(This question is a branch-out from the discussion in the comments of Template specialization of variable template and type deduction.)
[temp.expl.spec]/10 states that [emphasis mine]:
A trailing template-argument can be left unspecified in the template-id naming an explicit function template specialization provided it can be deduced from the function argument type. [ Example:
template<class T> class Array { /* ... */ }; template<class T> void sort(Array<T>& v); // explicit specialization for sort(Array<int>&) // with deduced template-argument of type int template<> void sort(Array<int>&);— end example ]
Which clearly applies to the (full) explicit specialization of foo(T) in the example below:
#include <iostream>
template <typename T>
void foo(T) { std::cout << "primary\n"; }
template <>
void foo(int) { std::cout << "int\n"; }
// OK ^<int> deduced from template argument deduction
// in this _declaration_, as of [temp.expl.spec]/10
int main()
{
const int a = 42;
foo(a); // int
// OK, <int> deduced from template argument deduction.
}
However, for both clang and GCC, for all the various ISO C++ version I have tested, this also applies in the example where there are no function arguments in the function template, and where its, say, type template parameter is solely present as the function template's return type:
#include <iostream>
template <typename T>
T bar() { std::cout << "primary\n"; return 0; }
template <>
int bar() { std::cout << "int\n"; return 42; }
// ^<int> deduced?
int main()
{
(void)bar<int>(); // int
// ^^^ OK, no template argument deduction.
}
I'm slightly confused with term "deduced" in the quote above, as it, afaics, doesn't refer to deduction in the sense of the typical (call site/instantiation) template argument deduction, but rather deduction in the context of a specialization declaration.
Question:
- Where is it covered in the ISO C++ Standard that the trailing template-argument in a declaration of an explicit function template specialization with a template argument only present as the return type, may in fact be omitted (deduced)?
