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c++: Reject pack expansion of assume attribute [PR109756]

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http://eel.is/c++draft/dcl.attr#grammar-4 says
"In an attribute-list, an ellipsis may appear only if that attribute's
specification permits it."
and doesn't explicitly permit it on any standard attribute.
The https://wg21.link/p1774r8 paper which introduced assume attribute says
"We could therefore hypothetically permit the assume attribute to directly
support pack expansion:
template <int... args>
void f() {
[[assume(args >= 0)...]];
}
However, we do not propose this. It would require substantial additional work
for a very rare use case. Note that this can instead be expressed with a fold
expression, which is equivalent to the above and works out of the box without
any extra effort:
template <int... args>
void f() {
[[assume(((args >= 0) && ...))]];
}
", but as the testcase shows, GCC 13+ ICEs on assume attribute followed by
... if it contains packs.
The following patch rejects those instead of ICE and for C++17 or later
suggests using fold expressions instead (it doesn't make sense to suggest
it for C++14 and earlier when we'd error on the fold expressions).

2023-05-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109756
	* cp-gimplify.cc (process_stmt_assume_attribute): Diagnose pack
	expansion of assume attribute.

	* g++.dg/cpp23/attr-assume11.C: New test.

(cherry picked from commit 2499540e9abb55079b5f7b7ccdac97fbc63d9ab4)
This commit is contained in:
Jakub Jelinek 2023-05-09 16:05:22 +02:00
parent 5fd2537b36
commit 7bd9a34e9f
2 changed files with 32 additions and 0 deletions
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10
gcc/cp/cp-gimplify.cc
View File

@ -3267,6 +3267,16 @@ process_stmt_assume_attribute (tree std_attrs, tree statement,
for (; attr; attr = lookup_attribute ("gnu", "assume", TREE_CHAIN (attr)))
{
tree args = TREE_VALUE (attr);
if (args && PACK_EXPANSION_P (args))
{
auto_diagnostic_group d;
error_at (attrs_loc, "pack expansion of %qE attribute",
get_attribute_name (attr));
if (cxx_dialect >= cxx17)
inform (attrs_loc, "use fold expression in the attribute "
"argument instead");
continue;
}
int nargs = list_length (args);
if (nargs != 1)
{

22
gcc/testsuite/g++.dg/cpp23/attr-assume11.C Normal file
View File

@ -0,0 +1,22 @@
// PR c++/109756
// { dg-do compile { target c++11 } }
template <int ...args>
void
foo ()
{
[[assume (1 > 0)...]]; // { dg-error "expansion pattern '\\\(1 > 0\\\)' contains no parameter packs" }
// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
[[assume (args > 0)...]]; // { dg-error "pack expansion of 'assume' attribute" }
// { dg-message "use fold expression in the attribute argument instead" "" { target c++17 } .-1 }
#if __cpp_fold_expressions >= 201603L
[[assume (((args > 0) && ...))]];
#endif
[[gnu::assume (1 > 0)...]]; // { dg-error "expansion pattern '\\\(1 > 0\\\)' contains no parameter packs" }
// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
[[gnu::assume (args > 0)...]]; // { dg-error "pack expansion of 'assume' attribute" }
// { dg-message "use fold expression in the attribute argument instead" "" { target c++17 } .-1 }
#if __cpp_fold_expressions >= 201603L
[[gnu::assume (((args > 0) && ...))]];
#endif
}