[NFC] Make Optional<T> trivially copyable when T is trivially copyable

This is a follow-up to r354246 and a reimplementation of https://reviews.llvm.org/D57097?id=186600
that should not trigger any UB thanks to the use of an union.

This may still be subject to the problem solved by std::launder, but I'm unsure how it interacts whith union.
/me plans to revert if this triggers any relevant bot failure. At least this validates in Release mode with
clang 6.0.1 and gcc 4.8.5.

llvm-svn: 354264

llvm/include/llvm/ADT/Optional.h

@@ -139,6 +139,78 @@ public:
   }
 };
 
+template <typename T> class OptionalStorage<T, true> {
+  union {
+    char empty;
+    T value;
+  };
+  bool hasVal = false;
+
+public:
+  ~OptionalStorage() = default;
+
+  OptionalStorage() noexcept : empty{} {}
+
+  OptionalStorage(OptionalStorage const &other) = default;
+  OptionalStorage(OptionalStorage &&other) = default;
+
+  OptionalStorage &operator=(OptionalStorage const &other) = default;
+  OptionalStorage &operator=(OptionalStorage &&other) = default;
+
+  template <class... Args>
+  explicit OptionalStorage(in_place_t, Args &&... args)
+      : value(std::forward<Args>(args)...), hasVal(true) {}
+
+  void reset() noexcept {
+    if (hasVal) {
+      value.~T();
+      hasVal = false;
+    }
+  }
+
+  bool hasValue() const noexcept { return hasVal; }
+
+  T &getValue() LLVM_LVALUE_FUNCTION noexcept {
+    assert(hasVal);
+    return value;
+  }
+  T const &getValue() const LLVM_LVALUE_FUNCTION noexcept {
+    assert(hasVal);
+    return value;
+  }
+#if LLVM_HAS_RVALUE_REFERENCE_THIS
+  T &&getValue() && noexcept {
+    assert(hasVal);
+    return std::move(value);
+  }
+#endif
+
+  template <class... Args> void emplace(Args &&... args) {
+    reset();
+    ::new ((void *)std::addressof(value)) T(std::forward<Args>(args)...);
+    hasVal = true;
+  }
+
+  OptionalStorage &operator=(T const &y) {
+    if (hasValue()) {
+      value = y;
+    } else {
+      ::new ((void *)std::addressof(value)) T(y);
+      hasVal = true;
+    }
+    return *this;
+  }
+  OptionalStorage &operator=(T &&y) {
+    if (hasValue()) {
+      value = std::move(y);
+    } else {
+      ::new ((void *)std::addressof(value)) T(std::move(y));
+      hasVal = true;
+    }
+    return *this;
+  }
+};
+
 } // namespace optional_detail
 
 template <typename T> class Optional {

llvm/unittests/ADT/OptionalTest.cpp

@@ -14,8 +14,15 @@
 
 #include <array>
 
+
 using namespace llvm;
 
+static_assert(is_trivially_copyable<Optional<int>>::value,
+          "trivially copyable");
+
+static_assert(is_trivially_copyable<Optional<std::array<int, 3>>>::value,
+              "trivially copyable");
+
 namespace {
 
 struct NonDefaultConstructible {
@@ -45,6 +52,10 @@ unsigned NonDefaultConstructible::CopyConstructions = 0;
 unsigned NonDefaultConstructible::Destructions = 0;
 unsigned NonDefaultConstructible::CopyAssignments = 0;
 
+static_assert(
+      !is_trivially_copyable<Optional<NonDefaultConstructible>>::value,
+      "not trivially copyable");
+
 // Test fixture
 class OptionalTest : public testing::Test {
 };
@@ -203,6 +214,10 @@ struct MultiArgConstructor {
 };
 unsigned MultiArgConstructor::Destructions = 0;
 
+static_assert(
+  !is_trivially_copyable<Optional<MultiArgConstructor>>::value,
+  "not trivially copyable");
+
 TEST_F(OptionalTest, Emplace) {
   MultiArgConstructor::ResetCounts();
   Optional<MultiArgConstructor> A;
@@ -250,6 +265,9 @@ unsigned MoveOnly::MoveConstructions = 0;
 unsigned MoveOnly::Destructions = 0;
 unsigned MoveOnly::MoveAssignments = 0;
 
+static_assert(!is_trivially_copyable<Optional<MoveOnly>>::value,
+              "not trivially copyable");
+
 TEST_F(OptionalTest, MoveOnlyNull) {
   MoveOnly::ResetCounts();
   Optional<MoveOnly> O;
@@ -351,6 +369,9 @@ private:
 unsigned Immovable::Constructions = 0;
 unsigned Immovable::Destructions = 0;
 
+static_assert(!is_trivially_copyable<Optional<Immovable>>::value,
+              "not trivially copyable");
+
 TEST_F(OptionalTest, ImmovableEmplace) {
   Optional<Immovable> A;
   Immovable::ResetCounts();