* [Bug c++/99963] [11 Regression] [concepts] template <concept> vs concept auto reports ambiguous overload
2021-04-07 20:02 [Bug c++/99963] New: [concepts] template <concept> vs concept auto reports ambiguous overload ldalessandro at gmail dot com
@ 2021-04-14 15:41 ` ppalka at gcc dot gnu.org
2021-04-14 19:12 ` jason at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: ppalka at gcc dot gnu.org @ 2021-04-14 15:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jason at gcc dot gnu.org,
| |ppalka at gcc dot gnu.org
Target Milestone|--- |11.0
Summary|[concepts] template |[11 Regression] [concepts]
|<concept> vs concept auto |template <concept> vs
|reports ambiguous overload |concept auto reports
| |ambiguous overload
--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Started with r11-1571. Reduced testcase that replaces the abbreviated function
templates with their corresponding non-abbreviated forms:
template <class T> concept C1 = true;
template <class T> concept C2 = C1<T> && true;
template <C1 T, C1 U> int f(T, U);
template <C1 T, C2 U> int f(U, T);
int x = f(0, 0); // error: ambiguous call
If I understand the wording of P2113 correctly:
If deduction against the other template succeeds for both transformed
templates, constraints can be considered as follows:
- ... if the corresponding template-parameters of the
template-parameter-lists are not equivalent ([temp.over.link]) or if the
function parameters that positionally correspond between the two templates are
not of the same type, neither template is more specialized than the other
then I think we're correct to reject the call as ambiguous because although the
second overload is more constrained than the first, their function parameter
lists aren't equivalent.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99963] [11 Regression] [concepts] template <concept> vs concept auto reports ambiguous overload
2021-04-07 20:02 [Bug c++/99963] New: [concepts] template <concept> vs concept auto reports ambiguous overload ldalessandro at gmail dot com
2021-04-14 15:41 ` [Bug c++/99963] [11 Regression] " ppalka at gcc dot gnu.org
@ 2021-04-14 19:12 ` jason at gcc dot gnu.org
2021-04-15 14:26 ` ldalessandro at gmail dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jason at gcc dot gnu.org @ 2021-04-14 19:12 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963
Jason Merrill <jason at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #1)
Yes. The problem is that you're mixing up which template parameter goes with
which function parameter.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99963] [11 Regression] [concepts] template <concept> vs concept auto reports ambiguous overload
2021-04-07 20:02 [Bug c++/99963] New: [concepts] template <concept> vs concept auto reports ambiguous overload ldalessandro at gmail dot com
2021-04-14 15:41 ` [Bug c++/99963] [11 Regression] " ppalka at gcc dot gnu.org
2021-04-14 19:12 ` jason at gcc dot gnu.org
@ 2021-04-15 14:26 ` ldalessandro at gmail dot com
2022-06-21 21:21 ` tabloid.adroit at gmail dot com
2022-06-23 18:47 ` tabloid.adroit at gmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: ldalessandro at gmail dot com @ 2021-04-15 14:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963
--- Comment #3 from Luke Dalessandro <ldalessandro at gmail dot com> ---
I understand. Thank you (I've forwarded this on to clang, which _does_ accept
the ambiguous form).
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99963] [11 Regression] [concepts] template <concept> vs concept auto reports ambiguous overload
2021-04-07 20:02 [Bug c++/99963] New: [concepts] template <concept> vs concept auto reports ambiguous overload ldalessandro at gmail dot com
` (2 preceding siblings ...)
