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

[Friedman, Eli via cfe-dev]

On 2/7/2017 5:31 PM, Vedant Kumar via cfe-dev wrote:
> 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)?
Yes, the assertion is correct
> If so, then I'd probably
> need to find some other way to perform this conversion.

The right way to perform the conversion is a series of two implicit 
casts: one of type CK_LValueToRValue, then another of type CK_IntegralCast.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
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