Memset is performed over HeapWord* which is "class {private: char* i;}" in

src/hotspot/share/gc/parallel/objectStartArray.cpp: In member function 'void ObjectStartArray::set_covered_region(MemRegion)':
src/hotspot/share/gc/parallel/objectStartArray.cpp:106:56: error: 'void* memset(void*, int, size_t)' writing to an object of type 'class HeapWord' with 'private' member 'HeapWord::i' [-Werror=class-memaccess]
     memset(_blocks_region.end(), clean_block, expand_by);
                                                        ^
In file included from /src/hotspot/share/utilities/align.hpp:28,
                 from src/hotspot/share/runtime/globals.hpp:29,
                 from src/hotspot/share/memory/allocation.hpp:28,
                 from src/hotspot/share/classfile/classLoaderData.hpp:28,
                 from src/hotspot/share/precompiled/precompiled.hpp:34:
src/hotspot/share/utilities/globalDefinitions.hpp:174:7: note: 'class HeapWord' declared here
 class HeapWord {
       ^~~~~~~~
   ... (rest of output omitted)

HeapWord is defined as a "fake" class (with a private char* member) that is never really instantiated. -Wclass-memaccess is being triggered because the use of memset is bypassing access control to the data member of HeapWord.

And clean_block is -1.

Possible fix is to add (char*) cast similar to src/hotspot/share/gc/parallel/mutableNUMASpace.cpp

Another approach is to add HeapWord constructor that sets i and use placement new.