[Bug c++/40127] Fails to identify template function with default args

[Bug c++/40127] Fails to identify template function with default args

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40127

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-01-15 20:01:35 UTC ---
This code is invalid (and other compilers agree with me).  What type is going
to be chosen for T?  void*, int*, any pointer type?  There is no way the
compiler can deduce the type of T from the arguments.

This code is valid though:
template<typename T>
void    foo(int i, void(*f)(T*) = 0, T* a = 0) {}
int  main() {
        foo<void>(5);
        return 0;
        }

AS the compiler does not need to deduce the template argument type.

[Bug c++/40127] Fails to identify template function with default args

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40127

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-01-15 23:16:53 UTC ---
(In reply to comment #0)
> I'm not sure whether argument types can be bound from default arguments in the
> standard,

They can't.

> but if they can't then I should get a diagnostic at the declaration
> of the template.

As Andrew showed, the template isn't invalid and can be called with an explicit
template argument list, so issuing a diagnostic at declaration would be wrong.

> The actual message is cryptic and unhelpful.

Current versions give a great diagnostic:

c.cc: In function ‘int main()’:
c.cc:4:14: error: no matching function for call to ‘foo(int)’
c.cc:4:14: note: candidate is:
c.cc:2:9: note: template<class T> void foo(int, void (*)(T*), T*)
c.cc:2:9: note:   template argument deduction/substitution failed:
c.cc:4:14: note:   couldn't deduce template parameter ‘T’