struct S { S () : s (42) {} ~S () {} int s; };
#ifdef TMPL
template <int N>
#endif
constexpr bool
foo ()
{
  if (__builtin_is_constant_evaluated ()) { return true; }
  S s;
  return true;
}
#ifdef TMPL
auto a = foo <42> ();
#else
auto a = foo ();
#endif
Both g++ and clang++ reject this with -std=c++20 if TMPL is not defined
g++ -S -std=c++20 -o /tmp/0.s /tmp/0.C
/tmp/0.C: In function ‘constexpr bool foo()’:
/tmp/0.C:9:5: error: variable ‘s’ of non-literal type ‘S’ in ‘constexpr’ function only available with ‘-std=c++23’ or ‘-std=gnu++23’
    9 |   S s;
      |     ^
/tmp/0.C:1:8: note: ‘S’ is not literal because:
    1 | struct S { S () : s (42) {} ~S () {} int s; };
      |        ^
/tmp/0.C:1:8: note:   ‘S’ does not have ‘constexpr’ destructor
clang++ -S -std=c++20 -o /tmp/0.s /tmp/0.C
/tmp/0.C:9:5: error: variable of non-literal type 'S' cannot be defined in a constexpr function before C++23
    9 |   S s;
      |     ^
/tmp/0.C:1:8: note: 'S' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors
    1 | struct S { S () : s (42) {} ~S () {} int s; };
      |        ^
1 error generated.
but 
g++ -S -DTMPL -std=c++20 -o /tmp/0.s /tmp/0.C
is quiet and
clang++ -S -DTMPL -std=c++20 -o /tmp/0.s /tmp/0.C
/tmp/0.C:9:5: error: variable of non-literal type 'S' cannot be defined in a constexpr function before C++23
    9 |   S s;
      |     ^
/tmp/0.C:1:8: note: 'S' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors
    1 | struct S { S () : s (42) {} ~S () {} int s; };
      |        ^
/tmp/0.C:13:10: error: no matching function for call to 'foo'
   13 | auto a = foo <42> ();
      |          ^~~~~~~~
/tmp/0.C:6:1: note: candidate template ignored: substitution failure [with N = 42]
    6 | foo ()
      | ^
2 errors generated.