[AST] Assert that no type class is polymorphic

[riccibruno]

Summary

Add a static_assert checking that no type class is polymorphic.
People should use LLVM style RTTI instead.

[aaron.ballman]

lib/AST/Type.cpp

299

This will squish your class name and error text together. I think you need a whitespace before "Type" and make that lowercase.

301

I'd feel more comfortable if there was a #undef TYPE here.

[riccibruno]

lib/AST/Type.cpp

299

This is on purpose, since what is substituted into
CLASS is the name of the type class, without the "Type",
eg: for FunctionProtoType, CLASS will be FunctionProto
and we want to stick the Type to it so that the error message will
be "FunctionProtoType should not ....".

301

I can certainly add one but it is already done in TypeNodes.def.

[aaron.ballman]

LGTM!

lib/AST/Type.cpp

299

Ah! Thank you for pointing that out.

301

Ah, nm then -- I didn't see we had that at the bottom of that file.

lib/AST/Type.cpp

 
 QualType QualType::getSingleStepDesugaredTypeImpl(QualType type,
                                                   const ASTContext &Context) {
   SplitQualType split = type.split();
   QualType desugar = split.Ty->getLocallyUnqualifiedSingleStepDesugaredType();
   return Context.getQualifiedType(desugar, split.Quals);
 }
 
+// Check that no type class is polymorphic. LLVM style RTTI should be used
+// instead. If absolutely needed an exception can still be added here by
+// defining the appropriate macro (but please don't do this).
+#define TYPE(CLASS, BASE) \
+  static_assert(!std::is_polymorphic<CLASS##Type>::value, \
+                #CLASS "Type should not be polymorphic!");
+#include "clang/AST/TypeNodes.def"
+
 QualType Type::getLocallyUnqualifiedSingleStepDesugaredType() const {
   switch (getTypeClass()) {
 #define ABSTRACT_TYPE(Class, Parent)
 #define TYPE(Class, Parent) \
   case Type::Class: { \
     const auto *ty = cast<Class##Type>(this); \
     if (!ty->isSugared()) return QualType(ty, 0); \
     return ty->desugar(); \