[NI] Make it possible to use nullable non-fixed type

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt

@@ -275,7 +275,7 @@ class NewConstraintSystemImpl(
 
     override fun bindingStubsForPostponedVariables(): Map<NewTypeVariable, NonFixedType> {
         checkState(State.BUILDING, State.COMPLETION)
-        return storage.postponedTypeVariables.associate { it to NonFixedType(it.freshTypeConstructor) }
+        return storage.postponedTypeVariables.associate { it to NonFixedType(it.freshTypeConstructor, it.defaultType.isMarkedNullable) }
     }
 
     override fun copyCurrentStorage(): ConstraintStorage {

core/descriptors/src/org/jetbrains/kotlin/types/NonFixedType.kt

@@ -8,25 +8,32 @@ package org.jetbrains.kotlin.types
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.resolve.scopes.MemberScope
 
-class NonFixedType(val originalTypeVariable: TypeConstructor) : SimpleType() {
+// This type is used as a stub for postponed type variables, which are important for coroutine inference
+class NonFixedType(
+    private val originalTypeVariable: TypeConstructor,
+    override val isMarkedNullable: Boolean,
     override val constructor: TypeConstructor =
-        ErrorUtils.createErrorTypeConstructor("Constructor for non fixed type: $originalTypeVariable")
+        ErrorUtils.createErrorTypeConstructor("Constructor for non fixed type: $originalTypeVariable"),
+    override val memberScope: MemberScope =
+        ErrorUtils.createErrorScope("Scope for non fixed type: $originalTypeVariable")
+) : SimpleType() {
 
     override val arguments: List<TypeProjection>
         get() = emptyList()
 
-    override val isMarkedNullable: Boolean
-        get() = false
-
-    override val memberScope: MemberScope =
-        ErrorUtils.createErrorScope("Scope for non fixed type: $originalTypeVariable")
-
     override val annotations: Annotations
         get() = Annotations.EMPTY
 
-    override fun replaceAnnotations(newAnnotations: Annotations): SimpleType = this
+    override fun replaceAnnotations(newAnnotations: Annotations): SimpleType {
+        error("Shouldn't be called on non-fixed type")
+    }
 
-    override fun makeNullableAsSpecified(newNullability: Boolean): SimpleType = this
+    override fun makeNullableAsSpecified(newNullability: Boolean): SimpleType {
+        return if (newNullability == isMarkedNullable)
+            this
+        else
+            NonFixedType(originalTypeVariable, newNullability, constructor, memberScope)
+    }
 
     override fun toString(): String {
         return "NonFixed: $originalTypeVariable"