Sema: implement existentialConformsToSelf using a request evaluator.

Add the request `ExistentialConformsToSelfRequest` to lazily determine
if an existential conforming to a protocol can be matched with a
generic type parameter constrained to that protocol.

Author: hborla

include/swift/AST/Decl.h

@@ -4120,7 +4120,20 @@ class ProtocolDecl final : public NominalTypeDecl {
     Bits.ProtocolDecl.RequiresClass = requiresClass;
   }
 
-  bool existentialConformsToSelfSlow();
+  /// Returns the cached result of \c existentialConformsToSelf or \c None if it
+  /// hasn't yet been computed.
+  Optional<bool> getCachedExistentialConformsToSelf() const {
+    if (Bits.ProtocolDecl.ExistentialConformsToSelfValid)
+      return Bits.ProtocolDecl.ExistentialConformsToSelf;
+
+    return None;
+  }
+
+  /// Caches the result of \c existentialConformsToSelf
+  void setCachedExistentialConformsToSelf(bool result) {
+    Bits.ProtocolDecl.ExistentialConformsToSelfValid = true;
+    Bits.ProtocolDecl.ExistentialConformsToSelf = result;
+  }
 
   bool existentialTypeSupportedSlow();
 
@@ -4134,6 +4147,7 @@ class ProtocolDecl final : public NominalTypeDecl {
   friend class SuperclassTypeRequest;
   friend class RequirementSignatureRequest;
   friend class ProtocolRequiresClassRequest;
+  friend class ExistentialConformsToSelfRequest;
   friend class TypeChecker;
 
 public:
@@ -4204,13 +4218,7 @@ class ProtocolDecl final : public NominalTypeDecl {
   /// This is only permitted if there is nothing "non-trivial" that we
   /// can do with the metatype, which means the protocol must not have
   /// any static methods and must be declared @objc.
-  bool existentialConformsToSelf() const {
-    if (Bits.ProtocolDecl.ExistentialConformsToSelfValid)
-      return Bits.ProtocolDecl.ExistentialConformsToSelf;
-
-    return const_cast<ProtocolDecl *>(this)
-             ->existentialConformsToSelfSlow();
-  }
+  bool existentialConformsToSelf() const;
 
   /// Does this protocol require a self-conformance witness table?
   bool requiresSelfConformanceWitnessTable() const;

include/swift/AST/TypeCheckRequests.h

@@ -201,6 +201,32 @@ class ProtocolRequiresClassRequest:
   void cacheResult(bool value) const;
 };
 
+/// Determine whether an existential conforming to a protocol can be matched
+/// with a generic type parameter constrained to that protocol.
+class ExistentialConformsToSelfRequest:
+    public SimpleRequest<ExistentialConformsToSelfRequest,
+                         bool(ProtocolDecl *),
+                         CacheKind::SeparatelyCached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  // Evaluation.
+  llvm::Expected<bool> evaluate(Evaluator &evaluator, ProtocolDecl *decl) const;
+
+public:
+  // Cycle handling.
+  void diagnoseCycle(DiagnosticEngine &diags) const;
+  void noteCycleStep(DiagnosticEngine &diags) const;
+
+  // Separate caching.
+  bool isCached() const { return true; }
+  Optional<bool> getCachedResult() const;
+  void cacheResult(bool value) const;
+};
+
 /// Determine whether the given declaration is 'final'.
 class IsFinalRequest :
     public SimpleRequest<IsFinalRequest,

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -20,6 +20,7 @@ SWIFT_TYPEID(EnumRawTypeRequest)
 SWIFT_TYPEID(OverriddenDeclsRequest)
 SWIFT_TYPEID(IsObjCRequest)
 SWIFT_TYPEID(ProtocolRequiresClassRequest)
+SWIFT_TYPEID(ExistentialConformsToSelfRequest)
 SWIFT_TYPEID(IsFinalRequest)
 SWIFT_TYPEID(IsDynamicRequest)
 SWIFT_TYPEID(RequirementRequest)

lib/AST/Decl.cpp

@@ -4329,43 +4329,9 @@ bool ProtocolDecl::requiresSelfConformanceWitnessTable() const {
   return isSpecificProtocol(KnownProtocolKind::Error);
 }
 
