[5.0] Allow Error to conform to itself

include/swift/AST/Decl.h

@@ -3975,6 +3975,9 @@ class ProtocolDecl final : public NominalTypeDecl {
              ->existentialConformsToSelfSlow();
   }
 
+  /// Does this protocol require a self-conformance witness table?
+  bool requiresSelfConformanceWitnessTable() const;
+
   /// Find direct Self references within the given requirement.
   ///
   /// \param allowCovariantParameters If true, 'Self' is assumed to be

include/swift/AST/Module.h

@@ -385,6 +385,11 @@ class ModuleDecl : public DeclContext, public TypeDecl {
   Optional<ProtocolConformanceRef>
   lookupConformance(Type type, ProtocolDecl *protocol);
 
+  /// Look for the conformance of the given existential type to the given
+  /// protocol.
+  Optional<ProtocolConformanceRef>
+  lookupExistentialConformance(Type type, ProtocolDecl *protocol);
+
   /// Find a member named \p name in \p container that was declared in this
   /// module.
   ///

lib/AST/Decl.cpp

@@ -3863,15 +3863,22 @@ bool ProtocolDecl::requiresClassSlow() {
   return Bits.ProtocolDecl.RequiresClass;
 }
 
+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()) {
-    Bits.ProtocolDecl.ExistentialConformsToSelf = false;
-    return false;
+    bool hasSelfConformance = requiresSelfConformanceWitnessTable();
+    Bits.ProtocolDecl.ExistentialConformsToSelf = hasSelfConformance;
+    return hasSelfConformance;
   }
 
   // Check whether this protocol conforms to itself.

lib/AST/Module.cpp

@@ -570,6 +570,66 @@ void ModuleDecl::getDisplayDecls(SmallVectorImpl<Decl*> &Results) const {
   FORWARD(getDisplayDecls, (Results));
 }
 
+Optional<ProtocolConformanceRef>
+ModuleDecl::lookupExistentialConformance(Type type, ProtocolDecl *protocol) {
+  assert(type->isExistentialType());
+
+  // If the existential type cannot be represented or the protocol does not
+  // conform to itself, there's no point in looking further.
+  if (!protocol->existentialConformsToSelf())
+    return None;
+
+  auto layout = type->getExistentialLayout();
+
+  // Due to an IRGen limitation, witness tables cannot be passed from an
+  // existential to an archetype parameter, so for now we restrict this to
+  // @objc protocols.
+  if (!layout.isObjC()) {
+    // There's a specific exception for protocols with self-conforming
+    // witness tables, but the existential has to be *exactly* that type.
+    // TODO: synthesize witness tables on-demand for protocol compositions
+    // that can satisfy the requirement.
+    if (protocol->requiresSelfConformanceWitnessTable() &&
+        type->is<ProtocolType>() &&
+        type->castTo<ProtocolType>()->getDecl() == protocol)
+      return ProtocolConformanceRef(protocol);
+
+    return None;
+  }
+
+  // If the existential is class-constrained, the class might conform
+  // concretely.
+  if (auto superclass = layout.explicitSuperclass) {
+    if (auto result = lookupConformance(superclass, protocol))
+      return result;
+  }
+
+  // Otherwise, the existential might conform abstractly.
+  for (auto proto : layout.getProtocols()) {
+    auto *protoDecl = proto->getDecl();
+
+    // If we found the protocol we're looking for, return an abstract
+    // conformance to it.
+    if (protoDecl == protocol)
+      return ProtocolConformanceRef(protocol);
+
+    // If the protocol has a superclass constraint, we might conform
+    // concretely.
+    if (auto superclass = protoDecl->getSuperclass()) {
+      if (auto result = lookupConformance(superclass, protocol))
+        return result;
+    }
+
+    // Now check refined protocols.
+    if (protoDecl->inheritsFrom(protocol))
+      return ProtocolConformanceRef(protocol);
+  }
+
+  // We didn't find our protocol in the existential's list; it doesn't
+  // conform.
+  return None;
+}
+
 Optional<ProtocolConformanceRef>
 ModuleDecl::lookupConformance(Type type, ProtocolDecl *protocol) {
   ASTContext &ctx = getASTContext();
@@ -609,52 +669,8 @@ ModuleDecl::lookupConformance(Type type, ProtocolDecl *protocol) {
   // An existential conforms to a protocol if the protocol is listed in the
   // existential's list of conformances and the existential conforms to
   // itself.
-  if (type->isExistentialType()) {
-    // If the existential type cannot be represented or the protocol does not
-    // conform to itself, there's no point in looking further.
-    if (!protocol->existentialConformsToSelf())
-      return None;
-
-    auto layout = type->getExistentialLayout();
-
-    // Due to an IRGen limitation, witness tables cannot be passed from an
-    // existential to an archetype parameter, so for now we restrict this to
-    // @objc protocols.
-    if (!layout.isObjC())
-      return None;
-
-    // If the existential is class-constrained, the class might conform
-    // concretely.
-    if (auto superclass = layout.explicitSuperclass) {
-      if (auto result = lookupConformance(superclass, protocol))
-        return result;
-    }
-
-    // Otherwise, the existential might conform abstractly.
-    for (auto proto : layout.getProtocols()) {
-      auto *protoDecl = proto->getDecl();
-
-      // If we found the protocol we're looking for, return an abstract
-      // conformance to it.
-      if (protoDecl == protocol)
-        return ProtocolConformanceRef(protocol);
-
-      // If the protocol has a superclass constraint, we might conform
-      // concretely.
-      if (auto superclass = protoDecl->getSuperclass()) {
-        if (auto result = lookupConformance(superclass, protocol))
-          return result;
-      }
-
-      // Now check refined protocols.
-      if (protoDecl->inheritsFrom(protocol))
-        return ProtocolConformanceRef(protocol);
-    }
-
-    // We didn't find our protocol in the existential's list; it doesn't
-    // conform.
-    return None;
-  }
+  if (type->isExistentialType())
+    return lookupExistentialConformance(type, protocol);
 
