[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))
     }

compiler/testData/diagnostics/foreignAnnotationsTests/tests/jsr305/nullabilityWarnings/kt65193.fir.kt

@@ -1,49 +0,0 @@
-// FILE: test/NonNullApi.java
-package test;
-
-import java.lang.annotation.*;
-import javax.annotation.Nonnull;
-import javax.annotation.meta.TypeQualifierDefault;
-
-@Target(ElementType.PACKAGE)
-@Retention(RetentionPolicy.RUNTIME)
-@Documented
-@Nonnull
-@TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER, ElementType.PACKAGE})
-public @interface NonNullApi {
-}
-
-// FILE: test/package-info.java
-@NonNullApi
-package test;
-
-// FILE: test/JpaSpecificationExecutor.java
-package test;
-
-import java.util.List;
-
-public interface JpaSpecificationExecutor<T> {
-    List<T> findAll();
-}
-
-// FILE: mockito/OngoingStubbing.java
-package mockito;
-
-public interface OngoingStubbing<T> {
-    OngoingStubbing<T> thenReturn(T var1);
-
-    public static <T> OngoingStubbing<T> when(T... methodCall) {
-        return null;
-    }
-}
-
-
-// FILE: test.kt
-package test
-
-import mockito.OngoingStubbing
-
-fun test(wrapper: JpaSpecificationExecutor<String>, l: List<String>, l2: List<Int>) {
-    OngoingStubbing.`when`(wrapper.findAll()).thenReturn(<!NULLABILITY_MISMATCH_BASED_ON_JAVA_ANNOTATIONS!>l<!>)
-    OngoingStubbing.`when`(wrapper.findAll(), l2).thenReturn(l)
-}

compiler/testData/diagnostics/foreignAnnotationsTests/tests/jsr305/nullabilityWarnings/kt65193.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // FILE: test/NonNullApi.java
 package test;