[Bug c++/106357] New: [12/13 Regression] direct-list-initialization of enum class from a variable of another enum class type fails

[enolan at alumni dot cmu.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106357

            Bug ID: 106357
           Summary: [12/13 Regression] direct-list-initialization of enum
                    class from a variable of another enum class type fails
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: enolan at alumni dot cmu.edu
  Target Milestone: ---

This example fails with GCC 12:

enum class A {
    a
};
enum class B {
    b
};

int main() {
    A a{};
    B b{a}; 
}

---

P0138 added this language to the standard for C++17 (see:
https://eel.is/c++draft/dcl.init.list#3.8)

> if T is an enumeration with a fixed underlying type ([dcl.enum]) U, the
> initializer-list has a single element v, v can be implicitly converted to U,
> and the initialization is direct-list-initialization, the object is
> initialized with the value T(v) ([expr.type.conv]);

The above test case works in GCC 7-11 but appears to have regressed in GCC 12
and on trunk.

---

> the exact version of GCC

gcc version 12.1.1 20220507 (Red Hat 12.1.1-1) (GCC)

> the system type

Fedora release 37 (Rawhide)

> the options given when GCC was configured/built

Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --enable-libstdcxx-backtrace
--with-linker-hash-style=gnu --enable-plugin --enable-initfini-array
--with-isl=/builddir/build/BUILD/gcc-12.1.1-20220507/obj-x86_64-redhat-linux/isl-install
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-offload-defaulted --enable-gnu-indirect-function --enable-cet
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
--with-build-config=bootstrap-lto --enable-link-serialization=1

> the complete command line that triggers the bug

g++ -std=c++17 poc2.cpp

> the compiler output (error messages, warnings, etc.)

poc2.cpp: In function ‘int main()’:
poc2.cpp:10:9: error: cannot convert ‘A’ to ‘B’ in initialization
   10 |     B b{a};
      |         ^
      |         |
      |         A

> the preprocessed file (*.i*) that triggers the bug, generated by adding -save-temps to the complete compilation command, or, in the case of a bug report for the GNAT frontend, a complete set of source files (see below).

# 0 "poc2.cpp"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "poc2.cpp"
enum class A {
    a
};
enum class B {
    b
};

int main() {
    A a{};
    B b{a};
}