Update OpBase.td AttrDef class to define default value for returnType and convertFromStorage (NFC)

[rriddle]

mlir/include/mlir/IR/OpBase.td

2709–2712

This description leads me to believe that this should just be $_self?

[mehdi_amini]

mlir/include/mlir/IR/OpBase.td

2709–2712

What would you write? (I just copy/pasted it from: https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/IR/OpBase.td#L791-L799 ). We should probably update both.

mlir/include/mlir/IR/OpBase.td

 // from the given C++ base class.
 class AttrDef<Dialect dialect, string name,
               string baseCppClass = "::mlir::Attribute">
     : DialectAttr<dialect, CPred<"">, /*descr*/"">,
       AttrOrTypeDef<"Attr", name, baseCppClass> {
   // The name of the C++ Attribute class.
   string cppClassName = name # "Attr";
 
+  // The underlying C++ value type
+  let returnType = dialect.cppNamespace # "::" # cppClassName;
+
+  // The call expression to convert from the storage type to the return
+  // type. For example, an enum can be stored as an int but returned as an
+  // enum class.
+  //
+  // Format: $_self will be expanded to the attribute.
+  //
+  // For example, `$_self.getValue().getSExtValue()` for `IntegerAttr val` will
+  // expand to `getAttrOfType<IntegerAttr>("val").getValue().getSExtValue()`.
+  let convertFromStorage = "$_self.cast<" # dialect.cppNamespace #
+                                 "::" # cppClassName # ">()";
+
   // A code block used to build the value 'Type' of an Attribute when
   // initializing its storage instance. This field is optional, and if not
   // present the attribute will have its value type set to `NoneType`. This code
   // block may reference any of the attributes parameters via
   // `$_<parameter-name`. If one of the parameters of the attribute is of type
   // `AttributeSelfTypeParameter`, this field is ignored.
   code typeBuilder = ?;