[cfe-dev] Handling direct-list-initialization of enums in C++1z mode

[Vedant Kumar via cfe-dev]

Hi,

After r264564, it became possible to perform direct-list-initialization of an
enum in C++1z mode, if the initializer and the enum share the same underlying
type. However, clang crashes on some code which relies on this feature:

  // clang++ -std=c++1z
  enum class A : unsigned {};
  A foo(unsigned x) { return A{x}; }

(We would reject this code in C++11/14 mode.)

The crash occurs in Sema::ImpCastExprToType, while we attempt to perform an
implicit conversion from 'unsigned int' ('x') to 'A'. The issue is that this is
an integral cast in an lvalue-to-rvalue conversion, which Sema asserts is not
possible. I'm not sure why exactly this kind of implicit cast is not allowed.

I'd like to send in a patch to fix this issue, but I'm not confident in my
simplistic fix which simply weakens the assertion in Sema. I'd like to know: is
the assertion really correct (lib/Sema/Sema.cpp:405)? If so, then I'd probably
need to find some other way to perform this conversion. If not, then I'd feel
more confident in my patch.

I've attached the WIP patch for reference.

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thanks,
vedant