AK: Simplify constructors and conversions from nullptr_t

Problem:
- Many constructors are defined as `{}` rather than using the ` =
  default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
  instead of requiring the caller to default construct. This violates
  the C++ Core Guidelines suggestion to declare single-argument
  constructors explicit
  (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).

Solution:
- Change default constructors to use the compiler-provided default
  constructor.
- Remove implicit conversion operators from `nullptr_t` and change
  usage to enforce type consistency without conversion.

Kernel/ACPI/MultiProcessorParser.cpp

@@ -41,7 +41,7 @@ OwnPtr<MultiProcessorParser> MultiProcessorParser::autodetect()
 {
     auto floating_pointer = find_floating_pointer();
     if (!floating_pointer.has_value())
-        return nullptr;
+        return {};
     return adopt_own(*new MultiProcessorParser(floating_pointer.value()));
 }
 

Kernel/CoreDump.cpp

@@ -45,12 +45,12 @@ OwnPtr<CoreDump> CoreDump::create(NonnullRefPtr<Process> process, const String&
 {
     if (!process->is_dumpable()) {
         dbgln("Refusing to generate CoreDump for non-dumpable process {}", process->pid().value());
-        return nullptr;
+        return {};
     }
 
     auto fd = create_target_file(process, output_path);
     if (!fd)
-        return nullptr;
+        return {};
     return adopt_own(*new CoreDump(move(process), fd.release_nonnull()));
 }
 

Kernel/Devices/SB16.cpp

@@ -227,7 +227,7 @@ void SB16::handle_irq(const RegisterState&)
 
 void SB16::wait_for_irq()
 {
-    m_irq_queue.wait_on(nullptr, "SB16");
+    m_irq_queue.wait_on({}, "SB16");
     disable_irq();
 }
 

Kernel/FileSystem/FIFO.cpp

@@ -75,7 +75,7 @@ KResultOr<NonnullRefPtr<FileDescription>> FIFO::open_direction_blocking(FIFO::Di
 
         if (m_writers == 0) {
             locker.unlock();
-            m_write_open_queue.wait_on(nullptr, "FIFO");
+            m_write_open_queue.wait_on({}, "FIFO");
             locker.lock();
         }
     }
@@ -85,7 +85,7 @@ KResultOr<NonnullRefPtr<FileDescription>> FIFO::open_direction_blocking(FIFO::Di
 
         if (m_readers == 0) {
             locker.unlock();
-            m_read_open_queue.wait_on(nullptr, "FIFO");
+            m_read_open_queue.wait_on({}, "FIFO");
             locker.lock();
         }
     }

Kernel/FileSystem/Plan9FileSystem.cpp

@@ -531,7 +531,7 @@ KResult Plan9FS::post_message(Message& message, RefPtr<ReceiveCompletion> comple
     while (size > 0) {
         if (!description.can_write()) {
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::WriteBlocker>(nullptr, description, unblock_flags).was_interrupted())
+            if (Thread::current()->block<Thread::WriteBlocker>({}, description, unblock_flags).was_interrupted())
                 return KResult(-EINTR);
         }
         auto data_buffer = UserOrKernelBuffer::for_kernel_buffer(const_cast<u8*>(data));
@@ -552,7 +552,7 @@ KResult Plan9FS::do_read(u8* data, size_t size)
     while (size > 0) {
         if (!description.can_read()) {
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::ReadBlocker>(nullptr, description, unblock_flags).was_interrupted())
+            if (Thread::current()->block<Thread::ReadBlocker>({}, description, unblock_flags).was_interrupted())
                 return KResult(-EINTR);
         }
         auto data_buffer = UserOrKernelBuffer::for_kernel_buffer(data);
@@ -621,7 +621,7 @@ KResult Plan9FS::post_message_and_wait_for_a_reply(Message& message)
     auto result = post_message(message, completion);
     if (result.is_error())
         return result;
-    if (Thread::current()->block<Plan9FS::Blocker>(nullptr, *this, message, completion).was_interrupted())
+    if (Thread::current()->block<Plan9FS::Blocker>({}, *this, message, completion).was_interrupted())
         return KResult(-EINTR);
 
     if (completion->result.is_error()) {

Kernel/KBuffer.h

@@ -128,7 +128,7 @@ public:
     {
         auto impl = KBufferImpl::try_create_with_size(size, access, name, strategy);
         if (!impl)
-            return nullptr;
+            return {};
         return adopt_own(*new KBuffer(impl.release_nonnull()));
     }
 
