C++ :: Argument Deduction Of Function And Class Template

When we use a function template, we use a function template like a regular function, for example,

Code:
template<class T>
void foo(T t1, T t2)
{
}
foo(1,3);

Based on the arguments passed to foo, the compiler can deduct the type T. But on the other hand, when we use a class template, we always need to specify the type, for example,

Code:
template<class T>
struct sum {
static void foo(T t1, T t2)
{
}
};
sum<int>::foo(1,3);

Here we can't call sum::foo(1,3), otherwise we get compiler errors. My question is why the compiler can't deduct the type based on the arguments passed to foo? In addition, if we call function template foo like this,

Code:
foo(1, '3');

Then we get compiler errors. We need to specify the type like foo<int>(1.'3'). Since '3' can be always treated as integer, why we need to specify the type here?