[PATCH] D93510: Fix llvm::Optional build breaks in MSVC using std::is_trivially_copyable

[James Player via Phabricator via llvm-commits]

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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.

According to the spec <https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable>, 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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D93510

Files:
  llvm/include/llvm/ADT/Optional.h


Index: llvm/include/llvm/ADT/Optional.h
===================================================================
--- llvm/include/llvm/ADT/Optional.h
+++ llvm/include/llvm/ADT/Optional.h
@@ -33,7 +33,9 @@
 struct in_place_t {};
 
 /// Storage for any type.
-template <typename T, bool = is_trivially_copyable<T>::value>
+template <typename T, bool = is_trivially_copyable<T>::value
+                          &&std::is_trivially_copy_constructible<T>::value
+                              &&std::is_trivially_copy_assignable<T>::value>
 class OptionalStorage {
   union {
     char empty;


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