Why can't the template argument be deduced when it is used as template parameter to another template?

Question

What is wrong in this code?

#include 

Answer 1

I don't think "we can't do this" argument is correct. If we slightly modify this example, the compiler is happily deducing arguments for us.

template
struct TMap //...

template 
struct tmap_t : TMap::Type {};

template
T test(tmap_t tmap) // ...

tmap_t tmap;  // ...
double d = test(tmap);  // compiles just fine.

I don't see a huge difference between the original example and mine. The real issue here seems that C++ treats typedefs and type declarations differently

Is this a good thing?