On Wed, Nov 15, 2023 at 12:09 PM Pavel Vazharov <freakpv_at_[hidden]> wrote:
> #include <cstdio>
>
> template <typename Tag = decltype([]{})>
> void fun()
> {
> static int counter = 0;
> std::printf("addr:%p value:%i\n", &counter, counter);
> }
>
> int main()
> {
> fun();
> fun();
> fun();
>
> return 0;
> }
>
> This prints something like
>
> addr:0x40401c value:0
> addr:0x404020 value:0
> addr:0x404024 value:0

Does the Standard mandate that every C++23 compiler behaves this way?
Or does it just so happen that this code behaves the way it behaves on
all the major compilers? What I mean is: Could the compiler create the
default template argument in one place and then share that type among
the instantiations? Or, does the Standard mandate that the compiler
produce a program that behaves as though:

     fun();
     fun();
     fun();

had been written as:

     fun< decltype([]{}) >();
     fun< decltype([]{}) >();
     fun< decltype([]{}) >();

With the way the C++23 Standard is currently written, could these
three lines have the same type as the default template parameter,
almost as though it had been written:

     auto some_static_duration_lambda_object = []{};
     fun< decltype(some_static_duration_lambda_object) >();
     fun< decltype(some_static_duration_lambda_object) >();
     fun< decltype(some_static_duration_lambda_object) >();