Integrate nullability into cone types, add & use FIR flexible type

FIR fake overrides are rendered now more precisely to test this process

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt

@@ -40,6 +40,18 @@ class ConeKotlinTypeProjectionOut(override val type: ConeKotlinType) : ConeKotli
         get() = ProjectionKind.OUT
 }
 
+enum class ConeNullability(val suffix: String) {
+    NULLABLE("?"),
+    UNKNOWN("!"),
+    NOT_NULL("");
+
+    val isNullable: Boolean get() = this != NOT_NULL
+
+    companion object {
+        fun create(isNullable: Boolean) = if (isNullable) NULLABLE else NOT_NULL
+    }
+}
+
 // We assume type IS an invariant type projection to prevent additional wrapper here
 // (more exactly, invariant type projection contains type)
 sealed class ConeKotlinType : ConeKotlinTypeProjection(), ConeTypedProjection {
@@ -50,12 +62,17 @@ sealed class ConeKotlinType : ConeKotlinTypeProjection(), ConeTypedProjection {
 
     override val type: ConeKotlinType
         get() = this
+
+    abstract val nullability: ConeNullability
 }
 
 class ConeKotlinErrorType(val reason: String) : ConeKotlinType() {
     override val typeArguments: Array<out ConeKotlinTypeProjection>
         get() = EMPTY_ARRAY
 
+    override val nullability: ConeNullability
+        get() = ConeNullability.UNKNOWN
+
     override fun toString(): String {
         return "<ERROR TYPE: $reason>"
     }
@@ -68,6 +85,9 @@ class ConeClassErrorType(val reason: String) : ConeClassLikeType() {
     override val typeArguments: Array<out ConeKotlinTypeProjection>
         get() = EMPTY_ARRAY
 
+    override val nullability: ConeNullability
+        get() = ConeNullability.UNKNOWN
+
     override fun toString(): String {
         return "<ERROR CLASS: $reason>"
     }
@@ -99,3 +119,11 @@ abstract class ConeFunctionType : ConeClassLikeType() {
     abstract val parameterTypes: List<ConeKotlinType>
     abstract val returnType: ConeKotlinType
 }
+
+class ConeFlexibleType(val lowerBound: ConeKotlinType, val upperBound: ConeKotlinType) : ConeKotlinType() {
+    override val typeArguments: Array<out ConeKotlinTypeProjection>
+        get() = emptyArray()
+
+    override val nullability: ConeNullability
+        get() = lowerBound.nullability.takeIf { it == upperBound.nullability } ?: ConeNullability.UNKNOWN
+}

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/ConeFunctionTypeImpl.kt

@@ -9,13 +9,17 @@ import org.jetbrains.kotlin.fir.symbols.ConeClassLikeSymbol
 import org.jetbrains.kotlin.fir.types.ConeFunctionType
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.ConeKotlinTypeProjection
+import org.jetbrains.kotlin.fir.types.ConeNullability
 
 class ConeFunctionTypeImpl(
     override val receiverType: ConeKotlinType?,
     override val parameterTypes: List<ConeKotlinType>,
     override val returnType: ConeKotlinType,
-    override val symbol: ConeClassLikeSymbol
+    override val symbol: ConeClassLikeSymbol,
+    isNullable: Boolean
 ) : ConeFunctionType() {
     override val typeArguments: Array<out ConeKotlinTypeProjection>
         get() = EMPTY_ARRAY
+
+    override val nullability: ConeNullability = ConeNullability.create(isNullable)
 }

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/impl/Impl.kt

@@ -11,19 +11,30 @@ import org.jetbrains.kotlin.fir.types.*
 
 open class ConeClassTypeImpl(
     override val symbol: ConeClassLikeSymbol,
-    override val typeArguments: Array<ConeKotlinTypeProjection>
-) : ConeClassLikeType()
+    override val typeArguments: Array<ConeKotlinTypeProjection>,
+    isNullable: Boolean
+) : ConeClassLikeType() {
+    override val nullability: ConeNullability = ConeNullability.create(isNullable)
+}
 
 class ConeAbbreviatedTypeImpl(
     override val abbreviationSymbol: ConeClassLikeSymbol,
     override val typeArguments: Array<ConeKotlinTypeProjection>,
-    override val directExpansion: ConeClassLikeType
+    override val directExpansion: ConeClassLikeType,
+    isNullable: Boolean
 ) : ConeAbbreviatedType() {
     override val symbol: ConeClassLikeSymbol
         get() = abbreviationSymbol
+
+    override val nullability: ConeNullability = ConeNullability.create(isNullable)
 }
 
-class ConeTypeParameterTypeImpl(override val symbol: ConeTypeParameterSymbol) : ConeTypeParameterType() {
+class ConeTypeParameterTypeImpl(
+    override val symbol: ConeTypeParameterSymbol,
+    isNullable: Boolean
+) : ConeTypeParameterType() {
     override val typeArguments: Array<out ConeKotlinTypeProjection>
         get() = EMPTY_ARRAY
+
+    override val nullability: ConeNullability = ConeNullability.create(isNullable)
 }