Fix llvm::Optional build breaks in MSVC using std::is_trivially_copyable

Current code breaks this version of MSVC due to a mismatch between `std::is_trivially_copyable` and `llvm::is_trivially_copyable` for `std::pair` instantiations.  Hence I was attempting to use `std::is_trivially_copyable` to set `llvm::is_trivially_copyable<T>::value`.

I spent some time root causing an `llvm::Optional` build error on MSVC 16.8.3 related to the change described above:

```
62>C:\src\ocg_llvm\llvm-project\llvm\include\llvm/ADT/BreadthFirstIterator.h(96,12): error C2280: 'llvm::Optional<std::pair<std::pair<unsigned int,llvm::Graph<4>::NodeSubset> *,llvm::Optional<llvm::Graph<4>::ChildIterator>>> &llvm::Optional<std::pair<std::pair<unsigned int,llvm::Graph<4>::NodeSubset> *,llvm::Optional<llvm::Graph<4>::ChildIterator>>>::operator =(const llvm::Optional<std::pair<std::pair<unsigned int,llvm::Graph<4>::NodeSubset> *,llvm::Optional<llvm::Graph<4>::ChildIterator>>> &)': attempting to reference a deleted function (compiling source file C:\src\ocg_llvm\llvm-project\llvm\unittests\ADT\BreadthFirstIteratorTest.cpp)
...
```
The "trivial" specialization of `optional_detail::OptionalStorage` assumes that the value type is trivially copy constructible and trivially copy assignable. The specialization is invoked based on a check of `is_trivially_copyable` alone, which does not imply both `is_trivially_copy_assignable` and `is_trivially_copy_constructible` are true.

[[ https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable | According to the spec ]], a deleted assignment operator does not make `is_trivially_copyable` false. So I think all these properties need to be checked explicitly in order to specialize `OptionalStorage` to the "trivial" version:
```
/// Storage for any type.
template <typename T, bool = std::is_trivially_copy_constructible<T>::value
                          && std::is_trivially_copy_assignable<T>::value>
class OptionalStorage {
```
Above fixed my build break in MSVC, but I think we need to explicitly check `is_trivially_copy_constructible` too since it might be possible the copy constructor is deleted.  Also would be ideal to move over to `std::is_trivially_copyable` instead of the `llvm` namespace verson.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D93510

1 files changed, 138 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/OptionalTest.cpp b/llvm/unittests/ADT/OptionalTest.cpp
index c7fa796..d40e212 100644
--- a/llvm/unittests/ADT/OptionalTest.cpp
+++ b/llvm/unittests/ADT/OptionalTest.cpp
@@ -8,6 +8,7 @@
 
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringMap.h"
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest-spi.h"
 #include "gtest/gtest.h"
@@ -390,6 +391,143 @@ TEST(OptionalTest, ImmovableEmplace) {
   EXPECT_EQ(0u, Immovable::Destructions);
 }
 
