[C++] Rare bad buffer content alignment if sizeof(T) != alignof(T) (#7520)

* [C++] Add a failing unit test for #7516 (Rare bad buffer content alignment if sizeof(T) != alignof(T))

* [C++] Fix final buffer alignment when using an array of structs
* A struct can have an arbitrary size and therefore sizeof(struct) == alignof(struct)
  does not hold anymore as for value primitives.
* This patch fixes this by introducing alignment parameters to various
  CreateVector*/StartVector calls.
* Closes #7516

src/idl_parser.cpp

@@ -1614,6 +1614,7 @@ CheckedError Parser::ParseVector(const Type &type, uoffset_t *ovalue,
   });
   ECHECK(err);
 
+  const size_t alignment = InlineAlignment(type);
   const size_t len = count * InlineSize(type) / InlineAlignment(type);
   const size_t elemsize = InlineAlignment(type);
   const auto force_align = field->attributes.Lookup("force_align");
@@ -1623,7 +1624,8 @@ CheckedError Parser::ParseVector(const Type &type, uoffset_t *ovalue,
     if (align > 1) { builder_.ForceVectorAlignment(len, elemsize, align); }
   }
 
-  builder_.StartVector(len, elemsize);
+  // TODO Fix using element alignment as size (`elemsize`)!
+  builder_.StartVector(len, elemsize, alignment);
   for (uoffset_t i = 0; i < count; i++) {
     // start at the back, since we're building the data backwards.
     auto &val = field_stack_.back().first;

src/reflection.cpp

@@ -689,9 +689,10 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
             FLATBUFFERS_FALLTHROUGH();  // fall thru
           default: {                    // Scalars and structs.
             auto element_size = GetTypeSize(element_base_type);
+            auto element_alignment = element_size; // For primitive elements
             if (elemobjectdef && elemobjectdef->is_struct())
               element_size = elemobjectdef->bytesize();
-            fbb.StartVector(vec->size(), element_size);
+            fbb.StartVector(vec->size(), element_size, element_alignment);
             fbb.PushBytes(vec->Data(), element_size * vec->size());
             offset = fbb.EndVector(vec->size());
             break;