[PATCH] D29930: Add `__is_direct_constructible` trait for checking safe reference initialization.

[Eric Fiselier via Phabricator via cfe-commits]

EricWF created this revision.

The STL types `std::pair` and `std::tuple` can both store reference types. However their constructors cannot adequately check if the initialization of reference types is safe.  For example:

  std::tuple<std::tuple<int> const&> t = 42;
  // The stored reference is already dangling.

This patch attempts to implement a traits that acts the same as `__is_constructible(type, argtypes...)` except when `type` is a reference. In which case the trait disallows initialization from a materialized temporary.  For example:

  static_assert(__is_constructible(int const&, long));
  static_assert(!__is_direct_constructible(int const&, long));

I don't think `__is_direct_constructible` is a good name, but I don't have any better ideas. All bikeshedding welcome.


https://reviews.llvm.org/D29930

Files:
  docs/LanguageExtensions.rst
  include/clang/Basic/TokenKinds.def
  include/clang/Basic/TypeTraits.h
  lib/Parse/ParseDeclCXX.cpp
  lib/Parse/ParseExpr.cpp
  lib/Sema/SemaExprCXX.cpp
  test/SemaCXX/type-traits.cpp

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