sem: Have sem::Type derive from sem::Node

Having a common base class will tighten up some of the dynamic casting that we do.

Bug: tint:724
Change-Id: I45c6f200e1ec242ddb08ce75bd1c4c037a21a38d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49882
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/program_builder.h

@@ -304,7 +304,10 @@ class ProgramBuilder {
   /// @param args the arguments to pass to the type constructor
   /// @returns the node pointer
   template <typename T, typename... ARGS>
-  traits::EnableIfIsType<T, sem::Node>* create(ARGS&&... args) {
+  traits::EnableIf<traits::IsTypeOrDerived<T, sem::Node>::value &&
+                       !traits::IsTypeOrDerived<T, sem::Type>::value,
+                   T>*
+  create(ARGS&&... args) {
     AssertNotMoved();
     return sem_nodes_.Create<T>(std::forward<ARGS>(args)...);
   }

src/sem/node.cc

@@ -19,6 +19,10 @@ TINT_INSTANTIATE_TYPEINFO(tint::sem::Node);
 namespace tint {
 namespace sem {
 
+Node::Node() = default;
+
+Node::Node(const Node&) = default;
+
 Node::~Node() = default;
 
 }  // namespace sem

src/sem/node.h

@@ -23,6 +23,12 @@ namespace sem {
 /// Node is the base class for all semantic nodes
 class Node : public Castable<Node> {
  public:
+  /// Constructor
+  Node();
+
+  /// Copy constructor
+  Node(const Node&);
+
   /// Destructor
   ~Node() override;
 };

src/sem/type.h

@@ -17,7 +17,7 @@
 
 #include <string>
 
-#include "src/castable.h"
+#include "src/sem/node.h"
 
 namespace tint {
 
@@ -31,7 +31,7 @@ namespace sem {
 enum class MemoryLayout { kUniformBuffer, kStorageBuffer };
 
 /// Base class for a type in the system
-class Type : public Castable<Type> {
+class Type : public Castable<Type, Node> {
  public:
   /// Move constructor
   Type(Type&&);