[Bug c++/91351] New: -fstrict-enums generates incorrect code

[fiesh at zefix dot tv]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91351

            Bug ID: 91351
           Summary: -fstrict-enums generates incorrect code
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fiesh at zefix dot tv
  Target Milestone: ---

The following code results in different runtime behavior depending on whether
it is compiled with -fstrict-enums or not:

#include <iostream>

enum E { e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, e10, e11, e12, e13, e14, e15,
e16, e17, e18, e19, e20, e21, e22, e23, e24, e25 };

void g()
{
        std::cout << "e11 || e12 || e13\n";
}
void j()
{
        std::cout << "default\n";
}

void f(E e)
{
        switch (e) {
                case e11:
                case e12:
                case e13: g(); break;
                case e24: break;
                case e14:
                case e15: break;
                default: j(); break;
        }
}

int main()
{
        volatile const E e{e3};
        f(e);
}


(The volatile is necessary to prevent optimization.)

Without -fstrict-enums: "default"
With -fstrict-enums: "e11 || e12 || e13"

Bisecting this behavior says the first bad commit is
37db4f8d72cfc87716a729b38aa2f42097cdbf1f

Also see 

https://godbolt.org/z/NmlKXq

for the generated assembly.