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There is one observable difference by explicitly casting `nullptr_t` to
`void*`: prvalues of type `nullptr_t` aren't read from (which seemed
unintentional):
static void* f(int x, ...) {
__builtin_va_list args;
__builtin_va_start(args, x);
void* arg = __builtin_va_arg(args, void*);
__builtin_va_end(args);
return arg;
}
int main() {
int x;
void* x_ptr = &x;
void* result = f(0, __builtin_bit_cast(decltype(nullptr), x_ptr));
__builtin_printf("%p\n%p\n%p\n", x_ptr, result, nullptr);
}
https://godbolt.org/z/Pfv8Wbhxj
An object of type `nullptr_t` is passed to the function, but when it is read
with `void* arg = __builtin_va_arg(args, void*);`, it is not a null pointer.
================
Comment at: clang/lib/Sema/SemaExpr.cpp:17328
+ PromoteType = Context.VoidPtrTy;
+ if (TInfo->getType()->isArrayType())
+ PromoteType = Context.getArrayDecayedType(TInfo->getType());
----------------
This warns if you call `va_arg(ap, double[2])`. However this might be valid
since the actual argument only has to be a "compatible type" and I think
`double _Complex` is compatible with `double[2]`. I think we should warn
anyways, since array rvalues are tricky to work with, and the user probably
passed a `double[2]` and should retrieve the `double*`.
================
Comment at: clang/test/Sema/format-strings-pedantic.c:21
+#elif !__is_identifier(nullptr)
+ printf("%p", nullptr); // expected-warning {{format specifies type 'void *'
but the argument has type 'nullptr_t'}}
#endif
----------------
In C2x, nullptr passed as an argument and retrieved via `va_arg(ap, void *)` is
valid (See C2x 7.16.1.1p2) and this is the same as `printf("%p", (void*)
nullptr)`, but this should still be fine as a "pedantic" warning
================
Comment at: clang/www/cxx_dr_status.html:4376
<td>Can <TT>nullptr</TT> be passed to an ellipsis?</td>
- <td class="none" align="center">Unknown</td>
+ <td class="unreleased" align="center">Clang 17</td>
</tr>
----------------
It may be better to mark this as "Yes", since in old clang versions passing
nullptr would work by virtue of it having representation of a null void*
pointer, and it only affected diagnostics
nullptr passed to a variadic function in C++ now converted to void*.
va_arg on array types and half precision floats now diagnosed.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D156054
Files:
clang/docs/ReleaseNotes.rst
clang/lib/Sema/SemaExpr.cpp
clang/test/CXX/drs/dr7xx.cpp
clang/test/Sema/format-strings-pedantic.c
clang/test/SemaCXX/varargs.cpp
clang/www/cxx_dr_status.html
Index: clang/www/cxx_dr_status.html
===================================================================
--- clang/www/cxx_dr_status.html
+++ clang/www/cxx_dr_status.html
@@ -4373,7 +4373,7 @@
<td><a href="https://cplusplus.github.io/CWG/issues/722.html";>722</a></td>
<td>CD2</td>
<td>Can <TT>nullptr</TT> be passed to an ellipsis?</td>
- <td class="none" align="center">Unknown</td>
+ <td class="unreleased" align="center">Clang 17</td>
</tr>
<tr id="726">
<td><a href="https://cplusplus.github.io/CWG/issues/726.html";>726</a></td>
Index: clang/test/SemaCXX/varargs.cpp
===================================================================
--- clang/test/SemaCXX/varargs.cpp
+++ clang/test/SemaCXX/varargs.cpp
@@ -55,6 +55,16 @@
(void)__builtin_va_arg(ap, unsigned int);
(void)__builtin_va_arg(ap, bool); // expected-warning {{second argument to 'va_arg' is of promotable type 'bool'; this va_arg has undefined behavior because arguments will be promoted to 'int'}}
+
+ (void)__builtin_va_arg(ap, float); // expected-warning {{second argument to 'va_arg' is of promotable type 'float'; this va_arg has undefined behavior because arguments will be promoted to 'double'}}
+ (void)__builtin_va_arg(ap, __fp16); // expected-warning {{second argument to 'va_arg' is of promotable type '__fp16'; this va_arg has undefined behavior because arguments will be promoted to 'double'}}
+
+#if __cplusplus >= 201103L
+ (void)__builtin_va_arg(ap, decltype(nullptr)); // expected-warning {{second argument to 'va_arg' is of promotable type 'decltype(nullptr)' (aka 'std::nullptr_t'); this va_arg has undefined behavior because arguments will be promoted to 'void *'}}
+#endif
+
+ (void)__builtin_va_arg(ap, int[3]); // expected-warning {{second argument to 'va_arg' is of promotable type 'int[3]'; this va_arg has undefined behavior because arguments will be promoted to 'int *'}}
+ (void)__builtin_va_arg(ap, const int[3]); // expected-warning {{second argument to 'va_arg' is of promotable type 'const int[3]'; this va_arg has undefined behavior because arguments will be promoted to 'const int *'}}
}
#if __cplusplus >= 201103L
Index: clang/test/Sema/format-strings-pedantic.c
===================================================================
--- clang/test/Sema/format-strings-pedantic.c
+++ clang/test/Sema/format-strings-pedantic.c
@@ -1,6 +1,7 @@
// RUN: %clang_cc1 -fsyntax-only -verify -Wno-format -Wformat-pedantic %s
// RUN: %clang_cc1 -xobjective-c -fblocks -fsyntax-only -verify -Wno-format -Wformat-pedantic %s
// RUN: %clang_cc1 -xc++ -fsyntax-only -verify -Wno-format -Wformat-pedantic %s
+// RUN: %clang_cc1 -std=c2x -fsyntax-only -verify -Wno-format -Wformat-pedantic %s
__attribute__((format(printf, 1, 2)))
int printf(const char *restrict, ...);
@@ -15,6 +16,8 @@
#endif
#ifdef __cplusplus
- printf("%p", nullptr); // expected-warning {{format specifies type 'void *' but the argument has type 'std::nullptr_t'}}
+ printf("%p", nullptr);
+#elif !__is_identifier(nullptr)
+ printf("%p", nullptr); // expected-warning {{format specifies type 'void *' but the argument has type 'nullptr_t'}}
#endif
}
Index: clang/test/CXX/drs/dr7xx.cpp
===================================================================
--- clang/test/CXX/drs/dr7xx.cpp
+++ clang/test/CXX/drs/dr7xx.cpp
@@ -53,6 +53,15 @@
#endif
}
+namespace dr722 { // dr722: 17
+#if __cplusplus >= 201103L
+ int x = __builtin_printf("%p", nullptr);
+ void f(__builtin_va_list args) {
+ __builtin_va_arg(args, decltype(nullptr)); // expected-warning {{second argument to 'va_arg' is of promotable type 'decltype(nullptr)' (aka 'std::nullptr_t'); this va_arg has undefined behavior because arguments will be promoted to 'void *'}}
+ }
+#endif
+}
+
namespace dr727 { // dr727: partial
struct A {
template<typename T> struct C; // expected-note 6{{here}}
Index: clang/lib/Sema/SemaExpr.cpp
===================================================================
--- clang/lib/Sema/SemaExpr.cpp
+++ clang/lib/Sema/SemaExpr.cpp
@@ -936,9 +936,9 @@
// enumeration, pointer, pointer to member, or class type, the program
// is ill-formed.
//
- // Since we've already performed array-to-pointer and function-to-pointer
- // decay, the only such type in C++ is cv void. This also handles
- // initializer lists as variadic arguments.
+ // Since we've already performed null pointer conversion, array-to-pointer
+ // decay and function-to-pointer decay, the only such type in C++ is cv
+ // void. This also handles initializer lists as variadic arguments.
if (Ty->isVoidType())
return VAK_Invalid;
@@ -1059,6 +1059,19 @@
E = ExprRes.get();
+ // C2x 6.5.2.2p6:
+ // The integer promotions are performed on each trailing argument, and
+ // trailing arguments that have type float are promoted to double. These are
+ // called the default argument promotions. No other conversions are
+ // performed implicitly.
+
+ // C++ [expr.call]p7, per DR722:
+ // An argument that has (possibly cv-qualified) type std::nullptr_t is
+ // converted to void* ([conv.ptr]).
+ if (E->getType()->isNullPtrType() && getLangOpts().CPlusPlus) {
+ E = ImpCastExprToType(E, Context.VoidPtrTy, CK_NullToPointer).get();
+ }
+
// Diagnostics regarding non-POD argument types are
// emitted along with format string checking in Sema::CheckFunctionCall().
if (isValidVarArgType(E->getType()) == VAK_Undefined) {
@@ -17307,8 +17320,13 @@
PromoteType = QualType();
}
}
- if (TInfo->getType()->isSpecificBuiltinType(BuiltinType::Float))
+ if (TInfo->getType()->isSpecificBuiltinType(BuiltinType::Float) ||
+ TInfo->getType()->isSpecificBuiltinType(BuiltinType::Half))
PromoteType = Context.DoubleTy;
+ if (TInfo->getType()->isNullPtrType() && getLangOpts().CPlusPlus)
+ PromoteType = Context.VoidPtrTy;
+ if (TInfo->getType()->isArrayType())
+ PromoteType = Context.getArrayDecayedType(TInfo->getType());
if (!PromoteType.isNull())
DiagRuntimeBehavior(TInfo->getTypeLoc().getBeginLoc(), E,
PDiag(diag::warn_second_parameter_to_va_arg_never_compatible)
Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -159,6 +159,8 @@
// -Wdeprecated-literal-operator diagnoses the extra space.
string operator "" _i18n(const char*, std::size_t);
// ^ an extra space
+- Implemented `DR722 <https://wg21.link/CWG722>`_ which promotes ``nullptr`` to ``void*``
+ when passed to a C-style variadic function.
C Language Changes
------------------
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