auto in function parameters just makes the function an abbreviated function
template. It's just a shorthand notation for a "regular" function template.

The following code is valid for example:

void func(auto arg = 0) { }

int main() {
    func<void*>();
}


With that knowledge, trying to deduce the type from the default argument
doesn't make sense. It would take away your ability to write templates
where you use a "symbolic" default argument like 0 or {} that is converted
to the specified type. It may be counter-intuitive for the abbreviated
syntax, but I think it would be equally confusing if there were major
differences between abbreviated and unabbreviated templates too.

On Sun, 22 Mar 2026 at 05:11, 叶易安 via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Hi, std-proposals mailing list. I'm a C++ learner, and i'm confused in a
> C++ grammar related question below. Could you please help and give some of
> your advice? Thanks!
>
> In this code,
>
> ===== tmp.cpp =====
> int func(auto arg = 42) { return int(arg); }
> int value = func(); // Expected: with default argument = 42, and `auto`
> deduced into `int`.
> =====
>
> We expect that the default argument is 42, and `auto` is **deduced** into
> `int`.
>
> But actually, in the current C++ standard, this `auto` can be deduced by
> user-provided arguments, but **cannot** be deduced by default-provided
> arguments. C++ complains about we cannot choose `int` as the default type.
> Let's see the outputs.
>
> ===== C++ draft =====
> https://eel.is/c++draft/dcl.fct.default, which does not mentioned this
> case.
> =====
>
> ===== cppreference =====
> https://cppreference.com/w/cpp/language/default_arguments.html, which
> does not mentioned this case.
> =====
>
> ===== g++ (version=15) =====
> shyeyian_at_macbook tmp % g++-15 -std=c++26 tmp.cpp -o tmp
> tmp.cpp:2:17: error: no matching function for call to 'func()'
> 2 | int value = func();
> | ~~~~^~
> tmp.cpp:2:17: note: there is 1 candidate
> tmp.cpp:1:5: note: candidate 1: 'template<class auto:1> int func(auto:1)'
> 1 | int func(auto arg = 42) { return arg * 2; }
> | ^~~~
> tmp.cpp:1:5: note: template argument deduction/substitution failed:
> tmp.cpp:2:17: note: couldn't deduce template parameter 'auto:1'
> 2 | int value = func();
> | ~~~~^~
> =====
>
> ===== clang++ (version=22) =====
> shyeyian_at_macbook tmp % clang++ -std=c++26 tmp.cpp -o tmp
> tmp.cpp:2:13: error: no matching function for call to 'func'
> 2 | int value = func();
> | ^~~~
> tmp.cpp:1:5: note: candidate template ignored: couldn't infer template
> argument
> 'arg:auto'
> 1 | int func(auto arg = 42) { return arg * 2; }
> | ^
> 1 error generated.
> =====
>
> Because the default argument is a value, it always has a type, and then
> this `auto` here is always deducable. So that, In my perspective, this
> default argument should work with default type as `int`.
>
> Thank you for any feedbacks!
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>