Fix null Java type argument problem & add relevant test

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaUtils.kt

@@ -68,7 +68,7 @@ internal fun FirTypeRef.toNotNullConeKotlinType(): ConeKotlinType =
         else -> ConeKotlinErrorType("Unexpected type reference in JavaClassUseSiteScope: ${this::class.java}")
     }
 
-internal fun JavaType.toNotNullConeKotlinType(session: FirSession): ConeLookupTagBasedType {
+internal fun JavaType?.toNotNullConeKotlinType(session: FirSession): ConeLookupTagBasedType {
     return toConeKotlinTypeWithNullability(session, isNullable = false)
 }
 
@@ -85,7 +85,7 @@ internal fun JavaClassifierType.toFirResolvedTypeRef(session: FirSession): FirRe
     )
 }
 
-internal fun JavaType.toConeKotlinTypeWithNullability(session: FirSession, isNullable: Boolean): ConeLookupTagBasedType {
+internal fun JavaType?.toConeKotlinTypeWithNullability(session: FirSession, isNullable: Boolean): ConeLookupTagBasedType {
     return when (this) {
         is JavaClassifierType -> {
             toConeKotlinTypeWithNullability(session, isNullable)
@@ -118,6 +118,10 @@ internal fun JavaType.toConeKotlinTypeWithNullability(session: FirSession, isNul
             val classId = ClassId(FqName("kotlin"), FqName("Any"), false)
             classId.toConeKotlinType(emptyArray(), isNullable)
         }
+        null -> {
+            val classId = ClassId(FqName("kotlin"), FqName("Any"), false)
+            classId.toConeKotlinType(emptyArray(), isNullable)
+        }
         else -> error("Strange JavaType: ${this::class.java}")
     }
 }
@@ -172,8 +176,9 @@ internal fun FirAbstractAnnotatedElement.addAnnotationsFrom(javaAnnotationOwner:
     }
 }
 
-internal fun JavaType.toConeProjection(session: FirSession, boundTypeParameter: FirTypeParameter?): ConeKotlinTypeProjection {
+internal fun JavaType?.toConeProjection(session: FirSession, boundTypeParameter: FirTypeParameter?): ConeKotlinTypeProjection {
     return when (this) {
+        null -> ConeStarProjection
         is JavaWildcardType -> {
             val bound = this.bound
             val argumentVariance = if (isExtends) OUT_VARIANCE else IN_VARIANCE

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/enhancement/EnhancementSignatureParts.kt

@@ -95,8 +95,8 @@ internal class EnhancementSignatureParts(
 
             if (type is FirJavaTypeRef) {
                 for (arg in type.type.typeArguments()) {
-                    if (arg is JavaWildcardType) {
-                        // TODO: wildcards
+                    if (arg is JavaWildcardType || arg == null) {
+                        // TODO: wildcards / raw types
                     } else {
                         add(arg.toFirJavaTypeRef(context.session))
                     }

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/enhancement/javaTypeUtils.kt

@@ -46,14 +46,14 @@ internal fun FirJavaTypeRef.enhance(
 // Example: for `A<B, C<D, E>>`, indices go as follows: `0 - A<...>, 1 - B, 2 - C<D, E>, 3 - D, 4 - E`,
 // which corresponds to the left-to-right breadth-first walk of the tree representation of the type.
 // For flexible types, both bounds are indexed in the same way: `(A<B>..C<D>)` gives `0 - (A<B>..C<D>), 1 - B and D`.
-private fun JavaType.enhancePossiblyFlexible(
+private fun JavaType?.enhancePossiblyFlexible(
     session: FirSession,
     annotations: List<FirAnnotationCall>,
     qualifiers: (Int) -> JavaTypeQualifiers,
     index: Int
 ): FirResolvedTypeRef {
     val type = this
-    val arguments = typeArguments()
+    val arguments = this?.typeArguments().orEmpty()
     return when (type) {
         is JavaClassifierType -> {
             val lowerResult = type.enhanceInflexibleType(
@@ -76,9 +76,9 @@ private fun JavaType.enhancePossiblyFlexible(
     }
 }
 
-private fun JavaType.subtreeSize(): Int {
+private fun JavaType?.subtreeSize(): Int {
     if (this !is JavaClassifierType) return 1
-    return 1 + typeArguments.sumBy { it.subtreeSize() }
+    return 1 + typeArguments.sumBy { it?.subtreeSize() ?: 0 }
 }
 
 private val KOTLIN_COLLECTIONS = FqName("kotlin.collections")
@@ -102,7 +102,7 @@ private fun ClassId.mutableToReadOnly(): ClassId? {
 private fun JavaClassifierType.enhanceInflexibleType(
     session: FirSession,
     annotations: List<FirAnnotationCall>,
-    arguments: List<JavaType>,
+    arguments: List<JavaType?>,
     position: TypeComponentPosition,
     qualifiers: (Int) -> JavaTypeQualifiers,
     index: Int
@@ -199,7 +199,7 @@ internal data class TypeAndDefaultQualifiers(
 internal fun FirTypeRef.typeArguments(): List<FirTypeProjection> =
     (this as? FirUserTypeRef)?.qualifier?.lastOrNull()?.typeArguments.orEmpty()
 
-internal fun JavaType.typeArguments(): List<JavaType> = (this as? JavaClassifierType)?.typeArguments.orEmpty()
+internal fun JavaType.typeArguments(): List<JavaType?> = (this as? JavaClassifierType)?.typeArguments.orEmpty()
 
 fun FirValueParameter.getDefaultValueFromAnnotation(): AnnotationDefaultValue? {
     annotations.find { it.resolvedFqName == JvmAnnotationNames.DEFAULT_VALUE_FQ_NAME }

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/scopes/JavaClassEnhancementScope.kt

@@ -102,7 +102,7 @@ class JavaClassEnhancementScope(
         original: ConeFunctionSymbol,
         name: Name
     ): FirFunctionSymbol {
-        val firMethod = (original as FirBasedSymbol<*>).fir as? FirJavaMethod ?: error("Can't make enhancement for $original")
+        val firMethod = (original as FirFunctionSymbol).fir as? FirJavaMethod ?: return original
 
         val memberContext = context.copyWithNewDefaultTypeQualifiers(typeQualifierResolver, jsr305State, firMethod.annotations)