Here f<int>() is not constexpr in C++20 (but is in C++23 after P2242R3) due to the local variable a of non-literal type A, so the testcase is invalid:

struct A { ~A(); };

void g();

template<class T>
constexpr int f() {
  if (__builtin_is_constant_evaluated())
    return 42;
  g();
  A a;
}

constexpr int n = f<int>();

But our diagnostic incorrectly blames the call to the non-constexpr g() for f<int>() being non-constexpr, rather than the variable a:

constexpr-nonliteral.C:13:25: error: ‘constexpr int f() [with T = int]’ called in a constant expression
   13 | constexpr int n = f<int>();
      |                   ~~~~~~^~
constexpr-nonliteral.C:6:15: note: ‘constexpr int f() [with T = int]’ is not usable as a ‘constexpr’ function because:
    6 | constexpr int f() {
      |               ^
constexpr-nonliteral.C:9:4: error: call to non-‘constexpr’ function ‘void g()’
    9 |   g();
      |   ~^~
constexpr-nonliteral.C:3:6: note: ‘void g()’ declared here
    3 | void g();
      |      ^