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

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CreateFreshTypeVariableSubstitutorStage.kt

@@ -15,7 +15,6 @@ import org.jetbrains.kotlin.fir.resolve.inference.model.ConeExplicitTypeParamete
 import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutor
 import org.jetbrains.kotlin.fir.resolve.substitution.substitutorByMap
 import org.jetbrains.kotlin.fir.scopes.impl.toConeType
-import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterLookupTag
 import org.jetbrains.kotlin.fir.symbols.ensureResolved
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.name.StandardClassIds
@@ -66,11 +65,7 @@ internal object CreateFreshTypeVariableSubstitutorStage : ResolutionStage() {
             when (val typeArgument = candidate.typeArgumentMapping[index]) {
                 is FirTypeProjectionWithVariance -> csBuilder.addEqualityConstraint(
                     freshVariable.defaultType,
-                    getTypePreservingFlexibilityWrtTypeVariable(
-                        typeArgument.typeRef.coneType,
-                        typeParameter,
-                        context.session
-                    ).fullyExpandedType(context.session),
+                    typeArgument.typeRef.coneType.fullyExpandedType(context.session),
                     ConeExplicitTypeParameterConstraintPosition(typeArgument)
                 )
                 is FirStarProjection -> csBuilder.addEqualityConstraint(
@@ -85,29 +80,6 @@ internal object CreateFreshTypeVariableSubstitutorStage : ResolutionStage() {
             }
         }
     }
-
-    private fun getTypePreservingFlexibilityWrtTypeVariable(
-        type: ConeKotlinType,
-        typeParameter: FirTypeParameterRef,
-        session: FirSession,
-    ): ConeKotlinType {
-        return if (typeParameter.shouldBeFlexible(session.typeContext)) {
-            val notNullType = type.withNullability(ConeNullability.NOT_NULL, session.typeContext) as ConeSimpleKotlinType
-            ConeFlexibleType(notNullType, notNullType.withNullability(ConeNullability.NULLABLE, session.typeContext))
-        } else {
-            type
-        }
-    }
-
-    private fun FirTypeParameterRef.shouldBeFlexible(context: ConeTypeContext): Boolean {
-        return symbol.resolvedBounds.any {
-            val type = it.coneType
-            type is ConeFlexibleType || with(context) {
-                (type.typeConstructor() as? ConeTypeParameterLookupTag)?.symbol?.fir?.shouldBeFlexible(context) ?: false
-            }
-        }
-    }
-
 }
 
 private fun createToFreshVariableSubstitutorAndAddInitialConstraints(

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/ConeConstraintSystemUtilContext.kt

@@ -21,7 +21,8 @@ import org.jetbrains.kotlin.types.model.TypeVariableMarker
 
 object ConeConstraintSystemUtilContext : ConstraintSystemUtilContext {
     override fun TypeVariableMarker.shouldBeFlexible(): Boolean {
-        // TODO
+        // In FIR, there's no need in hack with shouldTryUseDifferentFlexibilityForUpperType
+        // See org.jetbrains.kotlin.types.model.TypeSystemInferenceExtensionContext.useRefinedBoundsForTypeVariableInFlexiblePosition
         return false
     }