[Bug c++/69623] Invalid deduction of non-trailing template parameter pack
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623 --- Comment #9 from jim x --- 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.
[Bug c++/69623] Invalid deduction of non-trailing template parameter pack
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623
--- Comment #8 from Ed Catmur ---
(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
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623
--- Comment #7 from jim x ---
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
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623
Ed Catmur changed:
What|Removed |Added
CC||ed at catmur dot uk
--- Comment #6 from Ed Catmur ---
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 struct any { any(auto) {} };
template
auto g(std::index_sequence, auto... a) { return f...>(a...); }
template
auto h(auto... a) { return g(std::make_index_sequence(), 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
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623
--- Comment #5 from Andrew Pinski ---
Full testcase where both declarations should be an error:
template void f() { } // error
template void g() { } // error
[Bug c++/69623] Invalid deduction of non-trailing template parameter pack
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623 Andrew Pinski changed: What|Removed |Added CC||xmh970252187 at gmail dot com --- Comment #4 from Andrew Pinski --- *** Bug 103740 has been marked as a duplicate of this bug. ***
[Bug c++/69623] Invalid deduction of non-trailing template parameter pack
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623 Andrew Pinski changed: What|Removed |Added CC||kariya_mitsuru at hotmail dot com --- Comment #3 from Andrew Pinski --- *** Bug 61953 has been marked as a duplicate of this bug. ***
[Bug c++/69623] Invalid deduction of non-trailing template parameter pack
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623 Andrew Pinski changed: What|Removed |Added CC||smw at gcc dot gnu.org --- Comment #2 from Andrew Pinski --- *** Bug 83870 has been marked as a duplicate of this bug. ***
[Bug c++/69623] Invalid deduction of non-trailing template parameter pack
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69623 Andrew Pinski changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2016-08-21 Ever confirmed|0 |1 --- Comment #1 from Andrew Pinski --- Confirmed.
