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From: "filip.roseen at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/60177] New: Unable to deduce template base of derived class in function template accepting a simple-template-id
Date: Thu, 13 Feb 2014 14:38:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60177

            Bug ID: 60177
           Summary: Unable to deduce template base of derived class in
                    function template accepting a simple-template-id
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: filip.roseen at gmail dot com

template<class> struct Base { };

struct Derived : Base<void> { };

template<template<typename> class TT, typename T>
void func (TT<T>) { }

int main () {
  func (Derived { });
}


- - - - - - - - - - - - - - - - - - - - - - - - - -

main.cpp: In function 'int main()':
main.cpp:9:20: error: no matching function for call to 'func(Derived)'
   func (Derived { });
                    ^
main.cpp:9:20: note: candidate is:
main.cpp:6:6: note: template<template<class> class TT, class T> void
func(TT<T>)
 void func (TT<T>) { }
      ^
main.cpp:6:6: note:   template argument deduction/substitution failed:
main.cpp:9:20: note:   can't deduce a template for 'TT<T>' from non-template
type 'Derived'
   func (Derived { });

- - - - - - - - - - - - - - - - - - - - - - - - - -

gcc rejects the snippet which is a violation of [temp.deduct.call]/4 (`clang`
and `msvc` accepts the code).

`TT<T>` is a `simple-template-id` and the standard says that if a derived type
is passed to such context it should be deduced to a suitable base of that
passed type. 

Correct behavior is that the deduced template parameters for `func (Derived
{})` should be `TT = Base, T = void`.