+// Craft a class which is_trivially_copyable, but not
+// is_trivially_copy_constructible.
+struct NonTCopy {
+  NonTCopy() = default;
+
+  // Delete the volatile copy constructor to engage the "rule of 3" and delete
+  // any unspecified copy assignment or constructor.
+  NonTCopy(volatile NonTCopy const &) = delete;
+
+  // Leave the non-volatile default copy constructor unspecified (deleted by
+  // rule of 3)
+
+  // This template can serve as the copy constructor, but isn't chosen
+  // by =default in a class with a 'NonTCopy' member.
+  template <typename Self = NonTCopy>
+  NonTCopy(Self const &Other) : Val(Other.Val) {}
+
+  NonTCopy &operator=(NonTCopy const &) = default;
+
+  int Val{0};
+};
+
+#if defined(_MSC_VER) && _MSC_VER >= 1927 && !defined(__clang__)
+// Currently only true on recent MSVC releases.
+static_assert(std::is_trivially_copyable<NonTCopy>::value,
+              "Expect NonTCopy to be trivially copyable");
+
+static_assert(!std::is_trivially_copy_constructible<NonTCopy>::value,
+              "Expect NonTCopy not to be trivially copy constructible.");
+#endif // defined(_MSC_VER) && _MSC_VER >= 1927
+
+TEST(OptionalTest, DeletedCopyConstructor) {
+
+  // Expect compile to fail if 'trivial' version of
+  // optional_detail::OptionalStorage is chosen.
+  using NonTCopyOptT = Optional<NonTCopy>;
+  NonTCopyOptT NonTCopy1;
+
+  // Check that the Optional can be copy constructed.
+  NonTCopyOptT NonTCopy2{NonTCopy1};
+
+  // Check that the Optional can be copy assigned.
+  NonTCopy1 = NonTCopy2;
+}
+
+// Craft a class which is_trivially_copyable, but not
+// is_trivially_copy_assignable.
+class NonTAssign {
+public:
+  NonTAssign() = default;
+  NonTAssign(NonTAssign const &) = default;
+
+  // Delete the volatile copy assignment to engage the "rule of 3" and delete
+  // any unspecified copy assignment or constructor.
+  NonTAssign &operator=(volatile NonTAssign const &) = delete;
+
+  // Leave the non-volatile default copy assignment unspecified (deleted by rule
+  // of 3).
+
+  // This template can serve as the copy assignment, but isn't chosen
+  // by =default in a class with a 'NonTAssign' member.
+  template <typename Self = NonTAssign>
+  NonTAssign &operator=(Self const &Other) {
+    A = Other.A;
+    return *this;
+  }
+
+  int A{0};
+};
+
+#if defined(_MSC_VER) && _MSC_VER >= 1927 && !defined(__clang__)
+// Currently only true on recent MSVC releases.
+static_assert(std::is_trivially_copyable<NonTAssign>::value,
+              "Expect NonTAssign to be trivially copyable");
+
+static_assert(!std::is_trivially_copy_assignable<NonTAssign>::value,
+              "Expect NonTAssign not to be trivially assignable.");
+#endif // defined(_MSC_VER) && _MSC_VER >= 1927
+
+TEST(OptionalTest, DeletedCopyAssignment) {
+
+  // Expect compile to fail if 'trivial' version of
+  // optional_detail::OptionalStorage is chosen.
+  using NonTAssignOptT = Optional<NonTAssign>;
+  NonTAssignOptT NonTAssign1;
+
+  // Check that the Optional can be copy constructed.
+  NonTAssignOptT NonTAssign2{NonTAssign1};
+
+  // Check that the Optional can be copy assigned.
+  NonTAssign1 = NonTAssign2;
+}
+
+struct NoTMove {
+  NoTMove() = default;
+  NoTMove(NoTMove const &) = default;
+  NoTMove &operator=(NoTMove const &) = default;
+
+  // Delete move constructor / assignment.  Compiler should fall-back to the
+  // trivial copy constructor / assignment in the trivial OptionalStorage
+  // specialization.
+  NoTMove(NoTMove &&) = delete;
+  NoTMove &operator=(NoTMove &&) = delete;
+
+  int Val{0};
+};
+
+TEST(OptionalTest, DeletedMoveConstructor) {
+  using NoTMoveOptT = Optional<NoTMove>;
+
+  NoTMoveOptT NonTMove1;
+  NoTMoveOptT NonTMove2{std::move(NonTMove1)};
+
+  NonTMove1 = std::move(NonTMove2);
+
+  static_assert(
+      std::is_trivially_copyable<NoTMoveOptT>::value,
+      "Expect Optional<NoTMove> to still use the trivial specialization "
+      "of OptionalStorage despite the deleted move constructor / assignment.");
+}
+
+class NoCopyStringMap {
+public:
+  NoCopyStringMap() = default;
+
+private:
+  llvm::StringMap<std::unique_ptr<int>> Map;
+};
+
+TEST(OptionalTest, DeletedCopyStringMap) {
+  // Old versions of gcc (7.3 and prior) instantiate the copy constructor when
+  // std::is_trivially_copyable is instantiated.  This test will fail
+  // compilation if std::is_trivially_copyable is used in the OptionalStorage
+  // specialization condition by gcc <= 7.3.
+  Optional<NoCopyStringMap> TestInstantiation;
+}
+
 #if LLVM_HAS_RVALUE_REFERENCE_THIS
 
 TEST(OptionalTest, MoveGetValueOr) {