FIR: Minor. Refactor calculation of erased upper bound

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

@@ -179,25 +179,29 @@ private fun FirTypeParameter.getErasedUpperBound(
 
     val firstUpperBound = this.bounds.first().coneTypeUnsafe<ConeKotlinType>()
 
-    if (firstUpperBound is ConeClassLikeType) {
+    return getErasedVersionOfFirstUpperBound(firstUpperBound, mutableSetOf(this, potentiallyRecursiveTypeParameter), defaultValue)
-        return firstUpperBound.withArguments(firstUpperBound.typeArguments.map { ConeStarProjection }.toTypedArray())
 }
 
-    val alreadyVisited = mutableSetOf(potentiallyRecursiveTypeParameter, this)
+private tailrec fun getErasedVersionOfFirstUpperBound(
-    var current = (firstUpperBound as ConeTypeParameterType).lookupTag.typeParameterSymbol.fir
+    firstUpperBound: ConeKotlinType,
+    alreadyVisitedParameters: MutableSet<FirTypeParameter?>,
+    defaultValue: () -> ConeKotlinType
+): ConeKotlinType =
+    when (firstUpperBound) {
+        is ConeClassLikeType ->
+            firstUpperBound.withArguments(firstUpperBound.typeArguments.map { ConeStarProjection }.toTypedArray())
 
-    while (current !in alreadyVisited) {
+        is ConeTypeParameterType -> {
-        alreadyVisited += current
+            val current = firstUpperBound.lookupTag.typeParameterSymbol.fir
 
+            if (alreadyVisitedParameters.add(current)) {
                 val nextUpperBound = current.bounds.first().coneTypeUnsafe<ConeKotlinType>()
-        if (nextUpperBound is ConeClassLikeType) {
+                getErasedVersionOfFirstUpperBound(nextUpperBound, alreadyVisitedParameters, defaultValue)
-            return nextUpperBound.withArguments(nextUpperBound.typeArguments.map { ConeStarProjection }.toTypedArray())
+            } else {
+                defaultValue()
             }
-
-        current = (nextUpperBound as ConeTypeParameterType).lookupTag.typeParameterSymbol.fir
         }
-
+        else -> error("Unexpected kind of firstUpperBound: $firstUpperBound [${firstUpperBound::class}]")
-    return defaultValue()
     }