[Clang] Cannot use a static constexpr/consteval member function to initialize a static constexpr member

[YexuanXiao]

I've noticed that static constexpr/consteval member functions exhibit very strange behavior—they can initialize static non-constexpr members but fail to work for static constexpr members.
Example (Godbolt):

struct A
{
    static inline constexpr bool test_constexpr() noexcept
    {
        return true;
    }
    static inline constexpr bool t_expr = test_constexpr(); // can't compile
    static inline constexpr bool test_consteval() noexcept
    {
        return true;
    }
    static inline constexpr bool t_eval = test_consteval();  // can't compile
    static inline bool t_non_const = test_constexpr();
};
inline constexpr bool t_expr = A::test_constexpr();
inline constexpr bool t_eval = A::test_consteval();

Output:

<source>:8:34: error: constexpr variable 't_expr' must be initialized by a constant expression
    8 |     static inline constexpr bool t_expr = test_constexpr();
      |                                  ^        ~~~~~~~~~~~~~~~~
<source>:8:43: note: undefined function 'test_constexpr' cannot be used in a constant expression
    8 |     static inline constexpr bool t_expr = test_constexpr();
      |                                           ^
<source>:3:34: note: declared here
    3 |     static inline constexpr bool test_constexpr() noexcept
      |                                  ^
<source>:15:34: error: constexpr variable 't_eval' must be initialized by a constant expression
   15 |     static inline constexpr bool t_eval = test_consteval();
      |                                  ^        ~~~~~~~~~~~~~~~~
<source>:15:43: note: undefined function 'test_consteval' cannot be used in a constant expression
   15 |     static inline constexpr bool t_eval = test_consteval();
      |                                           ^
<source>:10:34: note: declared here
   10 |     static inline constexpr bool test_consteval() noexcept
      |                                  ^
2 errors generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: YexuanXiao (YexuanXiao)

I've noticed that consteval member functions exhibit very strange behavior—they can initialize static non-constexpr members but fail to work for static constexpr members. Example ([Godbolt](https://godbolt.org/z/Ynn5xf78s)): ```cpp struct A { static inline constexpr bool test_constexpr() noexcept { return true; } static inline constexpr bool t_expr = test_constexpr(); // can't compile static inline constexpr bool test_consteval() noexcept { return true; } static inline constexpr bool t_eval = test_consteval(); // can't compile static inline bool t_non_const = test_constexpr(); }; inline constexpr bool t_expr = A::test_constexpr(); inline constexpr bool t_eval = A::test_consteval(); ``` Output: ```cpp <source>:8:34: error: constexpr variable 't_expr' must be initialized by a constant expression 8 | static inline constexpr bool t_expr = test_constexpr(); | ^ ~~~~~~~~~~~~~~~~ <source>:8:43: note: undefined function 'test_constexpr' cannot be used in a constant expression 8 | static inline constexpr bool t_expr = test_constexpr(); | ^ <source>:3:34: note: declared here 3 | static inline constexpr bool test_constexpr() noexcept | ^ <source>:15:34: error: constexpr variable 't_eval' must be initialized by a constant expression 15 | static inline constexpr bool t_eval = test_consteval(); | ^ ~~~~~~~~~~~~~~~~ <source>:15:43: note: undefined function 'test_consteval' cannot be used in a constant expression 15 | static inline constexpr bool t_eval = test_consteval(); | ^ <source>:10:34: note: declared here 10 | static inline constexpr bool test_consteval() noexcept | ^ 2 errors generated. Compiler returned: 1 ```

[frederick-vs-ja]

Other implementations behave in the same way (Godbolt link). This perhaps deserves a CWG issue.

CWG2335 seems related but it covers different problems.

[frederick-vs-ja]

This perhaps deserves a CWG issue.

Looks like that we (implementations, possibly along with the standard) shouldn't support this.

Note that the initializer of a static constexpr data member of a class is not a complete-class context ([class.mem.general]/8), so the class itself is incomplete there, and the constant evaluation fails.

Given that the in-class function body of the member function is a a complete-class context, in order to accept your original (seemingly innocent) example, we need to establish wording to reject the following.

struct B
{
    static constexpr bool test_constexpr() noexcept
    {
        return !t_expr;
    }
    static constexpr bool t_expr = test_constexpr();
};

Also, the value of static constexpr data member of a class can be hardly lazily evaluated, as it can affect everything after it in the class.

Now I believe legitimation of the original example will need a very, very detailed paper targeting EWG.

[shafik]

We have a lot of open complete class context issues, the last time we discussed this in the context of reflection review we determined that we would want to address all the issues at once and that would require some work into a paper. I think @opensdh has some solid ideas about how to approach this but it is matter of someone finding the time to do this.