-bool ProtocolDecl::existentialConformsToSelfSlow() {
-  // Assume for now that the existential conforms to itself; this
-  // prevents circularity issues.
-  Bits.ProtocolDecl.ExistentialConformsToSelfValid = true;
-  Bits.ProtocolDecl.ExistentialConformsToSelf = true;
-
-  // If it's not @objc, it conforms to itself only if it has a
-  // self-conformance witness table.
-  if (!isObjC()) {
-    bool hasSelfConformance = requiresSelfConformanceWitnessTable();
-    Bits.ProtocolDecl.ExistentialConformsToSelf = hasSelfConformance;
-    return hasSelfConformance;
-  }
-
-  // Check whether this protocol conforms to itself.
-  for (auto member : getMembers()) {
-    if (member->isInvalid())
-      continue;
-
-    if (auto vd = dyn_cast<ValueDecl>(member)) {
-      if (!vd->isInstanceMember()) {
-        // A protocol cannot conform to itself if it has static members.
-        Bits.ProtocolDecl.ExistentialConformsToSelf = false;
-        return false;
-      }
-    }
-  }
-
-  // Check whether any of the inherited protocols fail to conform to
-  // themselves.
-  for (auto proto : getInheritedProtocols()) {
-    if (!proto->existentialConformsToSelf()) {
-      Bits.ProtocolDecl.ExistentialConformsToSelf = false;
-      return false;
-    }
-  }
-  return true;
+bool ProtocolDecl::existentialConformsToSelf() const {
+  return evaluateOrDefault(getASTContext().evaluator,
+    ExistentialConformsToSelfRequest{const_cast<ProtocolDecl *>(this)}, true);
 }
 
 /// Classify usages of Self in the given type.

lib/AST/TypeCheckRequests.cpp

@@ -211,6 +211,31 @@ void ProtocolRequiresClassRequest::cacheResult(bool value) const {
   decl->setCachedRequiresClass(value);
 }
 
+//----------------------------------------------------------------------------//
+// existentialConformsToSelf computation.
+//----------------------------------------------------------------------------//
+
+void ExistentialConformsToSelfRequest::diagnoseCycle(DiagnosticEngine &diags) const {
+  auto decl = std::get<0>(getStorage());
+  diags.diagnose(decl, diag::circular_protocol_def, decl->getName());
+}
+
+void ExistentialConformsToSelfRequest::noteCycleStep(DiagnosticEngine &diags) const {
+  auto requirement = std::get<0>(getStorage());
+  diags.diagnose(requirement, diag::kind_declname_declared_here,
+                 DescriptiveDeclKind::Protocol, requirement->getName());
+}
+
+Optional<bool> ExistentialConformsToSelfRequest::getCachedResult() const {
+  auto decl = std::get<0>(getStorage());
+  return decl->getCachedExistentialConformsToSelf();
+}
+
+void ExistentialConformsToSelfRequest::cacheResult(bool value) const {
+  auto decl = std::get<0>(getStorage());
+  decl->setCachedExistentialConformsToSelf(value);
+}
+
 //----------------------------------------------------------------------------//
 // isFinal computation.
 //----------------------------------------------------------------------------//

lib/Sema/TypeCheckDecl.cpp

@@ -1269,6 +1269,34 @@ ProtocolRequiresClassRequest::evaluate(Evaluator &evaluator,
   return false;
 }
 
+llvm::Expected<bool>
+ExistentialConformsToSelfRequest::evaluate(Evaluator &evaluator,
+                                           ProtocolDecl *decl) const {
+  // If it's not @objc, it conforms to itself only if it has a self-conformance
+  // witness table.
+  if (!decl->isObjC())
+    return decl->requiresSelfConformanceWitnessTable();
+
+  // Check whether this protocol conforms to itself.
+  for (auto member : decl->getMembers()) {
+    if (member->isInvalid()) continue;
+
+    if (auto vd = dyn_cast<ValueDecl>(member)) {
+      // A protocol cannot conform to itself if it has static members.
+      if (!vd->isInstanceMember())
+        return false;
+    }
+  }
+
+  // Check whether any of the inherited protocols fail to conform to themselves.
+  for (auto proto : decl->getInheritedProtocols()) {
+    if (!proto->existentialConformsToSelf())
+      return false;
+  }
+
+  return true;
+}
+
 llvm::Expected<bool>
 IsFinalRequest::evaluate(Evaluator &evaluator, ValueDecl *decl) const {
   if (isa<ClassDecl>(decl))