aaron.ballman added inline comments.
================
Comment at: clang/test/SemaCXX/varargs.cpp:34
enum Unscoped1 { One = 0x7FFFFFFF };
- (void)__builtin_va_arg(ap, Unscoped1); // ok
+ (void)__builtin_va_arg(ap, Unscoped1); // expected-warning {{second argument
to 'va_arg' is of promotable type 'Unscoped1'; this va_arg has undefined
behavior because arguments will be promoted to 'int'}}
----------------
MitalAshok wrote:
> MitalAshok wrote:
> > Unscoped1 is promoted to int when passed to a variadic function.
> >
> > The underlying type for Unscoped1 is unsigned int, so only Unscoped1 and
> > unsigned int are compatible, not Unscoped1 and int. An Unscoped1 passed to
> > a variadic function must be retrieved via va_arg(ap, int).
> >
> Although I guess the warning is now wrong because even though `void f(int x,
> ...) { std::va_list ap; va_start(ap, x); va_arg(ap, Unscoped1); }` `f(0,
> Unscoped1{2})` would be UB, `f(0, 2u)` would not be UB.
>
> The user still should be warned about it, so I could create a new warning
> "second argument to 'va_arg' is of promotable enumeration type 'Unscoped1';
> this va_arg may have undefined behavior because arguments of this enumeration
> type will be promoted to 'int', not the underlying type 'unsigned int'", and
> maybe suggest a fix `Unscoped1{va_arg(ap, unsigned)}`.
>
> Or we could ignore it and pretend that int and enums with underlying types
> unsigned are compatible for the purposes of va_arg
I think we shouldn't warn in this case because of C23 7.16.1.1p2:
> If type is not compatible with the type of the actual next argument (as
> promoted according to
> the default argument promotions), the behavior is undefined, except for the
> following cases:
> ...
> one type is compatible with a signed integer type, the other type is
> compatible with the
> corresponding unsigned integer type, and the value is representable in both
> types;
Coupled with C23 6.7.2.2p13:
> For all enumerations without a fixed underlying type, each enumerated type
> shall be compatible
> with char or a signed or an unsigned integer type that is not bool or a
> bit-precise integer type. The
> choice of type is implementation-defined., but shall be capable of
> representing the values of all
> the members of the enumeration.
WDYT?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D156054/new/
https://reviews.llvm.org/D156054
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