[Bug c++/110150] New: std::optional inside template leads to long compilation time

[Bug c++/110150] New: std::optional inside template leads to long compilation time

[stevenxia990430 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110150

            Bug ID: 110150
           Summary: std::optional inside template leads to long
                    compilation time
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stevenxia990430 at gmail dot com
  Target Milestone: ---

The following program is very slow to compile (>40 seconds). Tested on
GCC-trunk.

To quickly reproduce: https://gcc.godbolt.org/z/rsT3n38sY
```
#include <optional>
template<typename T>
void f(T x)
{
    f(std::optional<T>(x));
}
int main()
{
    using std::optional;
    optional<int> x;
    f(x);
    return 0;
}
```

[Bug c++/110150] std::optional inside template leads to long compilation time

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110150

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |compile-time-hog

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am not shocked because even this takes some time (not as much as the original
one):
```
template <typename T>
struct optional
{
        T a;
        optional();
        template<typename O>
        optional(const optional<O>&);
};

template<typename T>
void f(T x)
{
    f(optional<T>(x));
}
int main()
{
   // using std::optional;
    optional<int> x;
    f(x);
    return 0;
}
```

There is an enable if on the optional constructor in the std::optional case
which makes the time increase even more.

I doubt there is much to be optimized here except maybe lowering the template
depth limit (from the current 900, that is -ftemplate-depth=900 is the
default).

[Bug c++/110150] std::optional inside template leads to long compilation time

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110150

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
basically you are getting optional<optional<.....<int> ...>>>>

[Bug c++/110150] std::optional inside template leads to long compilation time

[stevenxia990430 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110150

--- Comment #3 from Steven Xia <stevenxia990430 at gmail dot com> ---
(In reply to Andrew Pinski from comment #2)
> basically you are getting optional<optional<.....<int> ...>>>>

Understood, thanks for the quick reply