archive.h

class IArchive
{
public:
...
  template <typename _Ty>
  bool TestTag() {
    // Only tagged types can be tested. If compilations fails here, it can
    // mean the file containing tag declaration is not included.
    static_assert(internal::Tag<const _Ty>::kTagLength != 0,
                  "Tag unknown for type.");

    const int tell = stream_->Tell();
    bool valid = internal::Tagger<const _Ty>::Validate(*this);
    stream_->Seek(tell, Stream::kSet);  // Rewinds before the tag test.
    return valid;
  }
};

ozz.nim

import os

const
  ozzIncludePath = currentSourcePath.parentDir().parentDir()/"lib/ozz-animation/include"

type
  Archive* {.importcpp: "ozz::io::IArchive", header: ozzIncludePath/"ozz/base/io/archive.h".} = object
  Skeleton* {.importcpp: "ozz::animation::Skeleton", header: ozzIncludePath/"ozz/animation/runtime/skeleton.h".} = object

proc constructArchive*(stream: ptr Stream): Archive {.importcpp: "ozz::io::IArchive(@)", header: ozzIncludePath/"ozz/base/io/archive.h".}
proc testTag*[T](a: Archive): bool {.importcpp: "#.TestTag<'*0>()", header: ozzIncludePath/"ozz/base/io/archive.h".}

rig.nim

# error:
# !  ~/d/junkers   *~…  nim cpp -r --gc:arc --passC:-Ilib/ozz-animation/include --passL:lib/ozz-animation/.build/src/base/libozz_base.a src/rig.nim
# Hint: used config file '/Users/zacharycarter/dev/Nim/config/nim.cfg' [Conf]
# Hint: used config file '/Users/zacharycarter/dev/Nim/config/config.nims' [Conf]
# ....................
# /Users/zacharycarter/dev/junkers/src/rig.nim(18, 17) template/generic instantiation of `testTag` from here
# /Users/zacharycarter/dev/junkers/src/ozz.nim(47, 15) Error: cannot instantiate: 'T'

proc rigLoadSkeleton(skeletonFilePath: cstring): bool =
  let file = constructFile(skeletonFilePath, "rb")
  if not file.opened():
    # TODO: Log error
    return false

  var memStream: MemoryStream
  memStream.resize(file.size())
  memStream.streamEnd = cint(file.size())
  file.read(memStream.buffer, file.size())

  let archive = constructArchive(addr memStream)
  if not archive.testTag[Skeleton]():
    # TODO: Log error
    return false

  result = true