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is_comparable_to_nullptr for better static_assert (#975)

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* is_comparable_to_nullptr for better static_assert

Trying `gsl::not_null<char> p2{ 0 };` on VS2019 the current implementation would trigger

>error C2446 : '!=' : no conversion from 'nullptr' to 'int'
>message: A native nullptr can only be converted to bool or , using reinterpret_cast, to an integral type
>message: see reference to class template instantiation 'gsl::not_null<char>' being compiled
>error C2955 : 'std::is_convertible' : use of class template requires template argument list
>message: see declaration of 'std::is_convertible'
>error C2039 : 'value' : is not a member of 'std::is_convertible<_From,_To>'
>error C2065 : 'value' : undeclared identifier


The new implementation gives much shorter and clearer message and does exactly as the `static_assert` intends to do:

> error C2338: T cannot be compared to nullptr.
> message : see reference to class template instantiation 'gsl::not_null<char *>' being compiled

* Update include/gsl/pointers

Co-authored-by: Casey Carter <cartec69@gmail.com>

Co-authored-by: Werner Henze <w.henze@avm.de>
Co-authored-by: Casey Carter <cartec69@gmail.com>
This commit is contained in:
beinhaerter 2021-02-24 23:39:13 +01:00 committed by GitHub
parent 176c92e802
commit ef0ffefe52
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1 changed files with 10 additions and 2 deletions
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12
include/gsl/pointers
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@ -32,6 +32,15 @@
namespace gsl
{
namespace details
{
template<typename T, typename = void>
struct is_comparable_to_nullptr : std::false_type {};
template <typename T>
struct is_comparable_to_nullptr<T, std::enable_if_t<std::is_convertible<decltype(std::declval<T>() != nullptr), bool>::value>> : std::true_type {};
} // namespace details
//
// GSL.owner: ownership pointers
//
@ -68,8 +77,7 @@ template <class T>
class not_null
{
public:
static_assert(std::is_convertible<decltype(std::declval<T>() != nullptr), bool>::value,
"T cannot be compared to nullptr.");
static_assert(details::is_comparable_to_nullptr<T>::value, "T cannot be compared to nullptr.");
template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
constexpr not_null(U&& u) : ptr_(std::forward<U>(u))