Example:
# dnf update foo.rpm bar
> Last metadata expiration check: 0:44:12 ago on Tue 31 Jul 2018 10:26:17 PM CEST.
> Can not load RPM file: foo.rpm.
> Segmentation fault (core dumped)

The most low-level explanation is a NULL pointer dereference
(most recent frames go first):

- brief stack trace (somewhat sifted by coredumpctl compared to
  58 frames shown in gdb; there was just a single thread):

> #0  0x00007f8d916f1f79 pool_whatprovides (libdnf.so.2)
> #1  0x00007f8d916f4d77 _ZN6libdnf5Query4Impl5applyEv (libdnf.so.2)
> #2  0x00007f8d916f5662 _ZN6libdnf5Query5emptyEv (libdnf.so.2)
> #3  0x00007f8d916f91d0 _ZN6libdnf8Solution15getBestSolutionEPKcP8_DnfSackP7_HyFormbbbbb (libdnf.so.2)
> #4  0x00007f8d91661203 hy_subject_get_best_solution (libdnf.so.2)
> #5  0x00007f8d8b0c93f6 get_best_parser (_hawkey.so)
> #6  0x00007f8d8b0c95cb get_best_solution (_hawkey.so)
> #7  0x00007f8d92936084 _PyMethodDef_RawFastCallKeywords (libpython3.7m.so.1.0)
> #8  0x00007f8d9295f69d _PyMethodDescr_FastCallKeywords (libpython3.7m.so.1.0)
> #9  0x00007f8d9299881b call_function (libpython3.7m.so.1.0)
> #10 0x00007f8d928ef8e6 _PyEval_EvalCodeWithName (libpython3.7m.so.1.0)
> #11 0x00007f8d92935971 _PyFunction_FastCallKeywords (libpython3.7m.so.1.0)
> #12 0x00007f8d92993ab4 call_function (libpython3.7m.so.1.0)
> #13 0x00007f8d929357ca function_code_fastcall (libpython3.7m.so.1.0)
> #14 0x00007f8d92993ab4 call_function (libpython3.7m.so.1.0)
> #15 0x00007f8d929357ca function_code_fastcall (libpython3.7m.so.1.0)
> #16 0x00007f8d92993ab4 call_function (libpython3.7m.so.1.0)
> #17 0x00007f8d929357ca function_code_fastcall (libpython3.7m.so.1.0)
> #18 0x00007f8d92993ab4 call_function (libpython3.7m.so.1.0)
> #19 0x00007f8d929357ca function_code_fastcall (libpython3.7m.so.1.0)
> #20 0x00007f8d92993c69 call_function (libpython3.7m.so.1.0)
> #21 0x00007f8d929357ca function_code_fastcall (libpython3.7m.so.1.0)
> #22 0x00007f8d92993c69 call_function (libpython3.7m.so.1.0)
> #23 0x00007f8d928ef8e6 _PyEval_EvalCodeWithName (libpython3.7m.so.1.0)
> #24 0x00007f8d92935971 _PyFunction_FastCallKeywords (libpython3.7m.so.1.0)
> #25 0x00007f8d92993c69 call_function (libpython3.7m.so.1.0)
> #26 0x00007f8d928ef8e6 _PyEval_EvalCodeWithName (libpython3.7m.so.1.0)
> #27 0x00007f8d92935971 _PyFunction_FastCallKeywords (libpython3.7m.so.1.0)
> #28 0x00007f8d92994a85 call_function (libpython3.7m.so.1.0)
> #29 0x00007f8d928ef8e6 _PyEval_EvalCodeWithName (libpython3.7m.so.1.0)
> #30 0x00007f8d928f08a3 PyEval_EvalCodeEx (libpython3.7m.so.1.0)
> #31 0x00007f8d928f08cb PyEval_EvalCode (libpython3.7m.so.1.0)
> #32 0x00007f8d92a199ae run_mod (libpython3.7m.so.1.0)
> #33 0x00007f8d92a19d46 PyRun_FileExFlags (libpython3.7m.so.1.0)
> #34 0x00007f8d92a1be08 PyRun_SimpleFileExFlags (libpython3.7m.so.1.0)
> #35 0x00007f8d92a1d7a2 pymain_run_file (libpython3.7m.so.1.0)
> #36 0x00007f8d92a1e0e0 _Py_UnixMain (libpython3.7m.so.1.0)
> #37 0x00007f8d9248f0e3 __libc_start_main (libc.so.6)
> #38 0x00005605f7e3d08a _start (python3.7)

