[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |smw at gcc dot gnu.org

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 83870 has been marked as a duplicate of this bug. ***

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |kariya_mitsuru at hotmail dot com

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 61953 has been marked as a duplicate of this bug. ***

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |xmh970252187 at gmail dot com

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 103740 has been marked as a duplicate of this bug. ***

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Full testcase where both declarations should be an error:

  template<class... T, class... U> void f() { } // error
  template<class... T, class U> void g() { } // error

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

Ed Catmur <ed at catmur dot uk> changed:

           What    |Removed                     |Added
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                 CC|                            |ed at catmur dot uk

--- Comment #6 from Ed Catmur <ed at catmur dot uk> ---
There is a fairly well-known idiom for extracting a specific element of an
argument pack:

auto f(auto..., auto a, auto...) { return a; }
template<auto> struct any { any(auto) {} };
template<std::size_t... I>
auto g(std::index_sequence<I...>, auto... a) { return f<any<I>...>(a...); }
template<unsigned I>
auto h(auto... a) { return g(std::make_index_sequence<I>(), a...); }

I believe the intent of http://eel.is/c++draft/temp#param-14.sentence-3 is that
this idiom should work; per http://eel.is/c++draft/temp#param-example-7 (as in
comment #5) a function template secondary template parameter pack is only
disallowed if it is nondeducible.  In the original description of this ticket
(not comment #5, which I agree is disallowed) T is specifiable and U is
deducible, so both are OK.

> [...] A template parameter pack of a function template shall not be followed by another template parameter unless that template parameter can be deduced from the parameter-type-list ([dcl.fct]) of the function template or has a default argument ([temp.deduct]). [...]

Or is the intent that T must be explicitly specified, i.e. that f can be called
as f<>() but not as f(), pace
http://eel.is/c++draft/temp#arg.explicit-4.sentence-3 ?

> If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted.

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[xmh970252187 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

--- Comment #7 from jim x <xmh970252187 at gmail dot com> ---
In a simple way, the rule just requires that, for a function template, the
template parameter that is declared after a template parameter pack should
either appear in the parameter-declaration-clause before the template
pack(deducible) or just have a default argument.

````
auto f(auto..., auto a, auto...) { return a; }
````
IIUC, this is just disallowed since all arguments would only match the first
function parameter pack.

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

--- Comment #8 from Ed Catmur <ed at catmur dot uk> ---
(In reply to jim x from comment #7)
> ````
> auto f(auto..., auto a, auto...) { return a; }
> ````
> IIUC, this is just disallowed since all arguments would only match the first
> function parameter pack.

Maybe? But all 4 major compilers currently permit it (with identical behavior),
so is it worth breaking existing code by disallowing it?

[Bug c++/69623] Invalid deduction of non-trailing template parameter pack

[xmh970252187 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623

--- Comment #9 from jim x <xmh970252187 at gmail dot com> ---
After reading [temp.deduct.call] p1 more carefully, I think you're right. Since
the first function parameter pack does not participate in template argument
deduction, the deduction will start to perform from the second function
parameter to the last.