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

[Vedant Kumar via cfe-dev]

> On Feb 7, 2017, at 5:58 PM, Friedman, Eli <efriedma at codeaurora.org> wrote:
> 
> 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.

That makes sense, seeing as 'x' in my example is not an rvalue!

I will try this and send in a patch.

thank you,
vedant

> 
> -Eli
> 
> -- 
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project