[Bug c++/114219] New: [expr.const] lvalue-to-rvalue conversion is not diagnosed to disqualify constant expressions for empty classes

["janschultke at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114219

            Bug ID: 114219
           Summary: [expr.const] lvalue-to-rvalue conversion is not
                    diagnosed to disqualify constant expressions for empty
                    classes
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janschultke at googlemail dot com
  Target Milestone: ---

https://godbolt.org/z/jhcP8WPn8

Code to reproduce
=================

struct S {};

constexpr int f(S s) {
    constexpr S _ = s;
    return 0;
}

S s;
constexpr int _ = f(s);



Issue description
=================

This code compiles, but should produce errors for both constexpr
initializations.

According to [expr.const] p5.9, the initializers of _ cannot be constant
expression because they contain lvalue-to-rvalue conversion of s, whose
lifetime did not begin within the evaluation of the constant expression, and
which is not usable in constant expressions.

It shouldn't matter whether S is an empty class or has members; the
lvalue-to-rvalue conversion in itself disqualifies expressions from being
constant expressions.