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From: "dgsteffen at numerica dot us" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/52522] New: Overloaded functions called with initializer lists considered ambiguous
Date: Wed, 07 Mar 2012 17:17:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D52522

             Bug #: 52522
           Summary: Overloaded functions called with initializer lists
                    considered ambiguous
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dgsteffen@numerica.us


Consider:

#include <vector>
using std::vector;
class Matrix{};
class Vector{};

Matrix horizontalConcat(const std::vector<Vector>& vectors);
Matrix horizontalConcat(const std::vector<Matrix>& matrices);

int main()
{
 Matrix a,b;
 horizontalConcat({a,b});
}

GCC 4.4 and 4.5 in C++0X mode (E.G., g++ testA.cpp -c -ansi -std=3Dc++0x ) =
accept
this code and do the right thing.  However, 4.6.3 produces:

/usr/local/compilers/4.6/bin/g++ testA.cpp -c -ansi -std=3Dc++0x=20

estA.cpp: In function =E2=80=98int main()=E2=80=99:
testA.cpp:25:24: error: call of overloaded =E2=80=98horizontalConcat(<brace=
-enclosed
initializer list>)=E2=80=99 is ambiguous
testA.cpp:25:24: note: candidates are:
testA.cpp:12:8: note: Matrix horizontalConcat(const std::vector<Vector>&)
testA.cpp:13:8: note: Matrix horizontalConcat(const std::vector<Matrix>&)

However, sending an initializer list containing 3 or more Matrices works fi=
ne.=20
Also, removing the vector<Vector> overloaded function removes the warning.

In principle there could be some ambiguity, when using an initializer list =
of
two elements, between calling vector's initializer list ctor or calling one=
 of
its other ctors, but here I don't see that either, since vector<Matrix> doe=
sn't
have a ctor that takes two matrices.

At any rate, either GCC 4.6 is incorrectly concluding that the function cal=
l is
ambiguous, or it's producing a very confusing error message.

Thanks very much.