template with varargs pragma causes AssertionDefect

[demotomohiro]

mytests/testvarargs.nim:

template foo() {.varargs.} =
  for x in unpack():
    discard x

./bin/nimony  c mytests/testvarargs.nim 
bin/nimsem  --isSystem m nifcache/sysvq0asl.1.nif nifcache/sysvq0asl.2.nif nifcache/sysvq0asl.2.idx.nif
bin/nimsem  m nifcache/tescllp2p1.1.nif nifcache/tescllp2p1.2.nif nifcache/tescllp2p1.2.idx.nif
cc -c   -Ivendor/mimalloc/include vendor/mimalloc/src/static.c -o nifcache/static.o
bin/hexer --bits:64 nifcache/tescllp2p1.2.nif
src/hexer/hexer.nim(94) hexer
src/hexer/hexer.nim(91) handleCmdLine
src/hexer/nifcgen.nim(1415) expand
src/hexer/pipeline.nim(39) transform
src/hexer/iterinliner.nim(411) elimForLoops
src/hexer/iterinliner.nim(378) transformStmt
src/hexer/iterinliner.nim(406) transformStmt
src/hexer/iterinliner.nim(378) transformStmt
src/hexer/iterinliner.nim(383) transformStmt
src/hexer/iterinliner.nim(360) transformForStmt
src/hexer/iterinliner.nim(263) inlineIterator
src/lib/nifcursors.nim symId
src/lib/nifstreams.nim(233) symId
nim-2.3.1/lib/std/assertions.nim(41) failedAssertImpl
nim-2.3.1/lib/std/assertions.nim(34) raiseAssert
Error: unhandled exception: src/lib/nifstreams.nim(233, 3) `n.kind in {Symbol, SymbolDef}`  [AssertionDefect]

echo template in syncio module cannot be compiled due to this issue.