[CPyCppyy] Add __template_args__ and signature-plus-template overload [ROOT-patch]

Source: ROOT

- __template_args__ read-only property on TemplateProxy for
  introspection of a method's template arguments
- "ss:__overload__" branch in tpp_overload so callers can select an
  overload by both signature and template arguments

Added for ROOT's Numba-introspection support.

Author: aaronj0

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

@@ -747,6 +747,21 @@ static int tpp_setuseffi(CPPOverload*, PyObject*, void*)
     return 0;                 // dummy (__useffi__ unused)
 }
 
+//-----------------------------------------------------------------------------
+static PyObject* tpp_gettemplateargs(TemplateProxy* self, void*) {
+    if (!self->fTemplateArgs) {
+        Py_RETURN_NONE;
+    }
+
+    Py_INCREF(self->fTemplateArgs);
+    return self->fTemplateArgs;
+}
+
+//-----------------------------------------------------------------------------
+static int tpp_settemplateargs(TemplateProxy*, PyObject*, void*) {
+    PyErr_SetString(PyExc_AttributeError, "__template_args__ is read-only");
+    return -1;
+}
 
 //----------------------------------------------------------------------------
 static PyMappingMethods tpp_as_mapping = {
@@ -757,7 +772,9 @@ static PyGetSetDef tpp_getset[] = {
     {(char*)"__doc__", (getter)tpp_doc, (setter)tpp_doc_set, nullptr, nullptr},
     {(char*)"__useffi__", (getter)tpp_getuseffi, (setter)tpp_setuseffi,
       (char*)"unused", nullptr},
-    {(char*)nullptr,   nullptr,         nullptr, nullptr, nullptr}
+    {(char*)"__template_args__", (getter)tpp_gettemplateargs, (setter)tpp_settemplateargs,
+      (char*)"the template arguments for this method", nullptr},
+    {(char*)nullptr,   nullptr,         nullptr, nullptr, nullptr},
 };
 
 
@@ -788,6 +805,7 @@ static PyObject* tpp_overload(TemplateProxy* pytmpl, PyObject* args)
 {
 // Select and call a specific C++ overload, based on its signature.
     const char* sigarg = nullptr;
+    const char* tmplarg = nullptr;
     PyObject* sigarg_tuple = nullptr;
     int want_const = -1;
 
@@ -818,6 +836,11 @@ static PyObject* tpp_overload(TemplateProxy* pytmpl, PyObject* args)
         scope = ((CPPClass*)pytmpl->fTI->fPyClass)->fCppType;
         cppmeth = Cppyy::GetMethodTemplate(
             scope, pytmpl->fTI->fCppName, proto.substr(1, proto.size()-2));
+    } else if (PyArg_ParseTuple(args, const_cast<char*>("ss:__overload__"), &sigarg, &tmplarg)) {
+        scope = ((CPPClass*)pytmpl->fTI->fPyClass)->fCppType;
+        std::string full_name = std::string(pytmpl->fTI->fCppName) + "<" + tmplarg + ">";
+
+        cppmeth = Cppyy::GetMethodTemplate(scope, full_name, sigarg);
     } else if (PyArg_ParseTuple(args, const_cast<char*>("O|i:__overload__"), &sigarg_tuple, &want_const)) {
         PyErr_Clear();
         want_const = PyTuple_GET_SIZE(args) == 1 ? -1 : want_const;