Prevent make_span from working with vectors and arrays of pointers (#8735)

* Prevent `make_span` from working with vectors and arrays of pointers

* support `make_structs_span` for little-endian

* fix build: add the required parentheses

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

Author: are-you-tilted-already

include/flatbuffers/array.h

@@ -39,10 +39,11 @@ class Array {
   typedef VectorConstIterator<T, return_type, uoffset_t> const_iterator;
   typedef VectorReverseIterator<const_iterator> const_reverse_iterator;
 
-  // If T is a LE-scalar or a struct (!scalar_tag::value).
+  // If T is a non-pointer and a LE-scalar or a struct (!scalar_tag::value).
   static FLATBUFFERS_CONSTEXPR bool is_span_observable =
-      (scalar_tag::value && (FLATBUFFERS_LITTLEENDIAN || sizeof(T) == 1)) ||
-      !scalar_tag::value;
+      !std::is_pointer<T>::value &&
+          ((scalar_tag::value && (FLATBUFFERS_LITTLEENDIAN || sizeof(T) == 1)) ||
+      !scalar_tag::value);
 
   FLATBUFFERS_CONSTEXPR uint16_t size() const { return length; }
 

include/flatbuffers/vector.h

@@ -172,7 +172,8 @@ class Vector {
       scalar_tag;
 
   static FLATBUFFERS_CONSTEXPR bool is_span_observable =
-      scalar_tag::value && (FLATBUFFERS_LITTLEENDIAN || sizeof(T) == 1);
+      scalar_tag::value && !std::is_pointer<T>::value &&
+      (FLATBUFFERS_LITTLEENDIAN || sizeof(T) == 1);
 
   SizeT size() const { return EndianScalar(length_); }
 
@@ -359,6 +360,22 @@ FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const uint8_t> make_bytes_span(
   return span<const uint8_t>(vec.Data(), vec.size() * sizeof(U));
 }
 
+#if FLATBUFFERS_LITTLEENDIAN
+
+template <class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<U> make_structs_span(
+    Vector<const U*>& vec) FLATBUFFERS_NOEXCEPT {
+  return span<U>(reinterpret_cast<U*>(vec.data()), vec.size());
+}
+
+template <class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const U> make_structs_span(
+    const Vector<const U*>& vec) FLATBUFFERS_NOEXCEPT {
+  return span<const U>(reinterpret_cast<const U*>(vec.data()), vec.size());
+}
+
+#endif
+
 // Convenient helper functions to get a span of any vector, regardless
 // of whether it is null or not (the field is not set).
 template <class U>
@@ -377,6 +394,23 @@ FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const U> make_span(
   return ptr ? make_span(*ptr) : span<const U>();
 }
 
+#if FLATBUFFERS_LITTLEENDIAN
+
+template <class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<U> make_structs_span(
+    Vector<const U*>* ptr) FLATBUFFERS_NOEXCEPT {
+  return ptr ? make_span(*ptr) : span<U>();
+}
+
+template <class U>
+FLATBUFFERS_CONSTEXPR_CPP11 flatbuffers::span<const U> make_structs_span(
+    const Vector<const U*>* ptr) FLATBUFFERS_NOEXCEPT {
+  return ptr ? make_span(*ptr) : span<const U>();
+}
+
+#endif
+
+
 // Represent a vector much like the template above, but in this case we
 // don't know what the element types are (used with reflection.h).
 class VectorOfAny {