2021-04-15 14:26 ` ldalessandro at gmail dot com
@ 2022-06-21 21:21 ` tabloid.adroit at gmail dot com
2022-06-23 18:47 ` tabloid.adroit at gmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: tabloid.adroit at gmail dot com @ 2022-06-21 21:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963
tabloid.adroit at gmail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |tabloid.adroit at gmail dot com
--- Comment #4 from tabloid.adroit at gmail dot com ---
(In reply to Patrick Palka from comment #1)
> Started with r11-1571. Reduced testcase that replaces the abbreviated
> function templates with their corresponding non-abbreviated forms:
>
> template <class T> concept C1 = true;
> template <class T> concept C2 = C1<T> && true;
>
> template <C1 T, C1 U> int f(T, U);
> template <C1 T, C2 U> int f(U, T);
>
> int x = f(0, 0); // error: ambiguous call
>
>
> If I understand the wording of P2113 correctly:
>
> If deduction against the other template succeeds for both transformed
> templates, constraints can be considered as follows:
> - ... if the corresponding template-parameters of the
> template-parameter-lists are not equivalent ([temp.over.link]) or if the
> function parameters that positionally correspond between the two templates
> are not of the same type, neither template is more specialized than the other
>
> then I think we're correct to reject the call as ambiguous because although
> the second overload is more constrained than the first, their function
> parameter lists aren't equivalent.
IMHO, `template <C1 T, C2 U> int f(U, T);` should win over `template <C1 T, C1
U> int f(T, U);`.
Based on interpreting the intent mentioned in
https://github.com/cplusplus/nbballot/issues/119 and the second example in
https://eel.is/c++draft/temp.fct#temp.func.order-example-6, the `corresponding`
(of the `corresponding template-parameters of ...`) relationship is based on
the mapping used during partial-ordering deduction. So the deduction between
`f(T, ..)` against `f(U, ..)` builds the <T,U> mapping, the deduction between
`f(.., U)` against `f(.., T)` builds the <U, T> mapping. The correspondence is
[T, U] against [U, T]. So `C1 T` is less constrained than `C2 U`, thus the
second `f` wins.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99963] [11 Regression] [concepts] template <concept> vs concept auto reports ambiguous overload
2021-04-07 20:02 [Bug c++/99963] New: [concepts] template <concept> vs concept auto reports ambiguous overload ldalessandro at gmail dot com
` (3 preceding siblings ...)
2022-06-21 21:21 ` tabloid.adroit at gmail dot com
@ 2022-06-23 18:47 ` tabloid.adroit at gmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: tabloid.adroit at gmail dot com @ 2022-06-23 18:47 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963
--- Comment #5 from Yuanfang Chen <tabloid.adroit at gmail dot com> ---
(In reply to Yuanfang Chen from comment #4)
> (In reply to Patrick Palka from comment #1)
> > Started with r11-1571. Reduced testcase that replaces the abbreviated
> > function templates with their corresponding non-abbreviated forms:
> >
> > template <class T> concept C1 = true;
> > template <class T> concept C2 = C1<T> && true;
> >
> > template <C1 T, C1 U> int f(T, U);
> > template <C1 T, C2 U> int f(U, T);
> >
> > int x = f(0, 0); // error: ambiguous call
> >
> >
> > If I understand the wording of P2113 correctly:
> >
> > If deduction against the other template succeeds for both transformed
> > templates, constraints can be considered as follows:
> > - ... if the corresponding template-parameters of the
> > template-parameter-lists are not equivalent ([temp.over.link]) or if the
> > function parameters that positionally correspond between the two templates
> > are not of the same type, neither template is more specialized than the other
> >
> > then I think we're correct to reject the call as ambiguous because although
> > the second overload is more constrained than the first, their function
> > parameter lists aren't equivalent.
>
> IMHO, `template <C1 T, C2 U> int f(U, T);` should win over `template <C1 T,
> C1 U> int f(T, U);`.
>
> Based on interpreting the intent mentioned in
> https://github.com/cplusplus/nbballot/issues/119 and the second example in
> https://eel.is/c++draft/temp.fct#temp.func.order-example-6, the
> `corresponding` (of the `corresponding template-parameters of ...`)
> relationship is based on the mapping used during partial-ordering deduction.
> So the deduction between `f(T, ..)` against `f(U, ..)` builds the <T,U>
> mapping, the deduction between `f(.., U)` against `f(.., T)` builds the <U,
> T> mapping. The correspondence is [T, U] against [U, T]. So `C1 T` is less
> constrained than `C2 U`, thus the second `f` wins.
Sorry. I spoke too soon. Template parameters reordering is not allowed for the
test case. The call is indeed ambiguous.
^ permalink raw reply [flat|nested] 6+ messages in thread