Kotlin: fix extraction of Java nested wildcards; wildcards in return types

This fixes two mistakes: return-type extraction not imposing a wildcard where a Java prototype explicitly uses one, and nested wildcard detection quietly failing due to not looking through a `JavaWildcardType` correctly.

I add a variant of the `kotlin_java_lowering_wildcards` test where Java prototypes are only seen from Kotlin, to be sure extraction is working as expected.

java/kotlin-extractor/src/main/kotlin/KotlinUsesExtractor.kt

@@ -963,9 +963,9 @@ open class KotlinUsesExtractor(
     private fun wildcardAdditionAllowed(v: Variance, t: IrType, addByDefault: Boolean, javaVariance: Variance?) =
         when {
             t.hasAnnotation(jvmWildcardAnnotation) -> true
-            !addByDefault -> false
             // If a Java declaration specifies a variance, introduce it even if it's pointless (e.g. ? extends FinalClass, or ? super Object)
             javaVariance == v -> true
+            !addByDefault -> false
             v == Variance.IN_VARIANCE -> !(t.isNullableAny() || t.isAny())
             v == Variance.OUT_VARIANCE -> extendsAdditionAllowed(t)
             else -> false
@@ -1006,8 +1006,9 @@ open class KotlinUsesExtractor(
                 null
         } ?: t
 
-    private fun getJavaTypeArgument(jt: JavaType, idx: Int) =
+    private fun getJavaTypeArgument(jt: JavaType, idx: Int): JavaType? =
         when(jt) {
+            is JavaWildcardType -> jt.bound?.let { getJavaTypeArgument(it, idx) }
             is JavaClassifierType -> jt.typeArguments.getOrNull(idx)
             is JavaArrayType -> if (idx == 0) jt.componentType else null
             else -> null

java/ql/integration-tests/all-platforms/kotlin/kotlin_java_lowering_wildcards/JavaDefns2.java

@@ -0,0 +1,18 @@
+public class JavaDefns2 {
+
+  // Currently known not to work: the Comparable<? extends X> case, which Kotlin sees as Comparable<*> because the
+  // wildcard goes the opposite direction to the variance declared on Comparable's type parameter.
+
+  public static void takesComparable(Comparable<CharSequence> invar, Comparable<? super CharSequence> contravar) { }
+
+  public static void takesNestedComparable(Comparable<Comparable<? super CharSequence>> innerContravar, Comparable<? super Comparable<CharSequence>> outerContravar) { }
+
+  public static void takesArrayOfComparable(Comparable<CharSequence>[] invar, Comparable<? super CharSequence>[] contravar) { }
+
+  public static Comparable<? super CharSequence> returnsWildcard() { return null; }
+
+  public static Comparable<CharSequence> returnsInvariant() { return null; }
+
+  public JavaDefns2(Comparable<CharSequence> invar, Comparable<? super CharSequence> contravar) { }
+
+}

java/ql/integration-tests/all-platforms/kotlin/kotlin_java_lowering_wildcards/kotlinuser.kt

@@ -7,4 +7,6 @@ fun user() {
   JavaDefns.takesArrayOfComparable(acs, acs)
 
   val constructed = JavaDefns(cs, cs)
+
+  JavaDefns2.takesComparable(cs, cs)
 }

java/ql/integration-tests/all-platforms/kotlin/kotlin_java_lowering_wildcards/test.expected

@@ -8,6 +8,16 @@
 | JavaDefns | takesComparable | invar | Comparable<CharSequence> |
 | JavaDefns | takesNestedComparable | innerContravar | Comparable<Comparable<? super CharSequence>> |
 | JavaDefns | takesNestedComparable | outerContravar | Comparable<? super Comparable<CharSequence>> |
+| JavaDefns2 | JavaDefns2 | p0 | Comparable<CharSequence> |
+| JavaDefns2 | JavaDefns2 | p1 | Comparable<? super CharSequence> |
+| JavaDefns2 | returnsInvariant | return | Comparable<CharSequence> |
+| JavaDefns2 | returnsWildcard | return | Comparable<? super CharSequence> |
+| JavaDefns2 | takesArrayOfComparable | p0 | Comparable<CharSequence>[] |
+| JavaDefns2 | takesArrayOfComparable | p1 | Comparable<? super CharSequence>[] |
+| JavaDefns2 | takesComparable | p0 | Comparable<CharSequence> |
+| JavaDefns2 | takesComparable | p1 | Comparable<? super CharSequence> |
+| JavaDefns2 | takesNestedComparable | p0 | Comparable<Comparable<? super CharSequence>> |
+| JavaDefns2 | takesNestedComparable | p1 | Comparable<? super Comparable<CharSequence>> |
 | KotlinDefns | returnsContravar | return | Comparable<CharSequence> |
 | KotlinDefns | returnsContravarForced | return | Comparable<? super CharSequence> |
 | KotlinDefns | returnsCovar | return | List<CharSequence> |

java/ql/integration-tests/all-platforms/kotlin/kotlin_java_lowering_wildcards/test.py

@@ -1,3 +1,5 @@
 from create_database_utils import *
 
+# Compile the JavaDefns2 copy outside tracing, to make sure the Kotlin view of it matches the Java view seen by the traced javac compilation of JavaDefns.java below.
+runSuccessfully(["javac", "JavaDefns2.java"])
 run_codeql_database_create(["kotlinc kotlindefns.kt", "javac JavaUser.java JavaDefns.java -cp .", "kotlinc -cp . kotlinuser.kt"], lang="java")

java/ql/integration-tests/all-platforms/kotlin/kotlin_java_lowering_wildcards/test.ql

@@ -1,7 +1,7 @@
 import java
 
 predicate isInterestingClass(Class c) {
-  [c, c.(NestedType).getEnclosingType()].getName().matches(["KotlinDefns%", "JavaDefns"])
+  [c, c.(NestedType).getEnclosingType()].getName().matches(["KotlinDefns%", "JavaDefns%"])
 }
 
 from Callable c, string paramOrReturnName, Type paramOrReturnType