g++ does not resolve decltype(nullptr) to base type nullptr_t, instead it generate following: 

  < 1><0x000005d3>    DW_TAG_unspecified_type
                      DW_AT_name                  "decltype(nullptr)"

g++ should generate something like this:

< 1><0x000005d3>    DW_TAG_base_type
                      DW_AT_name                  "nullptr_t"
                      DW_AT_encoding              DW_ATE_boolean
                      DW_AT_byte_size             0x00000004

main() {
    int i;
    decltype(nullptr) np;

    np = 0;
    i = sizeof(np);
}