From 52a7437808e44f7a4f70a9ad3663d7322e0b342d Mon Sep 17 00:00:00 2001
From: Vipul Cariappa <vipulcariappa@gmail.com>
Date: Wed, 23 Jul 2025 16:49:00 +0200
Subject: [PATCH] [cppyy] fix: instantiation of templated functions

when instantiating with another templated function
---
 .../cppyy/CPyCppyy/src/TemplateProxy.cxx      |  6 ++++
 .../pyroot/cppyy/cppyy/test/test_templates.py | 36 +++++++++++++++++++
 .../cling/lib/Interpreter/LookupHelper.cpp    |  4 ++-
 3 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/bindings/pyroot/cppyy/CPyCppyy/src/TemplateProxy.cxx b/bindings/pyroot/cppyy/CPyCppyy/src/TemplateProxy.cxx
index 20ea4929adfc5..e3ffbc3655901 100644
--- a/bindings/pyroot/cppyy/CPyCppyy/src/TemplateProxy.cxx
+++ b/bindings/pyroot/cppyy/CPyCppyy/src/TemplateProxy.cxx
@@ -142,6 +142,12 @@ PyObject* TemplateProxy::Instantiate(const std::string& fname,
             } else
                 PyErr_Clear();
 
+            if (!bArgSet && (Py_TYPE(itemi) == &TemplateProxy_Type)) {
+                TemplateProxy *tp = (TemplateProxy*)itemi;
+                PyObject *tmpl_name = CPyCppyy_PyText_FromFormat("decltype(%s%s)", tp->fTI->fCppName.c_str(), tp->fTemplateArgs ? CPyCppyy_PyText_AsString(tp->fTemplateArgs) : "");
+                PyTuple_SET_ITEM(tpArgs, i, tmpl_name);
+                bArgSet = true;
+            }
             if (!bArgSet) {
             // normal case (may well fail)
                 PyErr_Clear();
diff --git a/bindings/pyroot/cppyy/cppyy/test/test_templates.py b/bindings/pyroot/cppyy/cppyy/test/test_templates.py
index 9f41b8e968a14..429705e914594 100644
--- a/bindings/pyroot/cppyy/cppyy/test/test_templates.py
+++ b/bindings/pyroot/cppyy/cppyy/test/test_templates.py
@@ -1341,5 +1341,41 @@ def test01_reduce_binary(self):
         assert type(e1+e2) == cppyy.gbl.TypeReduction.Expr[int]
 
 
+class TestTEMPLATED_CALLBACK:
+    def setup_class(cls):
+        cls.test_dct = test_dct
+        import cppyy
+        cls.templates = cppyy.load_reflection_info(cls.test_dct)
+    
+    def test01_templated_callbacks(self):
+        import cppyy
+        from cppyy.gbl import std
+
+        cppyy.cppdef(r"""
+        bool foo() { return true; }
+
+        int bar(int a, int b) { return a + b; }
+
+        template <typename T, typename U>
+        T baz(T a, U b, std::string c) {
+            return (T)(a + b) + std::stoi(c);
+        }
+
+        template <typename F, typename R, typename... Args>
+        R returned_callback(F callable, R r, Args... args) {
+            return callable(r, std::forward<Args>(args)...);
+        }
+
+        template <typename F, typename... Args>
+        bool callback(F callable, Args&&... args) {
+            return callable(std::forward<Args>(args)...);
+        }
+        """)
+
+        assert cppyy.gbl.callback(cppyy.gbl.foo)
+        assert cppyy.gbl.returned_callback(cppyy.gbl.bar, 1, 1) == 2
+        assert cppyy.gbl.returned_callback(cppyy.gbl.baz[int, int], 1, 1, std.string("1")) == 3
+
+
 if __name__ == "__main__":
     exit(pytest.main(args=['-sv', '-ra', __file__]))
diff --git a/interpreter/cling/lib/Interpreter/LookupHelper.cpp b/interpreter/cling/lib/Interpreter/LookupHelper.cpp
index 5761c0bcd325c..2efe43daed333 100644
--- a/interpreter/cling/lib/Interpreter/LookupHelper.cpp
+++ b/interpreter/cling/lib/Interpreter/LookupHelper.cpp
@@ -1691,7 +1691,9 @@ namespace cling {
         clang::QualType QT = clang::Sema::GetTypeFromParser(Res.get(), &TSI);
         QT = QT.getCanonicalType();
         {
-          ExprValueKind VK = VK_PRValue;
+          // XValue is an object that can be "moved" whereas PRValue is temporary value
+          // This enables overloads that require the object to be moved
+          ExprValueKind VK = VK_XValue;
           if (QT->getAs<LValueReferenceType>()) {
             VK = VK_LValue;
           }