Define UnderlyingTypeRequest

Define a request for computing the interface type of the underlying type of a typealias.  This can be used in place of the declared interface type of the alias itself when it is nested to preserve the fragile validation ordering issues that can cause.

Author: CodaFi

include/swift/AST/Decl.h

@@ -2947,6 +2947,8 @@ class OpaqueTypeDecl : public GenericTypeDecl {
 /// TypeAliasDecl's always have 'MetatypeType' type.
 ///
 class TypeAliasDecl : public GenericTypeDecl {
+  friend class UnderlyingTypeRequest;
+  
   /// The location of the 'typealias' keyword
   SourceLoc TypeAliasLoc;
 

include/swift/AST/TypeCheckRequests.h

@@ -1181,6 +1181,28 @@ class GenericSignatureRequest :
   void cacheResult(GenericSignature *value) const;
 };
 
+/// Compute the interface type of the underlying definition type of a typealias declaration.
+class UnderlyingTypeRequest :
+    public SimpleRequest<UnderlyingTypeRequest,
+                         Type(TypeAliasDecl *),
+                         CacheKind::SeparatelyCached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  // Evaluation.
+  llvm::Expected<Type> evaluate(Evaluator &evaluator,
+                                TypeAliasDecl *decl) const;
+
+public:
+  // Caching.
+  bool isCached() const { return true; }
+  Optional<Type> getCachedResult() const;
+  void cacheResult(Type value) const;
+};
+
 // Allow AnyValue to compare two Type values, even though Type doesn't
 // support ==.
 template<>

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -132,5 +132,7 @@ SWIFT_REQUEST(TypeChecker, SynthesizeAccessorRequest,
               SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, TypeCheckFunctionBodyUntilRequest,
               bool(AbstractFunctionDecl *, SourceLoc), Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, UnderlyingTypeRequest, Type(TypeAliasDecl *),
+              SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, USRGenerationRequest, std::string(const ValueDecl *),
               Cached, NoLocationInfo)

lib/AST/TypeCheckRequests.cpp

@@ -822,3 +822,24 @@ void GenericSignatureRequest::cacheResult(GenericSignature *value) const {
   auto *GC = std::get<0>(getStorage());
   GC->GenericSigAndBit.setPointerAndInt(value, true);
 }
+
+//----------------------------------------------------------------------------//
+// IsImplicitlyUnwrappedOptionalRequest computation.
+//----------------------------------------------------------------------------//
+
+Optional<Type>
+UnderlyingTypeRequest::getCachedResult() const {
+  auto *typeAlias = std::get<0>(getStorage());
+  if (auto type = typeAlias->UnderlyingTy.getType())
+    return type;
+  return None;
+}
+
+void UnderlyingTypeRequest::cacheResult(Type value) const {
+  auto *typeAlias = std::get<0>(getStorage());
+  // lldb creates global typealiases containing archetypes
+  // sometimes...
+  if (value->hasArchetype() && typeAlias->isGenericContext())
+    value = value->mapTypeOutOfContext();
+  typeAlias->UnderlyingTy.setType(value);
+}