Confusing error messages about non-literal types being used in constant expressions

[tbaederr]

Consider this example code:

class Base {
public:
  virtual ~Base() {}
};
constexpr int test() {
  Base b;
  return 0;
}

clang -std=c++20 complains:

<source>:8:8: error: variable of non-literal type 'Base' cannot be defined in a constexpr function before C++2b
  Base b;
       ^
<source>:3:11: note: 'Base' is not literal because its destructor is not constexpr
  virtual ~Base() {}
          ^
1 error generated.

However, using -std=c++2b instead gives:

<source>:7:15: error: constexpr function never produces a constant expression [-Winvalid-constexpr]
constexpr int test() {
              ^
<source>:8:8: note: non-literal type 'Base' cannot be used in a constant expression
  Base b;
       ^
1 error generated.

See: https://godbolt.org/z/aEbYKxfjT

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

I think clang is just being more aggressive in diagnosing that test() can never be used in a constant expression, see this godbolt: https://godbolt.org/z/xcsEY98sf where I add constexpr to the variable declaration of b.

The error message could be improved.

[tbaederr]

Right, but why is is saying that part about using C++2b? Are non-literal types allowed in constexpr functions in C++2b or not?