[cppyy] Try instantiating empty template parameter list

bindings/pyroot/cppyy/CPyCppyy/src/TemplateProxy.cxx

@@ -656,7 +656,7 @@ static PyObject* tpp_call(TemplateProxy* pytmpl, PyObject* args, PyObject* kwds)
     if (result)
         TPPCALL_RETURN;
 
-// case 4: auto-instantiation from types of arguments
+// case 4.1: auto-instantiation from types of arguments
     for (auto pref : {Utility::kReference, Utility::kPointer, Utility::kValue}) {
         // TODO: no need to loop if there are no non-instance arguments; also, should any
         // failed lookup be removed?
@@ -671,6 +671,17 @@ static PyObject* tpp_call(TemplateProxy* pytmpl, PyObject* args, PyObject* kwds)
         if (!pcnt) break;         // preference never used; no point trying others
     }
 
+// case 4.2: auto-instantiation with zero template arguments (in case they have defaults)
+    std::string fullname = pytmpl->fTI->fCppName + "<>";
+    pymeth = pytmpl->Instantiate(fullname, args, nargsf, Utility::kNone);
+    if (pymeth) {
+    // attempt actual call; allow implicit conversion of arguments
+        result = CallMethodImp(pytmpl, pymeth, args, nargsf, kwds, true, sighash);
+        if (result)
+            TPPCALL_RETURN;
+    }
+    Utility::FetchError(errors);
+
 // case 5: low priority methods, such as ones that take void* arguments
     result = SelectAndForward(pytmpl, pytmpl->fTI->fLowPriority, args, nargsf, kwds,
         false /* implicitOkay */, false /* use_targs */, sighash, errors);

bindings/pyroot/cppyy/cppyy/test/test_regression.py

@@ -1,4 +1,4 @@
 import os, sys, pytest
 from pytest import mark, raises, skip
 from support import setup_make, IS_WINDOWS, ispypy, IS_MAC, IS_MAC_ARM
 
@@ -7,7 +7,7 @@
     helpout = []
 
     def setup_class(cls):
         import cppyy
 
         if sys.hexversion < 0x30d0000:
             def stringpager(text, cls=cls):
@@ -22,7 +22,7 @@
     def test01_kdcraw(self):
         """Doc strings for KDcrawIface (used to crash)."""
 
         import cppyy, pydoc
 
         # TODO: run a find for these paths
         qtpath = "/usr/include/qt5"
@@ -54,9 +54,9 @@
         if ispypy:
             skip('hangs (??) in pypy')
 
         import cppyy, pydoc
 
         assert not '__abstractmethods__' in dir(cppyy.gbl.gInterpreter)
         assert '__class__' in dir(cppyy.gbl.gInterpreter)
 
         self.__class__.helpout = []
@@ -68,7 +68,7 @@
 
         cppyy.cppdef("namespace cppyy_regression_test { void iii() {}; }")
 
         assert not 'iii' in cppyy.gbl.cppyy_regression_test.__dict__
         assert not '__abstractmethods__' in dir(cppyy.gbl.cppyy_regression_test)
         assert '__class__' in dir(cppyy.gbl.cppyy_regression_test)
         assert 'iii' in dir(cppyy.gbl.cppyy_regression_test)
@@ -1410,6 +1410,26 @@
         assert out == ""
         assert err == ""
 
+    def test48_variadic_conversion(self):
+        """Conversion for a variadic template function"""
+
+        import cppyy
+        cppyy.cppdef("""\
+        namespace regression_test48 {
+        template <typename... Ts> void f(std::pair<double, double>) {}
+        class A {
+        public:
+            template <typename... Ts>
+            void m(std::pair<double, double>) {}
+        };
+        }""")
+
+        r48 = cppyy.gbl.regression_test48
+
+        r48.f((1, 1))
+        a = r48.A()
+        a.m((1, 1))
+
     @mark.xfail(run=False, condition=IS_MAC_ARM or IS_WINDOWS == 64, reason="LLVM JIT fails to catch exceptions")
     def test49_overloads_with_runtime_errors(self):
         """Regression test for https://github.com/root-project/root/issues/17497