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* [Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter
@ 2021-09-20 16:45 joeloser93 at gmail dot com
2021-09-20 18:36 ` [Bug c++/102412] " ppalka at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: joeloser93 at gmail dot com @ 2021-09-20 16:45 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412
Bug ID: 102412
Summary: Template argument deduction fails when using concept
as defaulted non-type template parameter
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: joeloser93 at gmail dot com
Target Milestone: ---
When using a defaulted non-type template parameter whose value is from a
concept, GCC is unable to deduce the template correctly when calling the
function template. Clang accepts this code and it should be valid.
```
#include <concepts>
template<class T, class U>
concept different_from = not std::is_same_v<T, U>;
template<class T, auto B = different_from<T, int>>
void f() {}
template<class T, auto B = not std::is_same_v<T, int>>
void g() {}
int main() {
f<double>(); // fails to compile
g<double>(); // compiles
}
```
See it live at https://godbolt.org/z/n1zrzGPoq
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter
2021-09-20 16:45 [Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter joeloser93 at gmail dot com
@ 2021-09-20 18:36 ` ppalka at gcc dot gnu.org
2021-09-22 15:18 ` cvs-commit at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: ppalka at gcc dot gnu.org @ 2021-09-20 18:36 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ppalka at gcc dot gnu.org
Status|UNCONFIRMED |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |ppalka at gcc dot gnu.org
Target Milestone|--- |11.3
Last reconfirmed| |2021-09-20
Ever confirmed|0 |1
--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter
2021-09-20 16:45 [Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter joeloser93 at gmail dot com
2021-09-20 18:36 ` [Bug c++/102412] " ppalka at gcc dot gnu.org
@ 2021-09-22 15:18 ` cvs-commit at gcc dot gnu.org
2021-10-06 14:15 ` cvs-commit at gcc dot gnu.org
2021-10-06 14:19 ` ppalka at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-09-22 15:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412
--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:
https://gcc.gnu.org/g:9329344a6d81a6a5e3bd171167ebc7b158bb44f4
commit r12-3822-g9329344a6d81a6a5e3bd171167ebc7b158bb44f4
Author: Patrick Palka <ppalka@redhat.com>
Date: Wed Sep 22 11:16:53 2021 -0400
c++: concept-ids and value-dependence [PR102412]
The problem here is that uses_template_parms returns true for all
concept-ids (even those with non-dependent arguments), so when a concept-id
is used as a default template argument then during deduction the default
argument is considered dependent even after substituting into it, which
leads to deduction failure (from type_unification_real).
This patch fixes this by implementing the resolution of CWG 2446 which
says a concept-id is dependent only if its arguments are.
DR 2446
PR c++/102412
gcc/cp/ChangeLog:
* constexpr.c (cxx_eval_constant_expression)
<case TEMPLATE_ID_EXPR>: Check value_dependent_expression_p
instead of processing_template_decl.
* pt.c (value_dependent_expression_p) <case TEMPLATE_ID_EXPR>:
Return true only if any_dependent_template_arguments_p.
(instantiation_dependent_r) <case CALL_EXPR>: Remove this case.
<case TEMPLATE_ID_EXPR>: Likewise.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-nondep2.C: New test.
* g++.dg/cpp2a/concepts-nondep3.C: New test.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter
2021-09-20 16:45 [Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter joeloser93 at gmail dot com
2021-09-20 18:36 ` [Bug c++/102412] " ppalka at gcc dot gnu.org
2021-09-22 15:18 ` cvs-commit at gcc dot gnu.org
@ 2021-10-06 14:15 ` cvs-commit at gcc dot gnu.org
2021-10-06 14:19 ` ppalka at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-10-06 14:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:
https://gcc.gnu.org/g:5a411459b73b65d8a1235a16d6e2aac4eed17338
commit r11-9082-g5a411459b73b65d8a1235a16d6e2aac4eed17338
Author: Patrick Palka <ppalka@redhat.com>
Date: Wed Sep 22 11:16:53 2021 -0400
c++: concept-ids and value-dependence [PR102412]
The problem here is that uses_template_parms returns true for all
concept-ids (even those with non-dependent arguments), so when a concept-id
is used as a default template argument then during deduction the default
argument is considered dependent even after substituting into it, which
leads to deduction failure (from type_unification_real).
This patch fixes this by implementing the resolution of CWG 2446 which
says a concept-id is dependent only if its arguments are.
DR 2446
PR c++/102412
gcc/cp/ChangeLog:
* constexpr.c (cxx_eval_constant_expression)
<case TEMPLATE_ID_EXPR>: Check value_dependent_expression_p
instead of processing_template_decl.
* pt.c (value_dependent_expression_p) <case TEMPLATE_ID_EXPR>:
Return true only if any_dependent_template_arguments_p.
(instantiation_dependent_r) <case CALL_EXPR>: Remove this case.
<case TEMPLATE_ID_EXPR>: Likewise.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-nondep2.C: New test.
* g++.dg/cpp2a/concepts-nondep3.C: New test.
(cherry picked from commit 9329344a6d81a6a5e3bd171167ebc7b158bb44f4)
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter
2021-09-20 16:45 [Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter joeloser93 at gmail dot com
` (2 preceding siblings ...)
2021-10-06 14:15 ` cvs-commit at gcc dot gnu.org
@ 2021-10-06 14:19 ` ppalka at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: ppalka at gcc dot gnu.org @ 2021-10-06 14:19 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution|--- |FIXED
--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 11.3 and 12.
^ permalink raw reply [flat|nested] 5+ messages in thread
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2021-09-20 16:45 [Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter joeloser93 at gmail dot com
2021-09-20 18:36 ` [Bug c++/102412] " ppalka at gcc dot gnu.org
2021-09-22 15:18 ` cvs-commit at gcc dot gnu.org
2021-10-06 14:15 ` cvs-commit at gcc dot gnu.org
2021-10-06 14:19 ` ppalka at gcc dot gnu.org
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