[Bug c++/56842] New: Argument deduction failure note is empty for alias template

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56842

             Bug #: 56842
           Summary: Argument deduction failure note is empty for alias
                    template
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


For this code:

#include <type_traits>

struct Base { };

template<typename T>
struct Constraint1
: std::enable_if<std::is_base_of<Base, T>::value>
{ };

template <typename T,
         typename Requires = typename Constraint1<T>::type>
void foo() { }

struct A { };

int main()
{
    foo<A>();
}

GCC gives an excellent diagnostic:

b.cc: In function 'int main()':
b.cc:18:12: error: no matching function for call to 'foo()'
     foo<A>();
            ^
b.cc:18:12: note: candidate is:
b.cc:12:6: note: template<class T, class Requires> void foo()
 void foo() { }
      ^
b.cc:12:6: note:   template argument deduction/substitution failed:
b.cc:11:10: error: no type named 'type' in 'struct Constraint1<A>'
          typename Requires = typename Constraint1<T>::type>
          ^

However after changing it to use an alias template, which makes the declaration
of bar() much easier to read:

#include <type_traits>

struct Base { };

template<typename T>
  using Constraint2
    = typename std::enable_if<std::is_base_of<Base, T>::value>::type;

template <typename T,
         typename Requires = Constraint2<T>>
void bar() { }

struct A { };

int main()
{
    bar<A>();
}

GCC produces a useless diagnostic:

b.cc: In function 'int main()':
b.cc:16:12: error: no matching function for call to 'bar()'
     bar<A>();
            ^
b.cc:16:12: note: candidate is:
b.cc:10:6: note: template<class T, class Requires> void bar()
 void bar() { }
      ^
b.cc:10:6: note:   template argument deduction/substitution failed: