When to supply default arguments as template arguments

Question

template<typename T, typename U = T>
struct Test{};
template<typename T>
void func(Test<T>){  //#1
}
int main(){
  func(Test<int>{});  //#2
}

It seems to no rule in the standard that mentioned what situation the default argument is required for template-parameter.

In dcl.fct.default#1

If an initializer-clause is specified in a parameter-declaration this initializer-clause is used as a default argument. Default arguments will be used in calls where trailing arguments are missing.

In this section, the rule explicitly describe when to supply default arguments for a function call. However I haven't found a quote in the standard similar with the above sentence that describes when to supply default arguments as template arguments.

Such as Test<T> at #1. Maybe at #1, the specialization would be Test<T,T>, It's just inference. However, there's no formally terminology explicitly describe this in the standard.

The only quote that implies the default arguments as template arguments is in the following rule:
temp#arg-8

When a simple-template-id does not name a function, a default template-argument is implicitly instantiated when the value of that default argument is needed. [ Example:
template<typename T, typename U = int> struct S { };
S<bool>* p; // the type of p is S<bool, int>*
The default argument for U is instantiated to form the type S<bool, int>*. — end example ]

Consider the code at #1, Does the default argument need at #1? If it is(It seems to It need at that point because If I don't specify a default argument for template parameter U, then at the point #1 will occur an error. see godbolt outcome), According to the above quote, the default template-argument need to be implicitly instantiated, However at this point, T is a template-parameter and the definition of such function template instantiate nothing(It's just a function template definition at this point). So, How does the quote interpret this?

@StoryTeller-UnslanderMonica I have read that before write this question. My reading is that the quote is just about what situation the default argument will be implicitly instantiated — xmh0511
I do not actually. A reader of the standard is capable of understanding that the cases "when needed" includes the case where an explicit argument is not provided. It also potentially includes more (which I think is by design). — StoryTeller - Unslander Monica
The function template definition does not instantiate anything. That happens later during template argument deduction at the call site. Which come to thick of it is probably why the normative text goes with "when needed". — StoryTeller - Unslander Monica

Oliv Oliv · Accepted Answer · 2020-07-18T13:55:33

There is also this paragraph inside [temp.names]:

A template-id is valid if

there is an argument for each non-deducible non-pack parameter that does not have a default template-argument,

So one could consider that a default-argument is needed means a default-argument is needed in order to make the template-id valid and that when there lacks a template argument, the argument is the default argument value. But I have not found anything explicit in the standard for class templates. For function template this is more explicit. Probably nobody pointed out this hole in the standard because this is a common pattern: default is used in place of what is not user provided. Maybe it will not be changed because the definition of default in the english dictionary is already given:

IT. the way that something will happen or appear automatically, especially on a computer, if you do not make any different choices -- cambrige online dictionary

When to supply default arguments as template arguments

2 Answers