@@ -136,7 +136,7 @@ public:
     {
         auto impl = KBufferImpl::try_create_with_bytes(bytes, access, name, strategy);
         if (!impl)
-            return nullptr;
+            return {};
         return adopt_own(*new KBuffer(impl.release_nonnull()));
     }
 

Kernel/Lock.cpp

@@ -123,7 +123,7 @@ void Lock::lock(Mode mode)
                     ASSERT_NOT_REACHED();
                 }
                 m_lock.store(false, AK::memory_order_release);
-            } while (m_queue.wait_on(nullptr, m_name) == Thread::BlockResult::NotBlocked);
+            } while (m_queue.wait_on({}, m_name) == Thread::BlockResult::NotBlocked);
         } else {
             // I don't know *who* is using "m_lock", so just yield.
             Scheduler::yield_from_critical();

Kernel/Net/E1000NetworkAdapter.cpp

@@ -457,7 +457,7 @@ void E1000NetworkAdapter::send_raw(ReadonlyBytes payload)
             sti();
             break;
         }
-        m_wait_queue.wait_on(nullptr, "E1000NetworkAdapter");
+        m_wait_queue.wait_on({}, "E1000NetworkAdapter");
     }
 #ifdef E1000_DEBUG
     dbgln("E1000: Sent packet, status is now {:#02x}!", (u8)descriptor.status);

Kernel/Net/IPv4Socket.cpp

@@ -255,7 +255,7 @@ KResultOr<size_t> IPv4Socket::receive_byte_buffered(FileDescription& description
 
         locker.unlock();
         auto unblocked_flags = Thread::FileDescriptionBlocker::BlockFlags::None;
-        auto res = Thread::current()->block<Thread::ReadBlocker>(nullptr, description, unblocked_flags);
+        auto res = Thread::current()->block<Thread::ReadBlocker>({}, description, unblocked_flags);
         locker.lock();
 
         if (!((u32)unblocked_flags & (u32)Thread::FileDescriptionBlocker::BlockFlags::Read)) {
@@ -306,7 +306,7 @@ KResultOr<size_t> IPv4Socket::receive_packet_buffered(FileDescription& descripti
 
         locker.unlock();
         auto unblocked_flags = Thread::FileDescriptionBlocker::BlockFlags::None;
-        auto res = Thread::current()->block<Thread::ReadBlocker>(nullptr, description, unblocked_flags);
+        auto res = Thread::current()->block<Thread::ReadBlocker>({}, description, unblocked_flags);
         locker.lock();
 
         if (!((u32)unblocked_flags & (u32)Thread::FileDescriptionBlocker::BlockFlags::Read)) {

Kernel/Net/LocalSocket.cpp

@@ -192,7 +192,7 @@ KResult LocalSocket::connect(FileDescription& description, Userspace<const socka
     }
 
     auto unblock_flags = Thread::FileDescriptionBlocker::BlockFlags::None;
-    if (Thread::current()->block<Thread::ConnectBlocker>(nullptr, description, unblock_flags).was_interrupted()) {
+    if (Thread::current()->block<Thread::ConnectBlocker>({}, description, unblock_flags).was_interrupted()) {
         set_connect_side_role(Role::None);
         return KResult(-EINTR);
     }
@@ -329,7 +329,7 @@ KResultOr<size_t> LocalSocket::recvfrom(FileDescription& description, UserOrKern
         }
     } else if (!can_read(description, 0)) {
         auto unblock_flags = Thread::FileDescriptionBlocker::BlockFlags::None;
-        if (Thread::current()->block<Thread::ReadBlocker>(nullptr, description, unblock_flags).was_interrupted())
+        if (Thread::current()->block<Thread::ReadBlocker>({}, description, unblock_flags).was_interrupted())
             return KResult(-EINTR);
     }
     if (!has_attached_peer(description) && socket_buffer->is_empty())

