Reland "Make many base:: classes constexpr default constructible"

This is a reland of d74c23a2
Original change's description:
> Make many base:: classes constexpr default constructible

Same CL but extracted scoped_refptr changes to separate CL.

>
> Bug: 800760
> Change-Id: Ib713771347283825ff53a8517b38a8abccd9d1bb
> Reviewed-on: https://chromium-review.googlesource.com/860157
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#528805}

Bug: 800760
Change-Id: Ia989d978f632075c21f11cd81687f52b9e2bd2ca

TBR=dcheng@chromium.org

Change-Id: Ia989d978f632075c21f11cd81687f52b9e2bd2ca
Reviewed-on: https://chromium-review.googlesource.com/863622Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528934}

base/allocator/partition_allocator/spin_lock.cc

@@ -62,9 +62,6 @@
 namespace base {
 namespace subtle {
 
-SpinLock::SpinLock() = default;
-SpinLock::~SpinLock() = default;
-
 void SpinLock::LockSlow() {
   // The value of |kYieldProcessorTries| is cargo culted from TCMalloc, Windows
   // critical section defaults, and various other recommendations.

base/allocator/partition_allocator/spin_lock.h

@@ -22,8 +22,8 @@ namespace subtle {
 
 class BASE_EXPORT SpinLock {
  public:
-  SpinLock();
-  ~SpinLock();
+  constexpr SpinLock() = default;
+  ~SpinLock() = default;
   using Guard = std::lock_guard<SpinLock>;
 
   ALWAYS_INLINE void lock() {

base/android/scoped_java_ref.h

@@ -45,12 +45,12 @@ template<>
 class BASE_EXPORT JavaRef<jobject> {
  public:
   // Initializes a null reference. Don't add anything else here; it's inlined.
-  JavaRef() : obj_(nullptr) {}
+  constexpr JavaRef() : obj_(nullptr) {}
 
   // Allow nullptr to be converted to JavaRef. This avoids having to declare an
   // empty JavaRef just to pass null to a function, and makes C++ "nullptr" and
   // Java "null" equivalent.
-  JavaRef(std::nullptr_t) : JavaRef() {}
+  constexpr JavaRef(std::nullptr_t) : JavaRef() {}
 
   // Public to allow destruction of null JavaRef objects.
   // Don't add anything else here; it's inlined.
@@ -146,8 +146,8 @@ class JavaParamRef : public JavaRef<T> {
 template<typename T>
 class ScopedJavaLocalRef : public JavaRef<T> {
  public:
-  ScopedJavaLocalRef() : env_(nullptr) {}
-  ScopedJavaLocalRef(std::nullptr_t) : env_(nullptr) {}
+  constexpr ScopedJavaLocalRef() : env_(nullptr) {}
+  constexpr ScopedJavaLocalRef(std::nullptr_t) : env_(nullptr) {}
 
   // Non-explicit copy constructor, to allow ScopedJavaLocalRef to be returned
   // by value as this is the normal usage pattern.
@@ -234,8 +234,8 @@ class ScopedJavaLocalRef : public JavaRef<T> {
 template<typename T>
 class ScopedJavaGlobalRef : public JavaRef<T> {
  public:
-  ScopedJavaGlobalRef() {}
-  ScopedJavaGlobalRef(std::nullptr_t) {}
+  constexpr ScopedJavaGlobalRef() {}
+  constexpr ScopedJavaGlobalRef(std::nullptr_t) {}
 
   ScopedJavaGlobalRef(const ScopedJavaGlobalRef<T>& other) {
     this->Reset(other);

base/containers/circular_deque.h

@@ -417,7 +417,7 @@ class circular_deque {
   // ---------------------------------------------------------------------------
   // Constructor
 
-  circular_deque() = default;
+  constexpr circular_deque() = default;
 
   // Constructs with |count| copies of |value| or default constructed version.
   circular_deque(size_type count) { resize(count); }

base/containers/vector_buffer.h

@@ -37,7 +37,7 @@ namespace internal {
 template <typename T>
 class VectorBuffer {
  public:
-  VectorBuffer() = default;
+  constexpr VectorBuffer() = default;
 
 #if defined(__clang__) && !defined(__native_client__)
   // This constructor converts an uninitialized void* to a T* which triggers

base/mac/scoped_dispatch_object.h

@@ -15,7 +15,7 @@ namespace internal {
 
 template <typename T>
 struct ScopedDispatchObjectTraits {
-  static T InvalidValue() { return nullptr; }
+  static constexpr T InvalidValue() { return nullptr; }
   static T Retain(T object) {
     dispatch_retain(object);
     return object;

base/mac/scoped_nsobject.h

@@ -85,12 +85,12 @@ class scoped_nsprotocol
   using Traits = internal::ScopedNSProtocolTraits<NST>;
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
-  explicit scoped_nsprotocol(
+  explicit constexpr scoped_nsprotocol(
       NST object = Traits::InvalidValue(),
       base::scoped_policy::OwnershipPolicy policy = base::scoped_policy::ASSUME)
       : ScopedTypeRef<NST, Traits>(object, policy) {}
 #else
-  explicit scoped_nsprotocol(NST object = Traits::InvalidValue())
+  explicit constexpr scoped_nsprotocol(NST object = Traits::InvalidValue())
       : ScopedTypeRef<NST, Traits>(object, base::scoped_policy::RETAIN) {}
 #endif
 