   // Type variables have trivial conformances.
   if (type->isTypeVariableOrMember())
@@ -669,7 +685,7 @@ ModuleDecl::lookupConformance(Type type, ProtocolDecl *protocol) {
   auto nominal = type->getAnyNominal();
 
   // If we don't have a nominal type, there are no conformances.
-  if (!nominal) return None;
+  if (!nominal || isa<ProtocolDecl>(nominal)) return None;
 
   // Find the (unspecialized) conformance.
   SmallVector<ProtocolConformance *, 2> conformances;

lib/AST/ProtocolConformance.cpp

@@ -112,14 +112,16 @@ ProtocolConformanceRef::subst(Type origType,
   if (substType->isOpenedExistential())
     return *this;
 
-  // If the substituted type is an existential, we have a self-conforming
-  // existential being substituted in place of itself. There's no
-  // conformance information in this case, so just return.
-  if (substType->isObjCExistentialType())
-    return *this;
-
   auto *proto = getRequirement();
 
+  // If the type is an existential, it must be self-conforming.
+  if (substType->isExistentialType()) {
+    auto optConformance =
+      proto->getModuleContext()->lookupExistentialConformance(substType, proto);
+    assert(optConformance && "existential type didn't self-conform");
+    return *optConformance;
+  }
+
   // Check the conformance map.
   if (auto result = conformances(origType->getCanonicalType(),
                                  substType, proto)) {
@@ -1407,9 +1409,12 @@ DeclContext::getLocalConformances(
   if (!nominal)
     return result;
 
-  // Protocols don't have conformances.
-  if (isa<ProtocolDecl>(nominal))
+  // Protocols only have self-conformances.
+  if (auto protocol = dyn_cast<ProtocolDecl>(nominal)) {
+    if (protocol->requiresSelfConformanceWitnessTable())
+      return { protocol->getASTContext().getSelfConformance(protocol) };
     return { };
+  }
 
   // Update to record all potential conformances.
   nominal->prepareConformanceTable();

lib/IRGen/GenProto.cpp

@@ -2847,16 +2847,23 @@ llvm::Value *irgen::emitWitnessTableRef(IRGenFunction &IGF,
   // If we don't have concrete conformance information, the type must be
   // an archetype and the conformance must be via one of the protocol
   // requirements of the archetype. Look at what's locally bound.
+  ProtocolConformance *concreteConformance;
   if (conformance.isAbstract()) {
-    auto archetype = cast<ArchetypeType>(srcType);
-    return emitArchetypeWitnessTableRef(IGF, archetype, proto);
-  }
+    if (auto archetype = dyn_cast<ArchetypeType>(srcType))
+      return emitArchetypeWitnessTableRef(IGF, archetype, proto);
+
+    // Otherwise, this must be a self-conformance.
+    assert(proto->requiresSelfConformanceWitnessTable());
+    assert(cast<ProtocolType>(srcType)->getDecl() == proto);
+    concreteConformance = IGF.IGM.Context.getSelfConformance(proto);
 
   // All other source types should be concrete enough that we have
   // conformance info for them.  However, that conformance info might be
   // more concrete than we're expecting.
   // TODO: make a best effort to devirtualize, maybe?
-  auto concreteConformance = conformance.getConcrete();
+  } else {
+    concreteConformance = conformance.getConcrete();
+  }
   assert(concreteConformance->getProtocol() == proto);
 
   auto cacheKind =

lib/SILGen/SILGen.h

@@ -307,6 +307,9 @@ class LLVM_LIBRARY_VISIBILITY SILGenModule : public ASTVisitor<SILGenModule> {
   /// Emit the default witness table for a resilient protocol.
   void emitDefaultWitnessTable(ProtocolDecl *protocol);
 
+  /// Emit the self-conformance witness table for a protocol.
+  void emitSelfConformanceWitnessTable(ProtocolDecl *protocol);
+
   /// Emit the lazy initializer function for a global pattern binding
   /// declaration.
   SILFunction *emitLazyGlobalInitializer(StringRef funcName,

lib/SILGen/SILGenFunction.h

@@ -651,7 +651,8 @@ class LLVM_LIBRARY_VISIBILITY SILGenFunction
                            SubstitutionMap reqtSubs,
                            SILDeclRef witness,
                            SubstitutionMap witnessSubs,
-                           IsFreeFunctionWitness_t isFree);
+                           IsFreeFunctionWitness_t isFree,
+                           bool isSelfConformance);
 
   /// Convert a block to a native function with a thunk.
   ManagedValue emitBlockToFunc(SILLocation loc,