Kernel/Net/NetworkTask.cpp

@@ -114,7 +114,7 @@ void NetworkTask_main(void*)
     for (;;) {
         size_t packet_size = dequeue_packet(buffer, buffer_size, packet_timestamp);
         if (!packet_size) {
-            packet_wait_queue.wait_on(nullptr, "NetworkTask");
+            packet_wait_queue.wait_on({}, "NetworkTask");
             continue;
         }
         if (packet_size < sizeof(EthernetFrameHeader)) {

Kernel/Net/Routing.cpp

@@ -230,7 +230,7 @@ RoutingDecision route_to(const IPv4Address& target, const IPv4Address& source, c
     adapter->send({ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }, request);
 
     Optional<MACAddress> addr;
-    if (!Thread::current()->block<ARPTableBlocker>(nullptr, next_hop_ip, addr).was_interrupted()) {
+    if (!Thread::current()->block<ARPTableBlocker>({}, next_hop_ip, addr).was_interrupted()) {
         if (addr.has_value()) {
 #ifdef ROUTING_DEBUG
             klog() << "Routing: Got ARP response using adapter " << adapter->name().characters() << " for " << next_hop_ip.to_string().characters() << " (" << addr.value().to_string().characters() << ")";

Kernel/Net/TCPSocket.cpp

@@ -400,7 +400,7 @@ KResult TCPSocket::protocol_connect(FileDescription& description, ShouldBlock sh
     if (should_block == ShouldBlock::Yes) {
         locker.unlock();
         auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-        if (Thread::current()->block<Thread::ConnectBlocker>(nullptr, description, unblock_flags).was_interrupted())
+        if (Thread::current()->block<Thread::ConnectBlocker>({}, description, unblock_flags).was_interrupted())
             return KResult(-EINTR);
         locker.lock();
         ASSERT(setup_state() == SetupState::Completed);

Kernel/PerformanceEventBuffer.cpp

@@ -89,7 +89,7 @@ OwnPtr<KBuffer> PerformanceEventBuffer::to_json(ProcessID pid, const String& exe
 {
     KBufferBuilder builder;
     if (!to_json(builder, pid, executable_path))
-        return nullptr;
+        return {};
     return builder.build();
 }
 

Kernel/Random.cpp

@@ -69,7 +69,7 @@ KernelRng::KernelRng()
 void KernelRng::wait_for_entropy()
 {
     if (!resource().is_ready()) {
-        m_seed_queue.wait_on(nullptr, "KernelRng");
+        m_seed_queue.wait_on({}, "KernelRng");
     }
 }
 

Kernel/Storage/Partition/MBRPartitionTable.cpp

@@ -54,7 +54,7 @@ OwnPtr<MBRPartitionTable> MBRPartitionTable::try_to_initialize(const StorageDevi
 {
     auto table = make<MBRPartitionTable>(device, start_lba);
     if (!table->is_valid())
-        return nullptr;
+        return {};
     return table;
 }
 

Kernel/Storage/StorageManagement.cpp

@@ -90,16 +90,16 @@ OwnPtr<PartitionTable> StorageManagement::try_to_initialize_partition_table(cons
     if (mbr_table_or_result.error() == PartitionTable::Error::MBRProtective) {
         auto gpt_table_or_result = GUIDPartitionTable::try_to_initialize(device);
         if (gpt_table_or_result.is_error())
-            return nullptr;
+            return {};
         return move(gpt_table_or_result.value());
     }
     if (mbr_table_or_result.error() == PartitionTable::Error::ConatinsEBR) {
         auto ebr_table_or_result = EBRPartitionTable::try_to_initialize(device);
         if (ebr_table_or_result.is_error())
-            return nullptr;
+            return {};
         return move(ebr_table_or_result.value());
     }
-    return nullptr;
+    return {};
 }
 
 NonnullRefPtrVector<DiskPartition> StorageManagement::enumerate_disk_partitions() const

