BLI: remove implicit casts between some span types

Casting pointers from one type to another does change the
value of the pointer in some cases. Therefore, casting a span
that contains pointers of one type to a span that contains
pointers of another type, is not generally safe. In practice, this
issue mainly comes up when dealing with classes that have a
vtable.

There are some special cases that are still allowed. For example,
adding const to the pointer does not change the address.
Also, casting to a void pointer is fine.

In cases where implicit conversion is disabled, but one is sure
that the cast is valid, an explicit call of `span.cast<NewType>()`
can be used.

1 files changed, 3 insertions, 2 deletions

diff --git a/source/blender/blenlib/BLI_span.hh b/source/blender/blenlib/BLI_span.hh
index 3f410efe908..8011b2f9abc 100644
--- a/source/blender/blenlib/BLI_span.hh
+++ b/source/blender/blenlib/BLI_span.hh
@@ -100,7 +100,7 @@ template<typename T> class Span {
     BLI_assert(size >= 0);
   }
 
-  template<typename U, typename std::enable_if_t<is_convertible_pointer_v<U, T>> * = nullptr>
+  template<typename U, typename std::enable_if_t<is_span_convertible_pointer_v<U, T>> * = nullptr>
   constexpr Span(const U *start, int64_t size) : data_(static_cast<const T *>(start)), size_(size)
   {
     BLI_assert(size >= 0);
@@ -135,7 +135,8 @@ template<typename T> class Span {
    * Support implicit conversions like the ones below:
    *   Span<T *> -> Span<const T *>
    */
-  template<typename U, typename std::enable_if_t<is_convertible_pointer_v<U, T>> * = nullptr>
+
+  template<typename U, typename std::enable_if_t<is_span_convertible_pointer_v<U, T>> * = nullptr>
   constexpr Span(Span<U> array) : data_(static_cast<const T *>(array.data())), size_(array.size())
   {
   }