avoid constraint recursion in __basic_any

[ericniebler]

Description

closes #8037

as described in #8037, it is too easy when using any_resource and resource_ref to encounter mysterious compiler errors about recursive constraints. For example, the following code triggers the error:

struct derived_resource
{
  explicit derived_resource(cuda::mr::resource_ref<cuda::mr::device_accessible>);

  void* allocate(cuda::stream_ref, ::cuda::std::size_t, ::cuda::std::size_t);
  void deallocate(cuda::stream_ref, void*, ::cuda::std::size_t, ::cuda::std::size_t) noexcept;
  void* allocate_sync(::cuda::std::size_t, ::cuda::std::size_t);
  void deallocate_sync(void*, ::cuda::std::size_t, ::cuda::std::size_t) noexcept;
  bool operator==(derived_resource const&) const;
  bool operator!=(derived_resource const&) const;
  friend void get_property(derived_resource const&, cuda::mr::device_accessible) noexcept {}
};
static_assert(cuda::std::move_constructible<derived_resource>);

when asking if derived_resource is move-constructible, we check if there is a derived_resource constructor that accepts (an rvalue) derived_resource. there are two constructors that can take a derived_resource as an argument: the compiler-generated move constructor (the one we wanted to be used), and the constructor that takes a resource_ref because derived_resource is-a resource.

when checking if the derived_resource(resource_ref) constructor is viable, we instantiate the resource_ref(T&) constructor with T=derived_resource¹. that constructor checks, among other things, that the type T is move-constructible, completing the cycle in the type system.

the fix is to change __basic_any, the guts of resource_ref, to not constrain its constructor with checks for copyability or moveability. instead, we move those checks into the body of the constructor as static_asserts.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Footnotes

1. we shouldn't though because rvalues don't bind to lvalue references. i think that's a compiler bug. ↩

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