[FE] Skip CST computation if list of types has exactly one entry

Otherwise, we can get in a situation where the single item is flexible,
and we replace its attributes with the attribute of the lower bound,
which messes up `EnhancedTypeForWarningAttribute`.

#KT-65193 Fixed

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/NewCommonSuperTypeCalculator.kt

@@ -14,6 +14,15 @@ import org.jetbrains.kotlin.types.model.*
 
 object NewCommonSuperTypeCalculator {
     fun TypeSystemCommonSuperTypesContext.commonSuperType(types: List<KotlinTypeMarker>): KotlinTypeMarker {
+        // Skip computation if the list has only one element.
+        // It's not only an optimization, but it's required to not mess up attributes.
+        // See compiler/testData/diagnostics/foreignAnnotationsTests/tests/jsr305/nullabilityWarnings/kt65193.kt
+        // When the type is flexible, calling replaceCustomAttributes(unionTypeAttributes(types)) will take the attributes of the
+        // lower bound which will mess up `EnhancedTypeForWarningAttribute`s.
+        // It's not a problem if the list has multiple entries, because `EnhancedTypeForWarningAttribute.union` returns null,
+        // meaning that whenever we union multiple types, we remove any `EnhancedTypeForWarningAttribute`.
+        types.singleOrNull()?.let { return it }
+
         val maxDepth = types.maxOfOrNull { it.typeDepth() } ?: 0
         return commonSuperType(types, -maxDepth, true).replaceCustomAttributes(unionTypeAttributes(types))
     }