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[C2y] Claim conformance to WG14 N3363 (#130980)

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This paper clarifies two things:
  * a call to va_start must be within the body of a variadic function,
  * the va_list arguments must be lvalues

Clang has correctly diagnosed the first point since Clang 5.0. Clang
generally diagnoses the second point, except for va_copy. However, the
second point is not a constraint violation, so a diagnostic is not
strictly required. That said, it would be good for us to properly
diagnose va_copy.
This commit is contained in:
Aaron Ballman
2025-03-12 14:38:54 -04:00
committed by GitHub
parent 7f415e4441
commit ab53e1c8e5
2 changed files with 41 additions and 1 deletions
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40
clang/test/C/C2y/n3363.c Normal file
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@@ -0,0 +1,40 @@
// RUN: %clang_cc1 -triple x86_64-pc-linux -verify=expected,array-type -std=c2y -Wall -pedantic -ffreestanding %s
// RUN: %clang_cc1 -triple aarch64-pc-win32 -verify=expected,char-ptr-type -std=c2y -Wall -pedantic -ffreestanding %s
/* WG14 N3363: Clang 5
* stdarg.h wording... v3
*
* Clarifies that va_start can only be used within a function body, and that
* the macros and functions require an lvalue of type va_list instead of an
* rvalue.
*
* Clang has diagnosed this correctly in all cases except va_copy since
* Clang 5. va_copy still needs to be updated to issue a diagnostic about use
* of an rvalue. However, our lack of a diagnostic is still conforming because
* a diagnostic is not required (it's not a constraint violation).
*/
#include <stdarg.h>
// The va_list argument given to every macro defined in this subclause shall be
// an lvalue of this type or the result of array-to-pointer decay of such an
// lvalue.
void f(int a, ...) {
va_list rvalue(); // array-type-error {{function cannot return array type 'va_list' (aka '__builtin_va_list')}}
va_start(rvalue()); // / char-ptr-type-error {{non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')}}
va_arg(rvalue(), int); // char-ptr-type-error {{expression is not assignable}}
// FIXME: this should get two diagnostics about use of a non-lvalue.
va_copy(rvalue(), rvalue()); // char-ptr-type-error {{non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')}}
va_end(rvalue()); // char-ptr-type-error {{non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')}}
}
// The va_start macro may only be invoked in the compound-statement of the body
// of a variadic function.
void g(va_list ap, int [(va_start(ap), 1)], ...) { // expected-error {{'va_start' cannot be used outside a function}}
va_end(ap);
}
va_list ap;
struct S {
int array[(va_start(ap), 1)]; // expected-error {{'va_start' cannot be used outside a function}}
};

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clang/www/c_status.html
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@@ -263,7 +263,7 @@ conformance.</p>
<tr>
<td>stdarg.h wording... v3</td>
<td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3363.pdf">N3363</a></td>
<td class="unknown" align="center">Unknown</td>
<td class="full" align="center">Clang 5</td>
</tr>
<tr>
<td>Preprocessor integer expressions, v. 2</td>