现行规则草案中不参与推演的模板参数包推导为空的规范规则是什么

Question

template<typename ...T, typename U>
void fun(U){}
int main(){
   fun(0);
}

This snippet code is accepted by both GCC and Clang. The template parameter pack T does not participate in the template argument deduction in the context of function call, as per the following rules:
[temp.deduct.call]

Template argument deduction is done by comparing each function template parameter type (call it P) that contains template-parameters that participate in template argument deduction with the type of the corresponding argument of the call (call it A) as described below.

The pack T is contained by any function template parameter. If there were no other special rules specify, the deduction would fail according to:
[temp.deduct.type#2]

if any template argument remains neither deduced nor explicitly specified, template argument deduction fails.

However, such a case is ruled by the following rule in the current standard, that is: [temp.arg.explicit#4]

A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments .

So, the above case can be considered to deduce successfully which leaves the pack T with an empty set of template arguments.

However, the special rule in temp.arg.explicit#4 has been changed to a note in the current draft

[temp.arg.explicit#note-1]

[Note 1: A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments. — end note]

So, I wonder Is there any alternative normative rule in the current draft states that the pack T not otherwise deduced will be deduced to an empty set of template arguments?

Answer 1

The previously normative section of [temp.arg.explicit]/4

[...] A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments. [...]

was made into a non-normative note as part of P1787R6 .

As you've pointed out, as per [temp.deduct.type]/2 [ emphasis mine]:

Type deduction is done independently for each P / A pair [...], if any template argument remains neither deduced nor explicitly specified, template argument deduction fails.

[temp.arg.general] describes that a template parameter that is a template parameter pack may correspond to zero template arguments :

[...] When the parameter declared by the template is a template parameter pack, it will correspond to zero or more template-arguments .

and [temp.variadic]/1 explicitly mention that a template parameter pack may accept zero arguments:

A template parameter pack is a template parameter that accepts zero or more template arguments.

followed by a non-normative example of an empty argument list for an entity templated over a parameter pack template parameter:

 template<class... Types> struct Tuple { }; Tuple<> t0; // Types contains no arguments

Now, returning to [temp.arg.explicit]/4 :

If all of the template arguments can be deduced, they may all be omitted ; in this case, the empty template argument list <> itself may also be omitted.

Meaning the Tuple example above is can likewise omit the empty argument list

Tuple t0; // Also OK: Types contains no arguments

but where the key is that, as per [temp.arg.general] above, that a template parameter list may correspond to zero arguments, in which case there are no template arguments needed to be deduced.

If you look at your own example:

 template<typename...T, typename U> void fun(U){} int main(){ fun(0); // #1 }

you could likewise invoke #1 as:

fun<>(0);  // argument list for parameter pack is empty
           // -> no argument (beyond that for `U`) to deduce

highlighting that the deducible argument corresponding to the template parameter U can be omitted from the explicit template-arguments, whereas the remaining template-arguments are none ; namely, the argument list for the template parameter that is a template parameter pack is empty , and there are thus no remaining template arguments that needs to be deduced.

Thus

A trailing template parameter pack ([temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments.

is non-normative/redundant, explaining why it was wrapped into a [Note - [...] - end note] .