Kernel/Syscalls/execve.cpp

@@ -27,6 +27,7 @@
 #include <AK/LexicalPath.h>
 #include <AK/ScopeGuard.h>
 #include <AK/TemporaryChange.h>
+#include <AK/WeakPtr.h>
 #include <Kernel/FileSystem/Custody.h>
 #include <Kernel/FileSystem/FileDescription.h>
 #include <Kernel/PerformanceEventBuffer.h>
@@ -269,7 +270,7 @@ KResultOr<Process::LoadResult> Process::load_elf_object(FileDescription& object_
         executable_size,
         VirtualAddress(elf_image.program_header_table_offset()).offset(load_offset).get(),
         elf_image.program_header_count(),
-        master_tls_region ? master_tls_region->make_weak_ptr() : nullptr,
+        AK::try_make_weak_ptr(master_tls_region),
         master_tls_size,
         master_tls_alignment,
         stack_region->make_weak_ptr()

Kernel/Syscalls/read.cpp

@@ -51,7 +51,7 @@ ssize_t Process::sys$read(int fd, Userspace<u8*> buffer, ssize_t size)
     if (description->is_blocking()) {
         if (!description->can_read()) {
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::ReadBlocker>(nullptr, *description, unblock_flags).was_interrupted())
+            if (Thread::current()->block<Thread::ReadBlocker>({}, *description, unblock_flags).was_interrupted())
                 return -EINTR;
             if (!((u32)unblock_flags & (u32)Thread::FileBlocker::BlockFlags::Read))
                 return -EAGAIN;

Kernel/Syscalls/socket.cpp

@@ -115,7 +115,7 @@ int Process::sys$accept(int accepting_socket_fd, Userspace<sockaddr*> user_addre
     if (!socket.can_accept()) {
         if (accepting_socket_description->is_blocking()) {
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::AcceptBlocker>(nullptr, *accepting_socket_description, unblock_flags).was_interrupted())
+            if (Thread::current()->block<Thread::AcceptBlocker>({}, *accepting_socket_description, unblock_flags).was_interrupted())
                 return -EINTR;
         } else {
             return -EAGAIN;

Kernel/Syscalls/thread.cpp

@@ -132,7 +132,7 @@ int Process::sys$join_thread(pid_t tid, Userspace<void**> exit_value)
     // NOTE: pthread_join() cannot be interrupted by signals. Only by death.
     for (;;) {
         KResult try_join_result(KSuccess);
-        auto result = current_thread->block<Thread::JoinBlocker>(nullptr, *thread, try_join_result, joinee_exit_value);
+        auto result = current_thread->block<Thread::JoinBlocker>({}, *thread, try_join_result, joinee_exit_value);
         if (result == Thread::BlockResult::NotBlocked) {
             if (try_join_result.is_error())
                 return try_join_result.error();

Kernel/Syscalls/waitid.cpp

@@ -42,7 +42,7 @@ KResultOr<siginfo_t> Process::do_waitid(idtype_t idtype, int id, int options)
     }
 
     KResultOr<siginfo_t> result = KResult(KSuccess);
-    if (Thread::current()->block<Thread::WaitBlocker>(nullptr, options, idtype, id, result).was_interrupted())
+    if (Thread::current()->block<Thread::WaitBlocker>({}, options, idtype, id, result).was_interrupted())
         return KResult(-EINTR);
     ASSERT(!result.is_error() || (options & WNOHANG) || result.error() != KSuccess);
     return result;

Kernel/Syscalls/write.cpp

@@ -97,7 +97,7 @@ ssize_t Process::do_write(FileDescription& description, const UserOrKernelBuffer
                 return total_nwritten;
             }
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::WriteBlocker>(nullptr, description, unblock_flags).was_interrupted()) {
+            if (Thread::current()->block<Thread::WriteBlocker>({}, description, unblock_flags).was_interrupted()) {
                 if (total_nwritten == 0)
                     return -EINTR;
             }

Kernel/Tasks/FinalizerTask.cpp

@@ -36,7 +36,7 @@ void FinalizerTask::spawn()
         finalizer_thread, "FinalizerTask", [](void*) {
             Thread::current()->set_priority(THREAD_PRIORITY_LOW);
             for (;;) {
-                g_finalizer_wait_queue->wait_on(nullptr, "FinalizerTask");
+                g_finalizer_wait_queue->wait_on({}, "FinalizerTask");
 
                 if (g_finalizer_has_work.exchange(false, AK::MemoryOrder::memory_order_acq_rel) == true)
                     Thread::finalize_dying_threads();

