[Bug c++/59056] New: enable_if turns a non-ambiguous template into an ambiguous one

[walter.mascarenhas at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59056

            Bug ID: 59056
           Summary: enable_if turns a non-ambiguous template into an
                    ambiguous one
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: walter.mascarenhas at gmail dot com

Created attachment 31187
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31187&action=edit
a simple code illustrating the bug

The attached file shows that by turning a specialization of

template <class A, class Enable = void>
struct Foo {};

like this one (which works just fine)

template <class A>
struct Foo<A, void>{};

into 

template <class A>
struct Foo<A, std::enable_if< typename always_true<A>() >::type >{};

we can get ambigouities, even when typename always_true<A>() >::type
always resolves to void.

For more details, look at the attachement