On Thu, Mar 24, 2022 at 05:12:12PM -0400, Jason Merrill wrote:
> On 3/24/22 15:56, Marek Polacek wrote:
> > On Thu, Mar 24, 2022 at 12:02:29PM -0400, Jason Merrill wrote:
> > > On 3/24/22 11:49, Marek Polacek wrote:
> > > > I started looking into this PR because in GCC 4.9 we were able to
> > > > detect the invalid
> > > >
> > > > struct alignas(void) S{};
> > > >
> > > > but I broke it in r210262.
> > > >
> > > > It's ill-formed code in C++:
> > > > [dcl.align]/3: "An alignment-specifier of the form alignas(type-id) has
> > > > the same effect as alignas(alignof(type-id))", and [expr.align]/1:
> > > > "The operand shall be a type-id representing a complete object type,
> > > > or an array thereof, or a reference to one of those types." and void
> > > > is not a complete type.
> > > >
> > > > It's also invalid in C:
> > > > 6.7.5: _Alignas(type-name) is equivalent to
> > > > _Alignas(_Alignof(type-name))
> > > > 6.5.3.4: "The _Alignof operator shall not be applied to a function type
> > > > or an incomplete type."
> > > >
> > > > We have a GNU extension whereby we treat sizeof(void) as 1, but I assume
> > > > it doesn't apply to alignof, so I'd like to reject it in C too.
> > >
> > > That makes sense to me in principle, but we've allowed it since the
> > > beginning of version control, back when c_alignof was a separate function.
> > > Changing that seems questionable for a regression fix.
> >
> > Ok, that makes sense. How about rejecting alignof(void) in C++ only
> > now (where it is a regression), and maybe come back to this in GCC 13 for C?
>
> I'd probably just leave it alone for C and __alignof.
Fair enough.
> > PR c++/104944
> >
> > gcc/c-family/ChangeLog:
> >
> > * c-common.cc (c_sizeof_or_alignof_type): Do not allow alignof(void)
> > in C++.
> >
> > gcc/cp/ChangeLog:
> >
> > * typeck.cc (cxx_alignas_expr): Call cxx_sizeof_or_alignof_type with
> > complain == true.
>
> This hunk is OK. But let's put the diagnostic in
> cxx_sizeof_or_alignof_type, where it can depend on std_alignof.
Like so? With this patch __alignof only produces a pedwarn (there's no
__alignas to worry about).
-- >8 --
I started looking into this PR because in GCC 4.9 we were able to
detect the invalid
struct alignas(void) S{};
but I broke it in r210262.
It's ill-formed code in C++:
[dcl.align]/3: "An alignment-specifier of the form alignas(type-id) has
the same effect as alignas(alignof(type-id))", and [expr.align]/1:
"The operand shall be a type-id representing a complete object type,
or an array thereof, or a reference to one of those types." and void
is not a complete type.
It's also invalid in C:
6.7.5: _Alignas(type-name) is equivalent to _Alignas(_Alignof(type-name))
6.5.3.4: "The _Alignof operator shall not be applied to a function type
or an incomplete type."
We have a GNU extension whereby we treat sizeof(void) as 1, but I assume
it doesn't apply to alignof, at least in C++. However, __alignof__(void)
is still accepted with a -Wpedantic warning.
(We still say "invalid application of '__alignof__'" rather than
'alignas' but I felt that fixing that may not be suitable as part of this
patch.)
PR c++/104944
gcc/cp/ChangeLog:
* typeck.cc (cxx_sizeof_or_alignof_type): Diagnose alignof(void).
(cxx_alignas_expr): Call cxx_sizeof_or_alignof_type with
complain == true.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/alignas20.C: New test.
---
gcc/cp/typeck.cc | 21 +++++++++++++++------
gcc/testsuite/g++.dg/cpp0x/alignas20.C | 26 ++++++++++++++++++++++++++
2 files changed, 41 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/alignas20.C
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 516fa574ef6..26a7cb4b50d 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1873,9 +1873,9 @@ compparms (const_tree parms1, const_tree parms2)
}
-/* Process a sizeof or alignof expression where the operand is a
- type. STD_ALIGNOF indicates whether an alignof has C++11 (minimum alignment)
- or GNU (preferred alignment) semantics; it is ignored if op is
+/* Process a sizeof or alignof expression where the operand is a type.
+ STD_ALIGNOF indicates whether an alignof has C++11 (minimum alignment)
+ or GNU (preferred alignment) semantics; it is ignored if OP is
SIZEOF_EXPR. */
tree
@@ -1899,6 +1899,13 @@ cxx_sizeof_or_alignof_type (location_t loc, tree type,
enum tree_code op,
else
return error_mark_node;
}
+ else if (VOID_TYPE_P (type) && std_alignof)
+ {
+ if (complain)
+ error_at (loc, "invalid application of %qs to a void type",
+ OVL_OP_INFO (false, op)->name);
+ return error_mark_node;
+ }
bool dependent_p = dependent_type_p (type);
if (!dependent_p)
@@ -2132,11 +2139,13 @@ cxx_alignas_expr (tree e)
/* [dcl.align]/3:
When the alignment-specifier is of the form
- alignas(type-id ), it shall have the same effect as
- alignas(alignof(type-id )). */
+ alignas(type-id), it shall have the same effect as
+ alignas(alignof(type-id)). */
return cxx_sizeof_or_alignof_type (input_location,
- e, ALIGNOF_EXPR, true, false);
+ e, ALIGNOF_EXPR,
+ /*std_alignof=*/true,
+ /*complain=*/true);
/* If we reach this point, it means the alignas expression if of
the form "alignas(assignment-expression)", so we should follow
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas20.C
b/gcc/testsuite/g++.dg/cpp0x/alignas20.C
new file mode 100644
index 00000000000..01a55f3d4a4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas20.C
@@ -0,0 +1,26 @@
+// PR c++/104944
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wpedantic" }
+
+struct inc;
+
+struct alignas(inc) S1 { }; // { dg-error "invalid application" }
+struct alignas(void) S2 { }; // { dg-error "invalid application" }
+
+template <typename T>
+struct alignas(T) S4 {}; // { dg-error "invalid application" }
+
+template <typename T>
+struct alignas(T) S5 {}; // { dg-error "invalid application" }
+
+S4<void> s1;
+S5<inc> s2;
+
+void
+g ()
+{
+ auto s1 = alignof(void); // { dg-error "invalid application" }
+ auto s2 = alignof(const void); // { dg-error "invalid application" }
+ auto s3 = __alignof(void); // { dg-warning "invalid application" }
+ auto s4 = alignof(inc); // { dg-error "invalid application" }
+}
base-commit: 346ab5a54a831ad9c78afcbd8dfe98e0e07e3070
--
2.35.1
