[PATCH] D29181: Make TrailingObjects private inheritance of TrailingObjectsImpl protected

[Hugh Bellamy via Phabricator via llvm-commits]

hughbe created this revision.

There is an MSVC bug, that caused build errors porting the Swift compiler to Windows, involving trailing objects.

Example code is

  class Identifier {};
  class SourceLoc {};
  
  template<typename Derived>
  class TrailingCallArguments
    : private llvm::TrailingObjects<Derived, Identifier, SourceLoc> {
    using TrailingObjects = typename llvm::TrailingObjects<Derived, Identifier, SourceLoc>;
    friend TrailingObjects;
  
    size_t numTrailingObjects(
      typename TrailingObjects::template OverloadToken<Identifier>) const {
        return 1;
    }
  
    size_t numTrailingObjects(
      typename TrailingObjects::template OverloadToken<SourceLoc>) const {
      return 2;
    }
  };



> source_file.cpp(180): error C2751: 'llvm::TrailingObjects<BaseTy,TrailingTys...>::operator OverloadToken': the name of a function parameter cannot be qualified
>  source_file.cpp(190): note: see reference to class template instantiation 'TrailingCallArguments<Derived>' being compiled
>  source_file.cpp(180): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
>  source_file.cpp(185): error C2751: 'llvm::TrailingObjects<BaseTy,TrailingTys...>::operator OverloadToken': the name of a function parameter cannot be qualified
>  source_file.cpp(185): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
>  source_file.cpp(185): error C2535: 'size_t TrailingCallArguments<Derived>::numTrailingObjects(int) const': member function already defined or declared
>  source_file.cpp(179): note: see declaration of 'TrailingCallArguments<Derived>::numTrailingObjects'

There is a connect bug here: https://connect.microsoft.com/VisualStudio/feedback/details/3116517

The fix for this MSVC bug (that will likely be unresolved by VS 2017 is to directly refer to

  llvm::trailing_objects_internal::TrailingObjectsBase::OverloadToken<Identifier>

However, this requires us to give `protected` acess to `TrailingObjectsBase`.

I'm creating this revision, because Jordan Rose (Swift engineer at Apple) suggested I discuss this upstream.
You can see discussion with various members of the community here: https://github.com/apple/swift-llvm/pull/33
You can see the suggestion to bring discussion upstream here: https://github.com/apple/swift-llvm/pull/43
You can see the fix here: https://github.com/apple/swift/pull/5948, https://github.com/apple/swift/pull/5948/files#diff-323bdb4c6d842e3b3cd3b40e628c9771R669

Let me know what you think - it's brittle, but necessary to use TrailingObjects effectively in a generic context with MSVC.


https://reviews.llvm.org/D29181

Files:
  include/llvm/Support/TrailingObjects.h


Index: include/llvm/Support/TrailingObjects.h
===================================================================
--- include/llvm/Support/TrailingObjects.h
+++ include/llvm/Support/TrailingObjects.h
@@ -231,12 +231,12 @@
 /// See the file comment for details on the usage of the
 /// TrailingObjects type.
 template <typename BaseTy, typename... TrailingTys>
-class TrailingObjects : private trailing_objects_internal::TrailingObjectsImpl<
-                            trailing_objects_internal::AlignmentCalcHelper<
-                                TrailingTys...>::Alignment,
-                            BaseTy, TrailingObjects<BaseTy, TrailingTys...>,
-                            BaseTy, TrailingTys...> {
-
+class TrailingObjects
+    : protected trailing_objects_internal::TrailingObjectsImpl<
+          trailing_objects_internal::AlignmentCalcHelper<
+              TrailingTys...>::Alignment,
+          BaseTy, TrailingObjects<BaseTy, TrailingTys...>, BaseTy,
+          TrailingTys...> {
   template <int A, typename B, typename T, typename P, typename... M>
   friend class trailing_objects_internal::TrailingObjectsImpl;
 


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