`<mdspan>`: `std::extents`'s CTAD unconditionally constrains argument types to be default-constructible

[hewillk]

llvm-project/libcxx/include/__mdspan/extents.h

Lines 457 to 459 in f5031c6

	// Deduction guide for extents
	template <class... _IndexTypes>
	extents(_IndexTypes...) -> extents<size_t, size_t((_IndexTypes(), dynamic_extent))...>;

..which rejects the following code:

https://godbolt.org/z/47vfGne7s

#include <mdspan>

struct S {
  S(size_t);
  operator size_t() const noexcept;
};

int main() {
  int* p;
  std::mdspan ms(p, S{3}); // ok
  std::extents exts(S{3}); // error
}

[ldionne]

@crtrott or @dalg24 Can you folks take a look?

[crtrott]

Yeah this is a bug, let me try to fix this.

[llvmbot]

@llvm/issue-subscribers-bug

Author: Hewill Kang (hewillk)

https://github.com/llvm/llvm-project/blob/f5031c655ddab72bdeccf5f830a17448926254e2/libcxx/include/__mdspan/extents.h#L457-L459

..which rejects the following code:

https://godbolt.org/z/47vfGne7s

#include <mdspan>

struct S {
  S(size_t);
  operator size_t() const noexcept;
};

int main() {
  int* p;
  std::mdspan ms(p, S{3}); // ok
  std::extents exts(S{3}); // error
}

[crtrott]

I believe changing the deduction guide implementation to this:

template <class... _IndexTypes> 
extents(_IndexTypes...) -> extents<size_t, size_t((sizeof(_IndexTypes), dynamic_extent))...>;

should fix the problem. Any thoughts?

[ldionne]

Oh, I get it. We were using _IndexTypes() just as a way to get a variadic expansion but that was forcing _IndexTypes to be default constructible. Yeah your fix sounds good.

[crtrott]

Looks like I get warnings for left side of the operator not having any effect. This does not trigger warnings:

template <class... _IndexTypes> 
extents(_IndexTypes...) -> extents<size_t, size_t((sizeof(_IndexTypes)? dynamic_extent:dynamic_extent))...>;

Thoughts?

[crtrott]

Alternative would be introducing

constexpr size_t just_dynamic_extent(size_t) { return dynamic_extent; }

helper function.

[ldionne]

This would be the usual way of dealing with the warning (cast to void):

template <class... _IndexTypes> 
extents(_IndexTypes...) -> extents<size_t, size_t(((void)sizeof(_IndexTypes), dynamic_extent))...>;

[crtrott]

ah ok I like that.