FIR: replace constraint with NotFixedTypeToVariableSubstitutor properly

In this commit we add nullability to upper type of a substituted
constraint in the situation like (Stub<_L> <: SomeType),
where _L is fixed to nullable Stub<_L>?.
We have to change this constraint to L <: SomeType? and not to
L <: SomeType as before, otherwise nullability become broken
(direct substitution of Stub<_L> to L is illegal here).

#KT-50470 Fixed

compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/problems/expectedType.fir.txt

@@ -61,8 +61,8 @@ FILE: expectedType.kt
         public final val property: R|FirProperty| = R|<local>/property|
             public get(): R|FirProperty|
 
-        public final val expectedType: R|ConeKotlinType?|by <Inapplicable(INAPPLICABLE): /myLazy>#<R|ConeKotlinType|>(<L> = myLazy@fun <anonymous>(): R|ConeKotlinType| <inline=NoInline>  {
-            ^ this@R|/Session|.R|/Session.property|.R|/FirProperty.returnTypeRef|.<Inapplicable(INAPPLICABLE): /coneTypeSafe>#<R|ConeKotlinType|>()
+        public final val expectedType: R|ConeKotlinType?|by R|/myLazy|<R|ConeKotlinType?|>(<L> = myLazy@fun <anonymous>(): R|ConeKotlinType?| <inline=NoInline>  {
+            ^ this@R|/Session|.R|/Session.property|.R|/FirProperty.returnTypeRef|.R|/coneTypeSafe|<R|ConeKotlinType|>()
         }
         )
             public get(): R|ConeKotlinType?| {

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/substitution/Substitutors.kt

@@ -219,7 +219,7 @@ internal class TypeSubstitutorByTypeConstructor(
 
 // Note: builder inference uses TypeSubstitutorByTypeConstructor for not fixed type substitution
 class NotFixedTypeToVariableSubstitutorForDelegateInference(
-    private val bindings: Map<TypeVariableMarker, ConeKotlinType>,
+    val bindings: Map<TypeVariableMarker, ConeKotlinType>,
     typeContext: ConeTypeContext
 ) : AbstractConeSubstitutor(typeContext) {
     override fun substituteType(type: ConeKotlinType): ConeKotlinType? {

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

@@ -13,14 +13,12 @@ import org.jetbrains.kotlin.fir.resolve.calls.ResolutionContext
 import org.jetbrains.kotlin.fir.resolve.calls.candidate
 import org.jetbrains.kotlin.fir.resolve.inference.model.ConeBuilderInferenceSubstitutionConstraintPosition
 import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutor
+import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.resolve.calls.inference.components.ConstraintSystemCompletionContext
 import org.jetbrains.kotlin.resolve.calls.inference.model.ConstraintKind
 import org.jetbrains.kotlin.resolve.calls.inference.model.InitialConstraint
 import org.jetbrains.kotlin.resolve.calls.inference.model.NewConstraintSystemImpl
-import org.jetbrains.kotlin.types.model.StubTypeMarker
-import org.jetbrains.kotlin.types.model.TypeSubstitutorMarker
-import org.jetbrains.kotlin.types.model.TypeVariableMarker
-import org.jetbrains.kotlin.types.model.safeSubstitute
+import org.jetbrains.kotlin.types.model.*
 
 abstract class AbstractManyCandidatesInferenceSession(
     protected val resolutionContext: ResolutionContext
@@ -60,8 +58,10 @@ abstract class AbstractManyCandidatesInferenceSession(
         initialConstraint: InitialConstraint,
         callSubstitutor: ConeSubstitutor,
         nonFixedToVariablesSubstitutor: ConeSubstitutor,
+        fixedTypeVariables: Map<TypeConstructorMarker, KotlinTypeMarker>,
     ): Boolean {
-        val substitutedConstraintWith = initialConstraint.substitute(callSubstitutor).substitute(nonFixedToVariablesSubstitutor)
+        val substitutedConstraintWith =
+            initialConstraint.substitute(callSubstitutor).substitute(nonFixedToVariablesSubstitutor, fixedTypeVariables)
         val lower = substitutedConstraintWith.a // TODO: SUB
         val upper = substitutedConstraintWith.b // TODO: SUB
 
@@ -95,4 +95,31 @@ abstract class AbstractManyCandidatesInferenceSession(
             ConeBuilderInferenceSubstitutionConstraintPosition(this) // TODO
         )
     }
+
+    protected fun InitialConstraint.substitute(
+        substitutor: TypeSubstitutorMarker,
+        fixedTypeVariables: Map<TypeConstructorMarker, KotlinTypeMarker>
+    ): InitialConstraint {
+        val substituted = substitute(substitutor)
+        val a = a
+        // In situation when some type variable _T is fixed to Stub(_T)?,
+        // we are not allowed just to substitute Stub(_T) with T because nullabilities are different here!
+        // To compensate this, we have to substitute Stub(_T) <: SomeType constraint with T <: SomeType? adding nullability to upper type
+        if (a is ConeStubTypeForChainInference && substituted.a !is ConeStubTypeForChainInference) {
+            val constructor = a.constructor
+            val fixedTypeVariableType = fixedTypeVariables[constructor.variable.typeConstructor]
+            if (fixedTypeVariableType is ConeStubTypeForChainInference &&
+                fixedTypeVariableType.constructor === constructor &&
+                fixedTypeVariableType.isMarkedNullable
+            ) {
+                return InitialConstraint(
+                    substituted.a,
+                    (substituted.b as ConeKotlinType).withNullability(ConeNullability.NULLABLE, resolutionContext.typeContext),
+                    substituted.constraintKind,
+                    substituted.position
+                )
+            }
+        }
+        return substituted
+    }
 }

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

@@ -197,7 +197,10 @@ class FirBuilderInferenceSession(
 
         for (initialConstraint in storage.initialConstraints) {
             if (initialConstraint.position is BuilderInferencePosition) continue
-            if (integrateConstraintToSystem(commonSystem, initialConstraint, callSubstitutor, nonFixedToVariablesSubstitutor)) {
+            if (integrateConstraintToSystem(
+                    commonSystem, initialConstraint, callSubstitutor, nonFixedToVariablesSubstitutor, storage.fixedTypeVariables
+                )
+            ) {
                 introducedConstraint = true
             }
         }

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

@@ -300,7 +300,10 @@ class FirDelegatedPropertyInferenceSession(
         var introducedConstraint = false
 
         for (initialConstraint in storage.initialConstraints) {
-            if (integrateConstraintToSystem(commonSystem, initialConstraint, callSubstitutor, nonFixedToVariablesSubstitutor)) {
+            if (integrateConstraintToSystem(
+                    commonSystem, initialConstraint, callSubstitutor, nonFixedToVariablesSubstitutor, storage.fixedTypeVariables
+                )
+            ) {
                 introducedConstraint = true
             }
         }