[flang] Introduce BaseBoxType

[clementval]

Summary

Introduce a BaseBoxType to be used by BoxType and
the a new ClassType that is introduced in a follow up patch.

This patch is part of the implementation of the poltymorphic
entities.
https://github.com/llvm/llvm-project/blob/main/flang/docs/PolymorphicEntities.md

[PeteSteinfeld]

Aside from the two clang-format issues, all builds and tests correctly and looks good.

flang/include/flang/Optimizer/Dialect/FIRType.h

312

On my system, clang-format objects to this blank line.

flang/lib/Optimizer/Dialect/FIRType.cpp

933–934

On my system, clang-format puts all of this on a single line.

flang/include/flang/Optimizer/Dialect/FIRType.h

 #include "llvm/IR/Type.h"
 
 namespace fir {
 class FIROpsDialect;
 class KindMapping;
 using KindTy = unsigned;
 } // namespace fir
 
+//===----------------------------------------------------------------------===//
+// BaseBoxType
+//===----------------------------------------------------------------------===//
+
+namespace fir {
+
+/// This class provides a shared interface for box and class types.
+class BaseBoxType : public mlir::Type {
+public:
+  using mlir::Type::Type;
+
+  /// Returns the element type of this box type.
+  mlir::Type getEleTy() const;
+
+  /// Methods for support type inquiry through isa, cast, and dyn_cast.
+  static bool classof(mlir::Type type);
+};
+
+} // namespace fir
+
 #define GET_TYPEDEF_CLASSES
 #include "flang/Optimizer/Dialect/FIROpsTypes.h.inc"
 
 namespace llvm {
 class raw_ostream;
 class StringRef;
 template <typename>
 class ArrayRef;

 /// Does this function type has a result that requires binding the result value
 /// with a storage in a fir.save_result operation in order to use the result?
 bool hasAbstractResult(mlir::FunctionType ty);
 
 /// Convert llvm::Type::TypeID to mlir::Type
 mlir::Type fromRealTypeID(mlir::MLIRContext *context, llvm::Type::TypeID typeID,
                           fir::KindTy kind);
 
+inline bool BaseBoxType::classof(mlir::Type type) {
+  return type.isa<fir::BoxType>();
+}
+
 } // namespace fir
 
 #endif // FORTRAN_OPTIMIZER_DIALECT_FIRTYPE_H

flang/include/flang/Optimizer/Dialect/FIRTypes.td

 
 include "mlir/IR/AttrTypeBase.td"
 include "flang/Optimizer/Dialect/FIRDialect.td"
 
 //===----------------------------------------------------------------------===//
 // FIR Types
 //===----------------------------------------------------------------------===//
 
-class FIR_Type<string name, string typeMnemonic> : TypeDef<fir_Dialect, name> {
+class FIR_Type<string name, string typeMnemonic, list<Trait> traits = [],
+               string baseCppClass = "::mlir::Type">
+    : TypeDef<fir_Dialect, name, traits, baseCppClass> {
   let mnemonic = typeMnemonic;
 }
 
 def fir_Type : Type<CPred<"fir::isa_fir_or_std_type($_self)">,
     "FIR dialect type">;
 
 def fir_BoxCharType : FIR_Type<"BoxChar", "boxchar"> {
   let summary = "CHARACTER type descriptor.";
def fir_BoxProcType : FIR_Type<"BoxProc", "boxproc"> {
   }];
 
   let parameters = (ins "mlir::Type":$eleTy);
 
   let genVerifyDecl = 1;
   let hasCustomAssemblyFormat = 1;
 }
 
-def fir_BoxType : FIR_Type<"Box", "box"> {
+def fir_BoxType : FIR_Type<"Box", "box", [], "BaseBoxType"> {
   let summary = "The type of a Fortran descriptor";
 
   let description = [{
     Descriptors are tuples of information that describe an entity being passed
     from a calling context. This information might include (but is not limited
     to) whether the entity is an array, its size, or what type it has.
   }];
 

flang/lib/Optimizer/Dialect/FIRType.cpp

mlir::Type fir::fromRealTypeID(mlir::MLIRContext *context,
   default:
     mlir::emitError(mlir::UnknownLoc::get(context))
         << "unsupported type: !fir.real<" << kind << ">";
     return {};
   }
 }
 
 //===----------------------------------------------------------------------===//
+// BaseBoxType
+//===----------------------------------------------------------------------===//
+
+mlir::Type BaseBoxType::getEleTy() const {
+  return llvm::TypeSwitch<fir::BaseBoxType, mlir::Type>(*this)
+      .Case<fir::BoxType>([](auto type) { return type.getEleTy(); });
+}
+
+//===----------------------------------------------------------------------===//
 // FIROpsDialect
 //===----------------------------------------------------------------------===//
 
 void FIROpsDialect::registerTypes() {
   addTypes<BoxType, BoxCharType, BoxProcType, CharacterType, fir::ComplexType,
            FieldType, HeapType, fir::IntegerType, LenType, LogicalType,
            LLVMPointerType, PointerType, RealType, RecordType, ReferenceType,
            SequenceType, ShapeType, ShapeShiftType, ShiftType, SliceType,