[Bug c++/56506] variadic class template specialization not selected as best match

[mmehlich at semanticdesigns dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56506



--- Comment #1 from Michael Mehlich <mmehlich at semanticdesigns dot com> 
2013-03-02 18:55:43 UTC ---

Some more information using a function template instead of a class template:



-- Additional code

template<typename ... T> void foo(Y<Z<T...>,T>...) {

}



int main() {

    Y<Z<int,bool,char>,int> yi;

    Y<Z<int,bool,char>,bool> yb;

    Y<Z<int,bool,char>,char> yc;

    foo(yi,yb,yc);

}



-- Error message from gcc 4.7.2:

test.cpp: In function 'int main()':

test.cpp:31:14: error: no matching function for call to 'foo(Y<Z<int, bool,

char>, int>&, Y<Z<int, bool, char>, bool>&, Y<Z<int, bool, char>, char>&)'

test.cpp:31:14: note: candidate is:

test.cpp:15:31: note: template<class ... T> void foo(Y<Z<T ...>, T>...)

test.cpp:15:31: note:   template argument deduction/substitution failed:

test.cpp:31:14: note:   deduced conflicting types for parameter 'T' ('int,

bool, char' and 'int')

test.cpp:31:14: note:   'Y<Z<int, bool, char>, int>' is not derived from 'Y<Z<T

...>, T>'



It looks like the compiler decides to bind T to int when handling the

first argument instead of binding a "prefix" of T to int.