[metacling] Instantiate empty parameter packs

[hahnjo]

This effectively reverts the change of commit 4b00923. It is needed for working conversion with variadic templates from cppyy.

[github-actions]

Test Results

    19 files      19 suites   2d 16h 23m 0s ⏱️
 3 769 tests  3 337 ✅   0 💤   432 ❌
64 755 runs  63 178 ✅ 113 💤 1 464 ❌

For more details on these failures, see this check.

Results for commit 75f513d.

♻️ This comment has been updated with latest results.

[dpiparo]

lgtm

[vepadulano]

LGTM for the RDF part.

[hahnjo]

So this doesn't work on macOS, mostly because of std::variant related function templates where it actually fails to instantiate with the empty parameter pack. As a short-term workaround, it would be possible to only instantiate parameter packs when it's not the first parameter:

diff --git a/core/metacling/src/TClingMethodInfo.cxx b/core/metacling/src/TClingMethodInfo.cxx
index bb458163f70..692725d75c0 100644
--- a/core/metacling/src/TClingMethodInfo.cxx
+++ b/core/metacling/src/TClingMethodInfo.cxx
@@ -161,6 +161,9 @@ TClingCXXRecMethIter::InstantiateTemplateWithDefaults(const clang::RedeclarableT                                                                                                   
    llvm::SmallVector<TemplateArgument, 8> defaultTemplateArgs;
    for (const NamedDecl *templateParm: *templateParms) {
       if (templateParm->isTemplateParameterPack()) {
+         if (defaultTemplateArgs.size() == 0) {
+            return nullptr;
+         }
          defaultTemplateArgs.emplace_back(ArrayRef<TemplateArgument>{}); // empty pack.
       } else if (auto TTP = dyn_cast<TemplateTypeParmDecl>(templateParm)) {
          if (!TTP->hasDefaultArgument())

That would be get me what I need for the histograms, but potentially confusing why other member templates are not instantiated. Opinions?