[tint] static_assert that C in Castable<C[, B]> is correct

Fix a bug this caught.

Fixed: tint:1939
Change-Id: I61ea4eb66fe1f91b3bc17ba5d65aee7324aa555c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/132621
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/sem/statement.h

@@ -133,7 +133,7 @@ class Statement : public utils::Castable<Statement, Node> {
 
 /// CompoundStatement is the base class of statements that can hold other
 /// statements.
-class CompoundStatement : public utils::Castable<Statement, Statement> {
+class CompoundStatement : public utils::Castable<CompoundStatement, Statement> {
   public:
     /// Constructor
     /// @param declaration the AST node for this statement

src/tint/utils/castable.h

@@ -18,6 +18,7 @@
 #include <stdint.h>
 #include <functional>
 #include <tuple>
+#include <type_traits>
 #include <utility>
 
 #include "src/tint/utils/crc32.h"
@@ -73,7 +74,9 @@ static constexpr bool IsCastable =
         tint::utils::TypeInfo::HashCodeOf<CLASS>(),                           \
         tint::utils::TypeInfo::FullHashCodeOf<CLASS>(),                       \
     };                                                                        \
-    TINT_CASTABLE_POP_DISABLE_WARNINGS()
+    TINT_CASTABLE_POP_DISABLE_WARNINGS();                                     \
+    static_assert(std::is_same_v<CLASS, CLASS::Base::Class>,                  \
+                  #CLASS " does not derive from Castable<" #CLASS "[, BASE]>")
 
 /// Bit flags that can be passed to the template parameter `FLAGS` of Is() and As().
 enum CastFlags {
@@ -414,14 +417,16 @@ class CastableBase {
 template <typename CLASS, typename BASE = CastableBase>
 class Castable : public BASE {
   public:
-    /// A type alias for `CLASS` to easily access the `BASE` class members.
-    /// Base actually aliases to the Castable instead of `BASE` so that you can
-    /// use Base in the `CLASS` constructor.
+    /// A type alias to this Castable. Commonly used in derived type constructors to forward
+    /// constructor arguments to BASE.
     using Base = Castable;
 
     /// A type alias for `BASE`.
     using TrueBase = BASE;
 
+    /// A type alias for `CLASS`.
+    using Class = CLASS;
+
     /// Constructor
     /// @param arguments the arguments to forward to the base class.
     template <typename... ARGS>