[Bug c++/58354] New: variadic template ambigous

[Bug c++/58354] New: variadic template ambigous

[1zeeky at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

            Bug ID: 58354
           Summary: variadic template ambigous
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 1zeeky at gmail dot com

Created attachment 30761
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30761&action=edit
testcase

I have a templated function, which recursively calls itself.
The stopping criterion is provided via a std::integral_constant<std::size_t>
that gets incremented each iteration and upon a certain value should call an
overload that does not recurse.

There are two requirements to get g++ to claim that the call is ambiguous:
There must be an extra template-parameter that is explicitly specified when
recursing (typename T in the testcase) and variadic templates must be used.
If either is not present, it compiles fine.
If variadic templates are present, but no additional arguments are passed (like
the second parameter for foo (0) in the testcase), it compiles fine.

clang 3.3 compiles this code fine, and given that the mentioned extra
requirements must be met to make the call ambiguous I'm somewhat confident that
it is valid C++.

This is the specific output the attached testcase generates for g++ 4.8.1:

test.cpp: In instantiation of ‘void foo(IndexType<index>, Arguments ...) [with
T = int; long unsigned int index = 0ul; Arguments = {}; IndexType<index> =
std::integral_constant<long unsigned int, 0ul>]’:
test.cpp:15:25:   required from here
test.cpp:11:76: error: call of overloaded ‘foo(IndexType<1ul>, int)’ is
ambiguous
 void foo(IndexType<index>, Arguments...) { foo<T>(IndexType<index + 1>(), 0);
}
                                                                            ^
test.cpp:11:76: note: candidates are:
test.cpp:8:6: note: void foo(IndexType<1ul>, Arguments ...) [with T = int;
Arguments = {int}; IndexType<1ul> = std::integral_constant<long unsigned int,
1ul>]
 void foo(IndexType<1>, Arguments...) { }
      ^
test.cpp:11:6: note: void foo(IndexType<index>, Arguments ...) [with T = int;
long unsigned int index = 1ul; Arguments = {int}; IndexType<index> =
std::integral_constant<long unsigned int, 1ul>]
 void foo(IndexType<index>, Arguments...) { foo<T>(IndexType<index + 1>(), 0);
}
      ^
>From gcc-bugs-return-429211-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Sat Sep 07 12:29:37 2013
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Subject: [Bug fortran/58351] ICE using c_f_pointer subroutine and derived types
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX351

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |burnus at gcc dot gnu.org

--- Comment #2 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
Likely fixed by revision 197053 (r197010 works for me but with a patch).
According to comment #12

> ... by r197053, apparently. Since this looks way too large for backporting,
> I guess it will not be fixed on the 4.7 and 4.8 branches, right?

the fix won't be backported to 4.8.2.

[Bug c++/58354] variadic template ambiguous

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-09-07
     Ever confirmed|0                           |1

[Bug c++/58354] variadic template ambiguous

[officesamurai at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

Mikhail Kremnyov <officesamurai at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |officesamurai at gmail dot com

--- Comment #1 from Mikhail Kremnyov <officesamurai at gmail dot com> ---
There is a simpler case.

test.cpp:
template <typename T1, typename T2, typename... Types>
void foo(T2, Types...)
{
}

template <typename T1, typename... Types>
void foo(int, Types...)
{
}

int main()
{
  foo<void>(1, 2);
}

Command line:
g++ -std=c++11 test.cpp -o test

Output:
test.cpp: In function ‘int main()’:
test.cpp:13:17: error: call of overloaded ‘foo(int, int)’ is ambiguous
   foo<void>(1, 2);
                 ^
test.cpp:13:17: note: candidates are:
test.cpp:2:6: note: void foo(T2, Types ...) [with T1 = void; T2 = int; Types =
{int}]
 void foo(T2, Types...)
      ^
test.cpp:7:6: note: void foo(int, Types ...) [with T1 = void; Types = {int}]
 void foo(int, Types...)

I tried this with versions 4.8.1, 4.9.1, 4.9.2 - same result.
Clang has no problems with this code.
>From gcc-bugs-return-467542-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Nov 19 17:22:58 2014
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Subject: [Bug testsuite/63971] Some of  gcc.target/aarch64/test_frame_*.c tests fail now
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc971

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2014-11-19
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I have a patch which I will submit this weekend.

[Bug c++/58354] variadic template ambiguous

[1zeeky at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

--- Comment #2 from 1zeeky at gmail dot com ---
This has been fixed as of gcc 10.2.

[Bug c++/58354] variadic template ambiguous

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.2
           Keywords|                            |rejects-valid
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed by r11-1571 and r10-8343: Refinements to "more constrained".

Jason, was the change to tsubst_pack_expansion a drive-by fix that resolved
this bug?

            * pt.c (tsubst_pack_expansion): Fix getting a type parameter
            pack.


The rest of the commit is related to constrained functions, which aren't
relevant here.

Should we add this testcase before closing the bug?

[Bug c++/58354] variadic template ambiguous

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rbock at eudoxos dot de

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 77449 has been marked as a duplicate of this bug. ***

[Bug c++/58354] variadic template ambiguous

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lumosimann at gmail dot com

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 89085 has been marked as a duplicate of this bug. ***

[Bug c++/58354] variadic template ambiguous

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |oknenavin at outlook dot com

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 90642 has been marked as a duplicate of this bug. ***

[Bug c++/58354] variadic template ambiguous

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vexocide at gmail dot com

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 59135 has been marked as a duplicate of this bug. ***

[Bug c++/58354] variadic template ambiguous

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58354

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed.