[PATCH] D156054: [Clang][Sema] DR722 (nullptr and varargs) and missing -Wvarargs diagnostics

[Mital Ashok via Phabricator via cfe-commits]

MitalAshok 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'}}
 
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aaron.ballman wrote:
> 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?
This seems to have changed very recently between [[ https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf | N3096 ]] (April C23 draft) and [[ https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3149.zip | N3149 ]] (July C23 draft) by [[ https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3112.pdf | N3112 ]].

For reference, the old wording (also in C17) read:

> one type is a signed integer type, the other type is the corresponding unsigned integer type,
> and the value is representable in both types;

So with the current rules, yes they would be compatible and this shouldn't warn. I've changed it so it checks types with corresponding signedness.


Repository:
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  https://reviews.llvm.org/D156054/new/

https://reviews.llvm.org/D156054