io: Readd deprecated TBuffer::ReadClass signature.

core/.clang-format

@@ -0,0 +1,20 @@
+BasedOnStyle: InheritParentConfig
+
+AlignConsecutiveAssignments:
+  Enabled: true
+  AcrossEmptyLines: false
+  AcrossComments: true
+  AlignCompound: true
+  PadOperators: true
+AlignConsecutiveBitFields:
+  Enabled: true
+  AcrossEmptyLines: false
+  AcrossComments: false
+AlignConsecutiveDeclarations:
+  Enabled: true
+  # Part of clang-format 20.1.0
+  AlignFunctionDeclarations: true
+
+# Part of clang-format 22
+# IndentPPDirectives: Leave
+IndentPPDirectives: None

core/base/inc/TBuffer.h

@@ -26,6 +26,7 @@
 #include "TClass.h"
 #include "Bytes.h"
 
+#include <limits>
 #include <vector>
 #include <string>
 
@@ -228,6 +229,27 @@ class TBuffer : public TObject {
    virtual TVirtualArray *PopDataCache();
    virtual void           PushDataCache(TVirtualArray *);
 
+   TClass *ReadClass(const TClass *cl, UInt_t *objTag)
+      R__DEPRECATED(6, 42, "Use the overload with ULong64_t* for objTag instead")
+   {
+      if (objTag) {
+         ULong64_t objTag64 = 0;
+
+         auto result = ReadClass(cl, &objTag64);
+         if (objTag64 > std::numeric_limits<UInt_t>::max()) {
+            Error("ReadClass",
+                  "Object tag value %llu exceeds maximum of %u for 32-bit tag. Consider using the overload with "
+                  "ULong64_t* for objTag instead.",
+                  objTag64, std::numeric_limits<UInt_t>::max());
+            return result;
+         }
+         *objTag = static_cast<UInt_t>(objTag64);
+         return result;
+      } else {
+         return ReadClass(cl, static_cast<ULong64_t *>(nullptr));
+      }
+   }
+
    virtual TClass    *ReadClass(const TClass *cl = nullptr, ULong64_t *objTag = nullptr) = 0;
    virtual void       WriteClass(const TClass *cl) = 0;