- detailed trace of the 4 most recent frames:
> #0  0x00007f8d916f1f79 in pool_whatprovides (d=18721, pool=0x5605f9e75ea0) at /usr/include/solv/pool.h:327
>           r_id = 18721
>           match_in = {
>           num = 18721,
>           pset = <optimized out>,
>           reldep = 18721,
>           str = <optimized out>
>           }
>           __for_range = <optimized out>
>           pool = 0x5605f9e75ea0
>           p = <optimized out>
>           pp = <optimized out>

> 323│ static inline Id pool_whatprovides(Pool *pool, Id d)
> 324│ {
> 325│   if (!ISRELDEP(d))
> 326│     {
> 327├>      if (pool->whatprovides[d])
> 328│         return pool->whatprovides[d];
> 329│     }

> (gdb) p *pool
>> $1 = {
>>   appdata = 0x0,
>>   [...]
>>   whatprovides = 0x0,
>>   [...]
>> }
> (gdb) p d
>> $2 = 18721

> #1  0x00007f8d916f1f79 in libdnf::Query::Impl::filterProvidesReldep(libdnf::Filter const&, _Map*) (this=this@entry=0x5605f9ebd710, f=..., m=m@entry=0x7fffda91bd20) at /usr/src/debug/libdnf-0.16.1-2.fc29.x86_64/libdnf/sack/query.cpp:1407
>           r_id = 18721
>           match_in = {
>           num = 18721,
>           pset = <optimized out
>,
>           reldep = 18721,
>           str = <optimized out>
>           }
>           __for_range = <optimized out>
>           pool = 0x5605f9e75ea0
>           p = <optimized out>
>           pp = <optimized out>

> 1398│ void
> 1399│ Query::Impl::filterProvidesReldep(const Filter & f, Map *m)
> 1400│ {
> 1401│     Pool *pool = dnf_sack_get_pool(sack);
> 1402│     Id p, pp;
> 1403│
> 1404│     dnf_sack_make_provides_ready(sack);
> 1405│     for (auto match_in : f.getMatches()) {
> 1406│         Id r_id = match_in.reldep;
> 1407├>        FOR_PROVIDES(p, pp, r_id)
> 1408│             MAPSET(m, p);
> 1409│     }
> 1410│ }

> #2  0x00007f8d916f4d77 in libdnf::Query::Impl::apply() (this=0x5605f9ebd710) at /usr/src/debug/libdnf-0.16.1-2.fc29.x86_64/libdnf/sack/query.cpp:1800
>           f = {
>           pImpl = std::shared_ptr<libdnf::Filter::Impl
> (use count 2, weak count 0) = {
>             get() = 0x5605f9e5c7c0
>           }
>        	}
>        	__for_range = std::vector of length 1, capacity 1 = {{
>             pImpl = std::shared_ptr<libdnf::Filter::Impl
> (use count 2, weak count 0) = {
>               get() = 0x5605f9e5c7c0
>             }
>           }} 
>           pool = <optimized out>
>           m = {
>           map = 0x5605fa1a6f90 "",
>        	  size = 7595
>           }
>           __PRETTY_FUNCTION__ = "void libdnf::Query::Impl::apply()"

> 1798│             case HY_PKG_PROVIDES:
> 1799│                 assert(f.getMatchType() == _HY_RELDEP);
> 1800├>                filterProvidesReldep(f, &m);
> 1801│                 break;

> #3  0x00007f8d916f54ec in libdnf::Query::apply() (this=this@entry=0x5605f9e795d0) at /usr/include/c++/8/bits/unique_ptr.h:342

> 340│	   /// Return the stored pointer.
> 341│	   pointer
> 342├>      get() const noexcept
> 343│	   { return _M_t._M_ptr(); }

Note the other hawkey-related issue (_hawkey.so present in the trace
as well): [bug 1610456]