[FIR] Fix determining nullability of type variables and type parameters

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/LookupTagUtils.kt

@@ -17,7 +17,7 @@ fun ConeClassifierLookupTag.toSymbol(useSiteSession: FirSession): FirClassifierS
     when (this) {
         is ConeClassLikeLookupTag -> toSymbol(useSiteSession)
         is ConeClassifierLookupTagWithFixedSymbol -> this.symbol
-        else -> error("Unknown lookupTag type: ${this::class}")
+        else -> null
     }
 
 @OptIn(LookupTagInternals::class)

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

@@ -394,17 +394,24 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
         if (this.isMarkedNullable)
             return true
 
-        if (this is ConeFlexibleType && this.upperBound.isNullableType())
-            return true
-
-        if (this is ConeTypeParameterType /* || is TypeVariable */)
-            return hasNullableSuperType(type)
-
-        if (this is ConeIntersectionType && intersectedTypes.any { it.isNullableType() }) {
-            return true
+        return when (this) {
+            is ConeFlexibleType -> this.upperBound.isNullableType()
+            is ConeTypeParameterType -> lookupTag.symbol.allBoundsAreNullable()
+            is ConeTypeVariableType -> {
+                val symbol = lookupTag.toSymbol(session) ?: return false
+                when (symbol) {
+                    is FirClassSymbol -> false
+                    is FirTypeAliasSymbol -> symbol.fir.expandedConeType?.isNullableType() ?: false
+                    is FirTypeParameterSymbol -> symbol.allBoundsAreNullable()
+                }
+            }
+            is ConeIntersectionType -> intersectedTypes.all { it.isNullableType() }
+            else -> false
         }
+    }
 
-        return false
+    private fun FirTypeParameterSymbol.allBoundsAreNullable(): Boolean {
+        return fir.bounds.all { it.coneType.isMarkedNullable }
     }
 
     private fun TypeConstructorMarker.toFirRegularClass(): FirRegularClass? {

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

@@ -93,22 +93,6 @@ fun <T : ConeKotlinType> T.withAttributes(attributes: ConeAttributes): T {
     } as T
 }
 
-fun ConeTypeContext.hasNullableSuperType(type: ConeKotlinType): Boolean {
-    if (type is ConeClassLikeType) return false
-
-    if (type !is ConeLookupTagBasedType) return false // TODO?
-    val symbol = type.lookupTag
-    for (superType in symbol.supertypes()) {
-        if (superType.isNullableType()) return true
-    }
-//
-//    for (KotlinType supertype : getImmediateSupertypes(type)) {
-//        if (isNullableType(supertype)) return true;
-//    }
-
-    return false
-}
-
 fun <T : ConeKotlinType> T.withNullability(
     nullability: ConeNullability,
     typeContext: ConeInferenceContext? = null,