K2: repeat K1 representation for flexible type parameters

This commit changes the behavior of KT-59138 effectively declining it in 2.0.
However, we plan to implement KT-59138 behavior under a feature
flag in 2.0 (see KT-66447), and switch this feature on version 2.x.

Also, this commit implements the LC resolution about postponing
KT-57014 change. We don't have KT-57014 described behavior in 2.0 anymore.
However, we plan to implement a deprecation warning here, see KT-65578.

After this commit, 6 diagnostic tests become incorrectly broken:
- 5 tests from PurelyImplementedCollection group
- a test platformTypes/nullableTypeArgument.kt

This commit also breaks currently fixed-in-k2 KT-50134
(it is fixed again in the following commits),
as well as KT-58933 (it will remain not fixed till we enable KT-59138
behavior again).

#KT-65596 In Progress
#KT-57014 In Progress
#KT-58933 Submitted

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

@@ -15,6 +15,7 @@ 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.lazyResolveToPhase
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.name.StandardClassIds
@@ -55,7 +56,11 @@ internal object CreateFreshTypeVariableSubstitutorStage : ResolutionStage() {
             when (val typeArgument = candidate.typeArgumentMapping[index]) {
                 is FirTypeProjectionWithVariance -> csBuilder.addEqualityConstraint(
                     freshVariable.defaultType,
-                    typeArgument.typeRef.coneType.fullyExpandedType(context.session),
+                    getTypePreservingFlexibilityWrtTypeVariable(
+                        typeArgument.typeRef.coneType,
+                        typeParameter,
+                        context.session
+                    ).fullyExpandedType(context.session),
                     ConeExplicitTypeParameterConstraintPosition(typeArgument)
                 )
                 is FirStarProjection -> csBuilder.addEqualityConstraint(
@@ -76,6 +81,28 @@ internal object CreateFreshTypeVariableSubstitutorStage : ResolutionStage() {
             sink.yieldIfNeed()
         }
     }
+
+    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

@@ -8,9 +8,8 @@ package org.jetbrains.kotlin.fir.resolve.inference
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousFunction
 import org.jetbrains.kotlin.fir.resolve.inference.model.ConeArgumentConstraintPosition
 import org.jetbrains.kotlin.fir.resolve.inference.model.ConeFixVariableConstraintPosition
-import org.jetbrains.kotlin.fir.types.ConeKotlinType
-import org.jetbrains.kotlin.fir.types.coneType
-import org.jetbrains.kotlin.fir.types.coneTypeSafe
+import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterLookupTag
+import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.name.StandardClassIds
 import org.jetbrains.kotlin.resolve.calls.inference.components.ConstraintSystemUtilContext
 import org.jetbrains.kotlin.resolve.calls.inference.components.PostponedArgumentInputTypesResolver
@@ -22,9 +21,12 @@ import org.jetbrains.kotlin.types.model.TypeVariableMarker
 
 object ConeConstraintSystemUtilContext : ConstraintSystemUtilContext {
     override fun TypeVariableMarker.shouldBeFlexible(): Boolean {
-        // In FIR, there's no need in hack with shouldTryUseDifferentFlexibilityForUpperType
-        // See org.jetbrains.kotlin.types.model.TypeSystemInferenceExtensionContext.useRefinedBoundsForTypeVariableInFlexiblePosition
-        return false
+        if (this !is ConeTypeVariable) return false
+        val typeParameter =
+            (this.typeConstructor.originalTypeParameter as? ConeTypeParameterLookupTag)?.typeParameterSymbol?.fir ?: return false
+
+        // TODO: Take a look at org.jetbrains.kotlin.resolve.calls.components.CreateFreshVariablesSubstitutor.shouldBeFlexible
+        return typeParameter.bounds.any { it.coneType is ConeFlexibleType }
     }
 
     override fun TypeVariableMarker.hasOnlyInputTypesAttribute(): Boolean {