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c++: alignas and alignof void [PR104944]

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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' in the
void diagnostic, but I felt that fixing that may not be suitable as part of
this patch.  The "incomplete type" diagnostic still always prints
'__alignof__'.

	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.
This commit is contained in:
Marek Polacek 2022-03-23 17:12:29 -04:00
parent 8570cce7c7
commit d0b938a761
2 changed files with 41 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

21
gcc/cp/typeck.cc
View file

@ -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

26
gcc/testsuite/g++.dg/cpp0x/alignas20.C Normal file
View file

@ -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" }
}