Reintroduce class extensions test

To fix the case where e.g. an extension method for Collection<T> is
called on a List<Int>.

Author: fwcd

server/src/main/kotlin/org/javacs/kt/completion/Completions.kt

@@ -434,11 +434,19 @@ private fun isExtensionFor(type: KotlinType, extensionFunction: CallableDescript
     val receiverType = extensionFunction.extensionReceiverParameter?.type ?: return false
     return KotlinTypeChecker.DEFAULT.isSubtypeOf(type, receiverType)
         || (TypeUtils.getTypeParameterDescriptorOrNull(receiverType)?.isGenericExtensionFor(type) ?: false)
+        || (TypeUtils.getClassDescriptor(receiverType)?.isClassExtensionFor(type) ?: false)
 }
 
 private fun TypeParameterDescriptor.isGenericExtensionFor(type: KotlinType): Boolean =
     upperBounds.all { KotlinTypeChecker.DEFAULT.isSubtypeOf(type, it) }
 
+// Currently only needed for extensions such as e.g. Collection<T> applied to List<SomeConcreteType>
+// Direct subtype relationships will be detected by KotlinTypeChecker.isSubtypeOf
+private fun ClassDescriptor.isClassExtensionFor(type: KotlinType): Boolean {
+    fun matchesFqName(t: KotlinType) = TypeUtils.getClassDescriptor(t)?.fqNameSafe == fqNameSafe
+    return matchesFqName(type) || type.supertypes().any(::matchesFqName)
+}
+
 private val loggedHidden = CacheBuilder.newBuilder().expireAfterWrite(1, TimeUnit.MINUTES).build<Pair<Name, Name>, Unit>()
 
 private fun logHidden(target: DeclarationDescriptor, from: DeclarationDescriptor) {