FIR: Refine inference constraints when type variable in flexible position

That issue might be fixed via changing
TypeVariableMarker.shouldBeFlexible at ConeConstraintSystemUtilContext
but this and some other tricks have been added because of incorrect
handling of constraints where type variable has a flexible bound

^KT-51168 Fixed

core/compiler.common/src/org/jetbrains/kotlin/types/model/TypeSystemContext.kt

@@ -289,15 +289,19 @@ interface TypeSystemInferenceExtensionContext : TypeSystemContext, TypeSystemBui
         }
 
     /**
-     * For case Foo <: (T..T?) return LowerBound for new constraint LowerBound <: T
-     * In FE 1.0, in case nullable it was just Foo?, so constraint was Foo? <: T
+     * For case Foo <: (T..T?) return LowerConstraint for new constraint LowerConstraint <: T
+     * In K1, in case nullable it was just Foo?, so constraint was Foo? <: T
      * But it's not 100% correct because prevent having not-nullable upper constraint on T while initial (Foo? <: (T..T?)) is not violated
      *
-     * In FIR, we try to have a correct one: (Foo!!..Foo?) <: T
+     * In FIR, we try to have a correct one: (Foo & Any..Foo?) <: T
+     *
+     * The same logic applies for T! <: UpperConstraint, as well
+     * In K1, it was reduced to T <: UpperConstraint..UpperConstraint?
+     * In FIR, we use UpperConstraint & Any..UpperConstraint?
      *
      * In future once we have only FIR (or FE 1.0 behavior is fixed) this method should be inlined to the use-site
      */
-    fun SimpleTypeMarker.createConstraintPartForLowerBoundAndFlexibleTypeVariable(): KotlinTypeMarker
+    fun useRefinedBoundsForTypeVariableInFlexiblePosition(): Boolean
 
     fun createCapturedStarProjectionForSelfType(
         typeVariable: TypeVariableTypeConstructorMarker,

core/descriptors/src/org/jetbrains/kotlin/types/checker/ClassicTypeSystemContext.kt

@@ -25,6 +25,7 @@ import org.jetbrains.kotlin.resolve.substitutedUnderlyingType
 import org.jetbrains.kotlin.resolve.unsubstitutedUnderlyingType
 import org.jetbrains.kotlin.types.*
 import org.jetbrains.kotlin.types.error.ErrorTypeKind
+import org.jetbrains.kotlin.types.error.ErrorUtils
 import org.jetbrains.kotlin.types.model.*
 import org.jetbrains.kotlin.types.typeUtil.*
 import org.jetbrains.kotlin.utils.addIfNotNull
@@ -33,7 +34,6 @@ import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 import org.jetbrains.kotlin.types.typeUtil.isSignedOrUnsignedNumberType as classicIsSignedOrUnsignedNumberType
 import org.jetbrains.kotlin.types.typeUtil.isStubType as isSimpleTypeStubType
 import org.jetbrains.kotlin.types.typeUtil.isStubTypeForBuilderInference as isSimpleTypeStubTypeForBuilderInference
-import org.jetbrains.kotlin.types.error.ErrorUtils
 import org.jetbrains.kotlin.types.typeUtil.isStubTypeForVariableInSubtyping as isSimpleTypeStubTypeForVariableInSubtyping
 
 interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSystemCommonBackendContext {
@@ -876,12 +876,7 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
         return (baseType.memberScope as? SubstitutingScope)?.substitutor
     }
 
-    override fun SimpleTypeMarker.createConstraintPartForLowerBoundAndFlexibleTypeVariable(): KotlinTypeMarker =
-        if (this.isMarkedNullable()) {
-            this
-        } else {
-            createFlexibleType(this, this.withNullability(true))
-        }
+    override fun useRefinedBoundsForTypeVariableInFlexiblePosition(): Boolean = false
 
     override fun substitutionSupertypePolicy(type: SimpleTypeMarker): TypeCheckerState.SupertypesPolicy {
         require(type is SimpleType, type::errorMessage)