[cppyy] Allow passing of const ref to ptr args

This commit follows up on d35e114 wehere interfaces taking references to pointers were disallowed from being used in Python.
That was a nice change, but we should continue to allow immutable references to pointers since that prevents pointer rebinding and can be used safely from Python. This commit enables the same and updates the `VoidPtrRefConverter` to map only to the const case.

Author: aaronj0

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

@@ -3365,19 +3365,19 @@ CPyCppyy::Converter* CPyCppyy::CreateConverter(const std::string& fullType, cdim
     const std::string& cpd = TypeManip::compound(resolvedType);
     std::string realType   = TypeManip::clean_type(resolvedType, false, true);
 
-// mutable pointer references (T*&) are incompatible with Python's object model
-   if (cpd == "*&") {
-       return new NotImplementedConverter{PyExc_TypeError,
-           "argument type '" + resolvedType + "' is not supported: non-const references to pointers (T*&) allow a"
-           " function to replace the pointer itself. Python cannot represent this safely. Consider changing the"
-           " C++ API to return the new pointer or use a wrapper"};
-   }
-
 // accept unqualified type (as python does not know about qualifiers)
     h = gConvFactories.find((isConst ? "const " : "") + realType + cpd);
     if (h != gConvFactories.end())
         return (h->second)(dims);
 
+// mutable pointer references (T*&) are incompatible with Python's object model
+    if (!isConst && cpd == "*&") {
+        return new NotImplementedConverter{PyExc_TypeError,
+            "argument type '" + resolvedType + "' is not supported: non-const references to pointers (T*&) allow a"
+            " function to replace the pointer itself. Python cannot represent this safely. Consider changing the"
+            " C++ API to return the new pointer or use a wrapper"};
+    }
+
 // drop const, as that is mostly meaningless to python (with the exception
 // of c-strings, but those are specialized in the converter map)
     if (isConst) {
@@ -3827,7 +3827,8 @@ static struct InitConvFactories_t {
         gf["const std::wstring&"] =         gf["std::wstring"];
         gf["const " WSTRING1 "&"] =         gf["std::wstring"];
         gf["const " WSTRING2 "&"] =         gf["std::wstring"];
-        gf["void*&"] =                      (cf_t)+[](cdims_t) { static VoidPtrRefConverter c{};     return &c; };
+        // VoidPtrRefConverter should only be used for const references to pointers
+        gf["const void*&"] =                (cf_t)+[](cdims_t) { static VoidPtrRefConverter c{};     return &c; };
         gf["void**"] =                      (cf_t)+[](cdims_t d) { return new VoidPtrPtrConverter{d}; };
         gf["void ptr"] =                    gf["void**"];
         gf["PyObject*"] =                   (cf_t)+[](cdims_t) { static PyObjectConverter c{};       return &c; };