[Bug c++/98122] New: [regression] Accessing union member through pointer-to-member is not a constant expression

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98122

            Bug ID: 98122
           Summary: [regression] Accessing union member through
                    pointer-to-member is not a constant expression
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.cencora at gmail dot com
  Target Milestone: ---

Following code does not compile on gcc-10 and newer in all std C++14-20 modes.

This used to work in gcc-9 and earlier.

union foo
{
    int a;
};

constexpr bool test()
{
    foo f{ .a = 42 };

    constexpr auto memPtr = &foo::a;

    return (f.*memPtr) == 42;
}

// error: accessing value of 'f' through a 'int' glvalue in a constant
expression
//     return (f.*memPtr) == 42;
static_assert(test(), "");