[Bug c++/101295] New: constexpr destructor: ''result_decl' not supported by dump_expr<expression error>' is not a constant expression

[Bug c++/101295] New: constexpr destructor: ''result_decl' not supported by dump_expr<expression error>' is not a constant expression

[markus.kuehni at triviso dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101295

            Bug ID: 101295
           Summary: constexpr destructor: ''result_decl' not supported by
                    dump_expr<expression error>' is not a constant
                    expression
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: markus.kuehni at triviso dot ch
  Target Milestone: ---

Trying to conceptualize a constexpr capable, optimizer friendly linked list
reference tracking smart pointer, I get this error:

<source>: In function 'int main()':
<source>:49:16:   in 'constexpr' expansion of 'test()'
<source>:49:16:   in 'constexpr' expansion of
'(&<anonymous>)->myptr<Obj>::~myptr()'
<source>:49:16: error: ''result_decl' not supported by dump_expr<expression
error>' is not a constant expression
   49 |     return test();
...

I'm not sure whether the code is standards compliant, but the error message is
broken anyway, it seems. 

The same code works as I expect on clang, but creates an ICE on msvc. I tried
to create a minimal example that still shows the intention of the code. 

https://godbolt.org/z/sod6oYE8T

```
// Minimized code of a linked list reference tracking smart pointer.
template<typename T>
struct myptr {
private:
    T * p;
    myptr * next;
public:
    constexpr myptr(T * p) noexcept : p(p), next(p ? p->first : nullptr) {
        if (p) {
            p->first = this;
        }
    }
    constexpr myptr(const myptr & other) noexcept : myptr(other.p) {}

    constexpr ~myptr() noexcept {
        if (p) {
            myptr ** pnext = &(p->first);
            while (*pnext != this) { 
                pnext = &((*pnext)->next);
            }
            *pnext = next;
            next = nullptr;
            if (!p->first) {
                delete p;
            }
            p = nullptr;
        }
    }
};

struct Obj {
private:
    struct myptr<Obj> * first{};
    friend struct myptr<Obj>;
};

constexpr myptr<Obj> fun() {
    myptr<Obj> obj(new Obj);
    return obj;
}

// Force consteval to prove the myptr is constexpr capable.
consteval int test() {
    fun();
    return 42;
}

int main() {
    return test();
}
```

[Bug c++/101295] constexpr destructor: ''result_decl' not supported by dump_expr<expression error>' is not a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101295

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2023-04-13
      Known to fail|                            |10.4.0, 11.3.0, 12.2.0,
                   |                            |13.0
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.