template matching problem

[Norman J. Goldstein]

Below, is code that has 4 different conditional compilation blocks.
When the first block is chosen, the compilation breaks with the msg:

error: template-id ‘copy<>’ for
‘bool copy(WD&, WD::IoRefrType<A>)’
does not match any template declaration
template bool copy( WD&,

which refers to the inability of the compiler to explicitly instantiate,
as requested at the bottom of the source code.

However, when any of the last 3 blocks is chosen, instead,
the compilation is fine. I am using gcc 4.8.3 on linux x86-64.

Explanations or suggestions much appreciated!

/////////////////////// source code ///////////////
struct WD
{
template< typename T >
using IoRefrType = const T&;
};

class A;

#if 1 // Two template parameters: Error
template< typename S, typename T >
bool copy( S& s,
typename S:: template IoRefrType< T > t )
{ return true; }
#elif 1 // Only the first template parameter: OK
template< typename S >
bool copy( S& s,
typename S:: template IoRefrType<A> )
{ return true; }
#elif 1 // Only the second template parameter: OK
template< typename T >
bool copy( WD& ,
typename WD:: template IoRefrType< T > t )
{ return true; }
#else // Simpler two template parameters: OK
template< typename S, typename T >
bool copy( S& s,
const T& t )
{ return true; }
#endif

template bool
copy( WD&,
typename WD:: template IoRefrType<A> );
/////////////////////////////////////////////////////

[fefe]

In this template,

> template< typename S, typename T >
> bool copy( S& s,
> typename S:: template IoRefrType< T > t )
> { return true; }

in the type `typename S:: template IoRefrType<T>`, the template parameter `S` is in a non-deduced context, thus rendering the whole `typename S:: template IoRefrType<T>` non-deduced. This means, the type `T` cannot be automatic deduced.

You may try give the template arguments explicitly:

template bool
copy<WD, A>( WD&,

[Norman J. Goldstein]

This works! Thanks very much, and for the explanation.