`vir::struct_size_v` returns zero for structs that contain types with certain constructors

[the-argus]

The following does not compile due to the static assert failing:

class Ambiguous
{
    private:
        int i;
    public:
        constexpr Ambiguous(int _i) : i(_i) {}
};

struct ContainsAmbiguous
{
    float a;
    float b;
    Ambiguous ambiguous;
};

static_assert(vir::struct_size_v<ContainsAmbiguous> != 0);

To see what's wrong I tried reproducing the contents of requires clauses in vir::detail::brace_constructible_impl:

inline auto testinstance = ContainsAmbiguous{
    {vir::detail::anything_but_base_of<ContainsAmbiguous>{}},
    {vir::detail::anything_but_base_of<ContainsAmbiguous>{}},
    {vir::detail::anything_but_base_of<ContainsAmbiguous>{}}, // call to constructor of "Ambiguous" is ambiguous
};

Whereas this compiles:

inline auto testinstance = ContainsAmbiguous{
    vir::detail::anything_but_base_of<ContainsAmbiguous>{},
    vir::detail::anything_but_base_of<ContainsAmbiguous>{},
    vir::detail::anything_but_base_of<ContainsAmbiguous>{},
};

That's a recreation of this clause from brace_constructible_impl:

requires { T{{((void)Indexes, anything_but_base_of<T>())}...}; }

The additional curly braces being added in c1e9325ca880b908746f0c17c02df379889faa6d.

So it seems like although the additional braces prevent confusion due to brace elision they also opt into explicit constructors of the member variables which introduces ambiguity. Not sure what the correct approach here is, if there's a way to make both work at once.

[mattkretz]

Thank you for the detailed report. This looks tricky, but should be easy to solve with C++26. I'll have to add tests and experiment with possible workarounds.