Kernel/Thread.cpp

@@ -316,13 +316,13 @@ void Thread::relock_process(LockMode previous_locked, u32 lock_count_to_restore)
 auto Thread::sleep(clockid_t clock_id, const timespec& duration, timespec* remaining_time) -> BlockResult
 {
     ASSERT(state() == Thread::Running);
-    return Thread::current()->block<Thread::SleepBlocker>(nullptr, Thread::BlockTimeout(false, &duration, nullptr, clock_id), remaining_time);
+    return Thread::current()->block<Thread::SleepBlocker>({}, Thread::BlockTimeout(false, &duration, nullptr, clock_id), remaining_time);
 }
 
 auto Thread::sleep_until(clockid_t clock_id, const timespec& deadline) -> BlockResult
 {
     ASSERT(state() == Thread::Running);
-    return Thread::current()->block<Thread::SleepBlocker>(nullptr, Thread::BlockTimeout(true, &deadline, nullptr, clock_id));
+    return Thread::current()->block<Thread::SleepBlocker>({}, Thread::BlockTimeout(true, &deadline, nullptr, clock_id));
 }
 
 const char* Thread::state_string() const

Kernel/Thread.h

@@ -207,10 +207,6 @@ public:
             : m_infinite(true)
         {
         }
-        BlockTimeout(std::nullptr_t)
-            : m_infinite(true)
-        {
-        }
         explicit BlockTimeout(bool is_absolute, const timeval* time, const timespec* start_time = nullptr, clockid_t clock_id = CLOCK_MONOTONIC_COARSE)
             : m_clock_id(clock_id)
             , m_infinite(!time)

Kernel/VM/MemoryManager.cpp

@@ -385,7 +385,7 @@ OwnPtr<Region> MemoryManager::allocate_contiguous_kernel_region(size_t size, con
     ScopedSpinLock lock(s_mm_lock);
     auto range = kernel_page_directory().range_allocator().allocate_anywhere(size);
     if (!range.is_valid())
-        return nullptr;
+        return {};
     auto vmobject = ContiguousVMObject::create_with_size(size);
     return allocate_kernel_region_with_vmobject(range, vmobject, name, access, user_accessible, cacheable);
 }
@@ -396,10 +396,10 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(size_t size, const StringVi
     ScopedSpinLock lock(s_mm_lock);
     auto range = kernel_page_directory().range_allocator().allocate_anywhere(size);
     if (!range.is_valid())
-        return nullptr;
+        return {};
     auto vmobject = AnonymousVMObject::create_with_size(size, strategy);
     if (!vmobject)
-        return nullptr;
+        return {};
     return allocate_kernel_region_with_vmobject(range, vmobject.release_nonnull(), name, access, user_accessible, cacheable);
 }
 
@@ -409,10 +409,10 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region(PhysicalAddress paddr, size
     ScopedSpinLock lock(s_mm_lock);
     auto range = kernel_page_directory().range_allocator().allocate_anywhere(size);
     if (!range.is_valid())
-        return nullptr;
+        return {};
     auto vmobject = AnonymousVMObject::create_for_physical_range(paddr, size);
     if (!vmobject)
-        return nullptr;
+        return {};
     return allocate_kernel_region_with_vmobject(range, *vmobject, name, access, user_accessible, cacheable);
 }
 
@@ -422,10 +422,10 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region_identity(PhysicalAddress pa
     ScopedSpinLock lock(s_mm_lock);
     auto range = kernel_page_directory().identity_range_allocator().allocate_specific(VirtualAddress(paddr.get()), size);
     if (!range.is_valid())
-        return nullptr;
+        return {};
     auto vmobject = AnonymousVMObject::create_for_physical_range(paddr, size);
     if (!vmobject)
-        return nullptr;
+        return {};
     return allocate_kernel_region_with_vmobject(range, *vmobject, name, access, user_accessible, cacheable);
 }
 
@@ -453,7 +453,7 @@ OwnPtr<Region> MemoryManager::allocate_kernel_region_with_vmobject(VMObject& vmo
     ScopedSpinLock lock(s_mm_lock);
     auto range = kernel_page_directory().range_allocator().allocate_anywhere(size);
     if (!range.is_valid())
-        return nullptr;
+        return {};
     return allocate_kernel_region_with_vmobject(range, vmobject, name, access, user_accessible, cacheable);
 }