[PATCH] D55466: Change `tuple_size` from a `class` to a `struct` (see PR#39871)

[Marshall Clow via Phabricator]

mclow.lists created this revision.
mclow.lists added reviewers: EricWF, ldionne.

In the standard, it's a mix of things.  
libc++ currently consistently implements this as a `class`.
I talked to the editors, and Jonathan believes that this can be fixed editorially.
I'm assuming that it will end up as a struct, and so here's a patch to implement that.

This will affect people who have specialized `std::tuple_size` for their own uses, and compile with -Wmismatched-tags.
It will also affect people who use libc++ with MS compilers, where the ABI is different between `struct` and `class`

I don't recommend landing this until the change goes into the standard - which will probably be a week or so.
But I was in the neighborhood, and I figured I'd write this up.


https://reviews.llvm.org/D55466

Files:
  include/__tuple
  include/array
  include/tuple
  include/utility
  test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.fail.cpp
  test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_incomplete.pass.cpp
  test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_structured_bindings.pass.cpp

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