[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?

[pcanal]

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.

'What' is the mechanism that makes it fails? Is it invalid to do so? If it is invalid is there a (simple) way to inquire whether it is/will-be invalid?

[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.

'What' is the mechanism that makes it fails? Is it invalid to do so?

Yes, a variant requires at least one type:

A program that instantiates the definition of std::variant with no template arguments is ill-formed.

(from https://en.cppreference.com/w/cpp/utility/variant.html)

If it is invalid is there a (simple) way to inquire whether it is/will-be invalid?

No.

[hahnjo]

I don't see a good solution for the problems with std::variant.