Restore implicit wildcard types

The Kotlin compiler represents types like List<out CharSequence> internally as List<CharSequence> due to the fact that List's type parameter is covariant, and similarly Comparable<in CharSequence> where Comparable's type parameter is contravariant. However it restores use-site variance when emitting class files, so we must do the same thing for
compatability with Java code.

Note this is a partial solution because it will also add wildcards to Java .class files that *could* have a variance / wildcard but don't -- for example, a Java method could really take an invariant Comparable<CharSequence>, which is only achievable in Kotlin via the @JvmSuppressWildcards annotation. We also don't yet support
@JvmSuppressWildcards given on a surrounding class or function.

Author: smowton

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

@@ -497,7 +497,9 @@ open class KotlinFileExtractor(
                 else
                     null
             } ?: vp.type
-            val substitutedType = typeSubstitution?.let { it(maybeErasedType, TypeContext.OTHER, pluginContext) } ?: maybeErasedType
+            val typeWithWildcards = addJavaLoweringWildcards(maybeErasedType, true)
+            val substitutedType = typeSubstitution?.let { it(typeWithWildcards, TypeContext.OTHER, pluginContext) } ?: typeWithWildcards
+
             val id = useValueParameter(vp, parent)
             if (extractTypeAccess) {
                 extractTypeAccessRecursive(substitutedType, location, id, -1)
@@ -704,7 +706,7 @@ open class KotlinFileExtractor(
 
                 val paramsSignature = allParamTypes.joinToString(separator = ",", prefix = "(", postfix = ")") { it.javaResult.signature!! }
 
-                val adjustedReturnType = getAdjustedReturnType(f)
+                val adjustedReturnType = addJavaLoweringWildcards(getAdjustedReturnType(f), false)
                 val substReturnType = typeSubstitution?.let { it(adjustedReturnType, TypeContext.RETURN, pluginContext) } ?: adjustedReturnType
 
                 val locId = locOverride ?: getLocation(f, classTypeArgsIncludingOuterClasses)

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

@@ -6,6 +6,7 @@ import com.github.codeql.utils.versions.isRawType
 import com.semmle.extractor.java.OdasaOutput
 import org.jetbrains.kotlin.backend.common.extensions.IrPluginContext
 import org.jetbrains.kotlin.backend.common.ir.allOverridden
+import org.jetbrains.kotlin.backend.common.ir.isFinalClass
 import org.jetbrains.kotlin.backend.common.lower.parentsWithSelf
 import org.jetbrains.kotlin.backend.jvm.ir.getJvmNameFromAnnotation
 import org.jetbrains.kotlin.backend.jvm.ir.propertyIfAccessor
@@ -850,6 +851,49 @@ open class KotlinUsesExtractor(
                 (f.name.asString() == "addAll" && overridesFunctionDefinedOn(f, "kotlin.collections", "MutableCollection")) ||
                 (f.name.asString() == "addAll" && overridesFunctionDefinedOn(f, "kotlin.collections", "MutableList"))
 
+
+    private fun wildcardAdditionAllowed(v: Variance, t: IrType, inParameterContext: Boolean) =
+        when {
+            t.hasAnnotation(FqName("kotlin.jvm.JvmWildcard")) -> true
+            !inParameterContext -> false // By default, wildcards are only automatically added for method parameters.
+            t.hasAnnotation(FqName("kotlin.jvm.JvmSuppressWildcards")) -> false
+            v == Variance.IN_VARIANCE -> !(t.isNullableAny() || t.isAny())
+            v == Variance.OUT_VARIANCE -> ((t as? IrSimpleType)?.classOrNull?.owner?.isFinalClass) != true
+            else -> false
+        }
+
+    private fun addJavaLoweringArgumentWildcards(p: IrTypeParameter, t: IrTypeArgument, inParameterContext: Boolean): IrTypeArgument =
+        (t as? IrTypeProjection)?.let {
+            val newBase = addJavaLoweringWildcards(it.type, inParameterContext)
+            val newVariance =
+                if (it.variance == Variance.INVARIANT &&
+                    p.variance != Variance.INVARIANT &&
+                    wildcardAdditionAllowed(p.variance, it.type, inParameterContext))
+                    p.variance
+                else
+                    it.variance
+            if (newBase !== it.type || newVariance != it.variance)
+                makeTypeProjection(newBase, newVariance)
+            else
+                null
+        } ?: t
+
+    fun addJavaLoweringWildcards(t: IrType, inParameterContext: Boolean): IrType =
+        (t as? IrSimpleType)?.let {
+            val typeParams = it.classOrNull?.owner?.typeParameters ?: return t
+            val newArgs = typeParams.zip(it.arguments).map { pair ->
+                addJavaLoweringArgumentWildcards(
+                    pair.first,
+                    pair.second,
+                    inParameterContext
+                )
+            }
+            return if (newArgs.zip(it.arguments).all { pair -> pair.first === pair.second })
+                t
+            else
+                it.toBuilder().also { builder -> builder.arguments = newArgs }.buildSimpleType()
+        } ?: t
+
     /*
      * This is the normal getFunctionLabel function to use. If you want
      * to refer to the function in its source class then
@@ -956,8 +1000,10 @@ open class KotlinUsesExtractor(
             // Collection.remove(Object) because Collection.remove(Collection::E) in the Kotlin universe.
             // If this has happened, erase the type again to get the correct Java signature.
             val maybeAmendedForCollections = if (overridesCollectionsMethod) eraseCollectionsMethodParameterType(it.value.type, name, it.index) else it.value.type
+            // Add any wildcard types that the Kotlin compiler would add in the Java lowering of this function:
+            val withAddedWildcards = addJavaLoweringWildcards(maybeAmendedForCollections, true)
             // Now substitute any class type parameters in:
-            val maybeSubbed = maybeAmendedForCollections.substituteTypeAndArguments(substitutionMap, TypeContext.OTHER, pluginContext)
+            val maybeSubbed = withAddedWildcards.substituteTypeAndArguments(substitutionMap, TypeContext.OTHER, pluginContext)
             // Finally, mimic the Java extractor's behaviour by naming functions with type parameters for their erased types;
             // those without type parameters are named for the generic type.
             val maybeErased = if (functionTypeParameters.isEmpty()) maybeSubbed else erase(maybeSubbed)
@@ -969,6 +1015,8 @@ open class KotlinUsesExtractor(
                 pluginContext.irBuiltIns.unitType
             else
                 erase(returnType.substituteTypeAndArguments(substitutionMap, TypeContext.RETURN, pluginContext))
+        // Note that `addJavaLoweringWildcards` is not required here because the return type used to form the function
+        // label is always erased.
         val returnTypeId = useType(labelReturnType, TypeContext.RETURN).javaResult.id
         // This suffix is added to generic methods (and constructors) to match the Java extractor's behaviour.
         // Comments in that extractor indicates it didn't want the label of the callable to clash with the raw