@@ -149,12 +149,12 @@ class scoped_nsobject : public scoped_nsprotocol<NST*> {
   using Traits = typename scoped_nsprotocol<NST*>::Traits;
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
-  explicit scoped_nsobject(
+  explicit constexpr scoped_nsobject(
       NST* object = Traits::InvalidValue(),
       base::scoped_policy::OwnershipPolicy policy = base::scoped_policy::ASSUME)
       : scoped_nsprotocol<NST*>(object, policy) {}
 #else
-  explicit scoped_nsobject(NST* object = Traits::InvalidValue())
+  explicit constexpr scoped_nsobject(NST* object = Traits::InvalidValue())
       : scoped_nsprotocol<NST*>(object) {}
 #endif
 
@@ -198,12 +198,12 @@ class scoped_nsobject<id> : public scoped_nsprotocol<id> {
   using Traits = typename scoped_nsprotocol<id>::Traits;
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
-  explicit scoped_nsobject(
+  explicit constexpr scoped_nsobject(
       id object = Traits::InvalidValue(),
       base::scoped_policy::OwnershipPolicy policy = base::scoped_policy::ASSUME)
       : scoped_nsprotocol<id>(object, policy) {}
 #else
-  explicit scoped_nsobject(id object = Traits::InvalidValue())
+  explicit constexpr scoped_nsobject(id object = Traits::InvalidValue())
       : scoped_nsprotocol<id>(object) {}
 #endif
 

base/mac/scoped_typeref.h

@@ -53,7 +53,7 @@ class ScopedTypeRef {
  public:
   typedef T element_type;
 
-  explicit ScopedTypeRef(
+  explicit constexpr ScopedTypeRef(
       __unsafe_unretained T object = Traits::InvalidValue(),
       base::scoped_policy::OwnershipPolicy policy = base::scoped_policy::ASSUME)
       : object_(object) {

base/memory/ref_counted.h

@@ -148,8 +148,9 @@ class BASE_EXPORT RefCountedThreadSafeBase {
   bool HasOneRef() const;
 
  protected:
-  explicit RefCountedThreadSafeBase(StartRefCountFromZeroTag) {}
-  explicit RefCountedThreadSafeBase(StartRefCountFromOneTag) : ref_count_(1) {
+  explicit constexpr RefCountedThreadSafeBase(StartRefCountFromZeroTag) {}
+  explicit constexpr RefCountedThreadSafeBase(StartRefCountFromOneTag)
+      : ref_count_(1) {
 #if DCHECK_IS_ON()
     needs_adopt_ref_ = true;
 #endif

base/test/test_discardable_memory_allocator.cc

@@ -53,10 +53,6 @@ class DiscardableMemoryImpl : public DiscardableMemory {
 
 }  // namespace
 
-TestDiscardableMemoryAllocator::TestDiscardableMemoryAllocator() = default;
-
-TestDiscardableMemoryAllocator::~TestDiscardableMemoryAllocator() = default;
-
 std::unique_ptr<DiscardableMemory>
 TestDiscardableMemoryAllocator::AllocateLockedDiscardableMemory(size_t size) {
   return std::make_unique<DiscardableMemoryImpl>(size);

base/test/test_discardable_memory_allocator.h

@@ -17,8 +17,7 @@ namespace base {
 // DiscardableMemory instances backed by heap memory.
 class TestDiscardableMemoryAllocator : public DiscardableMemoryAllocator {
  public:
-  TestDiscardableMemoryAllocator();
-  ~TestDiscardableMemoryAllocator() override;
+  constexpr TestDiscardableMemoryAllocator() = default;
 
   // Overridden from DiscardableMemoryAllocator:
   std::unique_ptr<DiscardableMemory> AllocateLockedDiscardableMemory(

base/threading/platform_thread.h

@@ -55,13 +55,9 @@ class PlatformThreadRef {
 #elif defined(OS_POSIX)
   typedef pthread_t RefType;
 #endif
-  PlatformThreadRef()
-      : id_(0) {
-  }
+  constexpr PlatformThreadRef() : id_(0) {}
 
-  explicit PlatformThreadRef(RefType id)
-      : id_(id) {
-  }
+  explicit constexpr PlatformThreadRef(RefType id) : id_(id) {}
 
   bool operator==(PlatformThreadRef other) const {
     return id_ == other.id_;
@@ -85,9 +81,9 @@ class PlatformThreadHandle {
   typedef pthread_t Handle;
 #endif
 
-  PlatformThreadHandle() : handle_(0) {}
+  constexpr PlatformThreadHandle() : handle_(0) {}
 
-  explicit PlatformThreadHandle(Handle handle) : handle_(handle) {}
+  explicit constexpr PlatformThreadHandle(Handle handle) : handle_(handle) {}
 
   bool is_equal(const PlatformThreadHandle& other) const {
     return handle_ == other.handle_;