[PATCH] D41977: [libc++] Fix PR20855 -- libc++ incorrectly diagnoses illegal reference binding in std::tuple.

[Eric Fiselier via Phabricator via cfe-commits]

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

See https://bugs.llvm.org/show_bug.cgi?id=20855

Libc++ goes out of it's way to diagnose `std::tuple` constructions which are UB due to lifetime bugs caused by reference creation. For example:

  // The 'const std::string&' is created *inside* the tuple constructor, and its lifetime is over before the end of the constructor call.
  std::tuple<int, const std::string&> t(std::make_tuple(42, "abc"));

However, we are over-aggressive and we incorrectly diagnose cases such as:

  void foo(std::tuple<int const&, int const&> const&);
  foo(std::make_tuple(42, 42));

This patch fixes the incorrectly diagnosed cases, as well as converting the diagnostic to use the newly added Clang trait `__reference_binds_to_temporary`. The new trait allows us to diagnose cases we previously couldn't such as:

  std::tuple<int, const std::string&> t(42, "abc");


https://reviews.llvm.org/D41977

Files:
  include/tuple
  test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.fail.cpp
  test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp

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