[Clang][C++] It is not possible to initialize constants using the 'constexpr' static member of a function reference parameter

[polarlinda6]

struct A {
  constexpr static int value = 0;
};

void func(A& _) {
  constexpr auto result = _.value;
}

auto main() -> int {
  A a;
  func(a);
}

error message:

<source>:6:18: error: constexpr variable 'result' must be initialized by a constant expression
    6 |   constexpr auto result = _.value;
      |                  ^        ~~~~~~~
<source>:6:27: note: function parameter '_' with unknown value cannot be used in a constant expression
    6 |   constexpr auto result = _.value;
      |                           ^
<source>:5:14: note: declared here
    5 | void func(A& _) {
      |              ^

Additional Information:

Version: clang version 18.1.8
Link: godbolt

PS: The code compiles successfully on gcc and msvc.

[MitalAshok]

This is P2280R4 which Clang doesn't implement yet. It's tracked by #63139

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: PolarLinda6 (PolarLinda6)

```cpp struct A { constexpr static int value = 0; };

void func(A& _) {
constexpr auto result = _.value;
}

auto main() -> int {
A a;
func(a);
}

#### error message:

```txt
<source>:6:18: error: constexpr variable 'result' must be initialized by a constant expression
    6 |   constexpr auto result = _.value;
      |                  ^        ~~~~~~~
<source>:6:27: note: function parameter '_' with unknown value cannot be used in a constant expression
    6 |   constexpr auto result = _.value;
      |                           ^
<source>:5:14: note: declared here
    5 | void func(A& _) {
      |              ^

Additional Information:

Version: clang version 18.1.8
Link: godbolt

PS: The code compiles successfully on gcc and msvc.