isMarkedNullable()) {
                     val typeVariableTypeConstructor = typeVariable.typeConstructor()
                     val subTypeConstructor = subType.typeConstructor()
-                    val resultType = if (
-                        !subTypeConstructor.isTypeVariable() &&
-                        typeVariableTypeConstructor.isTypeVariable() &&
-                        (typeVariableTypeConstructor as TypeVariableTypeConstructorMarker).isContainedInInvariantOrContravariantPositions()
-                    ) {
-                        subType.withNullability(false)
-                    } else {
+                    val needToMakeDefNotNull = subTypeConstructor.isTypeVariable() ||
+                            typeVariableTypeConstructor !is TypeVariableTypeConstructorMarker ||
+                            !typeVariableTypeConstructor.isContainedInInvariantOrContravariantPositions()
+
+                    val resultType = if (needToMakeDefNotNull) {
                         subType.makeDefinitelyNotNullOrNotNull()
+                    } else {
+                        if (!isInferenceCompatibilityEnabled && subType is CapturedTypeMarker) {
+                            subType.withNotNullProjection()
+                        } else {
+                            subType.withNullability(false)
+                        }
                     }
-                    if (resultType is CapturedTypeMarker) resultType.withNotNullProjection() else resultType
+                    if (isInferenceCompatibilityEnabled && resultType is CapturedTypeMarker) resultType.withNotNullProjection() else resultType
                 } else subType
 
             is FlexibleTypeMarker -> {

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintInjector.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.resolve.calls.inference.components
 
 
 import org.jetbrains.kotlin.resolve.calls.inference.ConstraintSystemOperation
+import org.jetbrains.kotlin.resolve.calls.inference.InferenceCompatibilityChecker
 import org.jetbrains.kotlin.resolve.calls.inference.model.*
 import org.jetbrains.kotlin.resolve.calls.inference.model.ConstraintKind.*
 import org.jetbrains.kotlin.types.AbstractTypeApproximator
@@ -19,7 +20,8 @@ import kotlin.math.max
 
 class ConstraintInjector(
     val constraintIncorporator: ConstraintIncorporator,
-    val typeApproximator: AbstractTypeApproximator
+    val typeApproximator: AbstractTypeApproximator,
+    val inferenceCompatibilityChecker: InferenceCompatibilityChecker
 ) {
     private val ALLOWED_DEPTH_DELTA_FOR_INCORPORATION = 1
 
@@ -129,6 +131,8 @@ class ConstraintInjector(
         // We use `var` intentionally to avoid extra allocations as this property is quite "hot"
         private var possibleNewConstraints: MutableList<Pair<TypeVariableMarker, Constraint>>? = null
 
+        override val isInferenceCompatibilityEnabled = inferenceCompatibilityChecker.isCompatibilityModeEnabled
+
         private var baseLowerType = position.initialConstraint.a
         private var baseUpperType = position.initialConstraint.b