FIR: Fix inference case with flexible captured types

Previsously, errors have been ignored because we ignored errors raised
from the completion phase

See the comment above the createConstraintPartForLowerBoundAndFlexibleTypeVariable

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt

@@ -492,4 +492,7 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext, ConeTypeCo
         }
         return intersectionType.withAlternative(secondCandidate)
     }
+
+    override fun SimpleTypeMarker.createConstraintPartForLowerBoundAndFlexibleTypeVariable(): KotlinTypeMarker =
+        createFlexibleType(this.makeSimpleTypeDefinitelyNotNullOrNotNull(), this.withNullability(true))
 }

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/components/AbstractTypeCheckerContextForConstraintSystem.kt

@@ -237,11 +237,7 @@ abstract class AbstractTypeCheckerContextForConstraintSystem(override val typeSy
                 when (subType) {
                     is SimpleTypeMarker ->
                         // Foo <: T! -- (Foo!! .. Foo) <: T
-                        if (subType.isMarkedNullable()) {
-                            subType // prefer nullable type to flexible one: `Foo? <: (T..T?)` => lowerConstraint = `Foo?`
-                        } else {
-                            createFlexibleType(subType, subType.withNullability(true))
-                        }
+                        subType.createConstraintPartForLowerBoundAndFlexibleTypeVariable()
 
                     is FlexibleTypeMarker ->
                         // (Foo..Bar) <: T! -- (Foo!! .. Bar) <: T

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

@@ -7,7 +7,6 @@ package org.jetbrains.kotlin.types.model
 
 import org.jetbrains.kotlin.types.AbstractTypeCheckerContext
 import org.jetbrains.kotlin.types.Variance
-import kotlin.collections.ArrayList
 import kotlin.contracts.ExperimentalContracts
 import kotlin.contracts.contract
 
@@ -238,6 +237,17 @@ interface TypeSystemInferenceExtensionContext : TypeSystemContext, TypeSystemBui
 
     @OptIn(ExperimentalStdlibApi::class)
     fun KotlinTypeMarker.extractTypeVariables() = buildSet { extractTypeVariables(this) }
+
+    /**
+     * 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
+     * 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 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
 }
 
 

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

@@ -741,6 +741,13 @@ interface ClassicTypeSystemContext : TypeSystemInferenceExtensionContext, TypeSy
     override fun getKFunctionTypeConstructor(parametersNumber: Int, isSuspend: Boolean): TypeConstructorMarker {
         return getKFunctionDescriptor(builtIns, parametersNumber, isSuspend).typeConstructor
     }
+
+    override fun SimpleTypeMarker.createConstraintPartForLowerBoundAndFlexibleTypeVariable(): KotlinTypeMarker =
+        if (this.isMarkedNullable()) {
+            this
+        } else {
+            createFlexibleType(this, this.withNullability(true))
+        }
 }
 
 fun TypeVariance.convertVariance(): Variance {