[clang] Incorrect constexpr member function definition order dependency

[pdimov]

The following code

class X
{
public:

    constexpr int f( int x ) const
    {
        return g( x );
    }

private:

    template<class T>
    constexpr T g( T x ) const
    {
        return x;
    }
};

constexpr int x = X().f( 1 );

is rejected by Clang trunk (and all previous versions AFAICS) with

<source>:7:16: note: undefined function 'g<int>' cannot be used in a constant expression
    7 |         return g( x );
      |                ^

and in order to fix that, the definition of g needs to be moved above that of f.

But I don't think this order dependency is required by the standard, and other compilers accept the code.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Peter Dimov (pdimov)

The following code ``` class X { public:

constexpr int f( int x ) const
{
    return g( x );
}

private:

template<class T>
constexpr T g( T x ) const
{
    return x;
}

};

constexpr int x = X().f( 1 );

is [rejected](https://godbolt.org/z/ox1E8nG7M) by Clang trunk (and all previous versions AFAICS) with

<source>:7:16: note: undefined function 'g<int>' cannot be used in a constant expression
7 | return g( x );
| ^

and in order to fix that, the definition of `g` needs to be moved above that of `f`.

But I don't think this order dependency is required by the standard, and [other](https://godbolt.org/z/qMe1dPMTM) [compilers](https://godbolt.org/z/dnrz95orn) accept the code.
</details>

[zyn0217]

#73232