Allow Error to conform to itself.

Most of the foundation for this was laid in earlier patches.

Author: rjmccall

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

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

@@ -585,6 +585,15 @@ ModuleDecl::lookupExistentialConformance(Type type, ProtocolDecl *protocol) {
   // 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;
   }
 
@@ -676,7 +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

@@ -1409,9 +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,

lib/SILGen/SILGenPoly.cpp

@@ -3575,15 +3575,23 @@ SILGenFunction::emitVTableThunk(SILDeclRef derived,
 enum class WitnessDispatchKind {
   Static,
   Dynamic,
-  Class
+  Class,
+  Witness
 };
 
-static WitnessDispatchKind getWitnessDispatchKind(SILDeclRef witness) {
+static WitnessDispatchKind getWitnessDispatchKind(SILDeclRef witness,
+                                                  bool isSelfConformance) {
   auto *decl = witness.getDecl();
 
+  if (isSelfConformance) {
+    assert(isa<ProtocolDecl>(decl->getDeclContext()));
+    return WitnessDispatchKind::Witness;
+  }
+
   ClassDecl *C = decl->getDeclContext()->getSelfClassDecl();
-  if (!C)
+  if (!C) {
     return WitnessDispatchKind::Static;
+  }
 
   // If the witness is dynamic, go through dynamic dispatch.
   if (decl->isObjCDynamic()) {
@@ -3630,6 +3638,7 @@ getWitnessFunctionType(SILGenModule &SGM,
   switch (witnessKind) {
   case WitnessDispatchKind::Static:
   case WitnessDispatchKind::Dynamic:
+  case WitnessDispatchKind::Witness:
     return SGM.Types.getConstantInfo(witness).SILFnType;
   case WitnessDispatchKind::Class:
     return SGM.Types.getConstantOverrideType(witness);
@@ -3638,18 +3647,36 @@ getWitnessFunctionType(SILGenModule &SGM,
   llvm_unreachable("Unhandled WitnessDispatchKind in switch.");
 }
 
+static std::pair<CanType, ProtocolConformanceRef>
+getSelfTypeAndConformanceForWitness(SILDeclRef witness, SubstitutionMap subs) {
+  auto protocol = cast<ProtocolDecl>(witness.getDecl()->getDeclContext());
+  auto selfParam = protocol->getProtocolSelfType()->getCanonicalType();
+  auto type = subs.getReplacementTypes()[0];
+  auto conf = *subs.lookupConformance(selfParam, protocol);
+  return {type->getCanonicalType(), conf};
+}
+
 static SILValue
 getWitnessFunctionRef(SILGenFunction &SGF,
                       SILDeclRef witness,
                       CanSILFunctionType witnessFTy,
                       WitnessDispatchKind witnessKind,
+                      SubstitutionMap witnessSubs,
                       SmallVectorImpl<ManagedValue> &witnessParams,
                       SILLocation loc) {
   switch (witnessKind) {
   case WitnessDispatchKind::Static:
     return SGF.emitGlobalFunctionRef(loc, witness);
   case WitnessDispatchKind::Dynamic:
     return SGF.emitDynamicMethodRef(loc, witness, witnessFTy).getValue();
+  case WitnessDispatchKind::Witness: {
+    auto typeAndConf =
+      getSelfTypeAndConformanceForWitness(witness, witnessSubs);
+    return SGF.B.createWitnessMethod(loc, typeAndConf.first,
+                                     typeAndConf.second,
+                                     witness,
+                            SILType::getPrimitiveObjectType(witnessFTy));
+  }
   case WitnessDispatchKind::Class: {
     SILValue selfPtr = witnessParams.back().getValue();
     return SGF.emitClassMethodRef(loc, selfPtr, witness, witnessFTy);
@@ -3659,13 +3686,32 @@ getWitnessFunctionRef(SILGenFunction &SGF,
   llvm_unreachable("Unhandled WitnessDispatchKind in switch.");
 }
 
+static ManagedValue
+emitOpenExistentialInSelfConformance(SILGenFunction &SGF, SILLocation loc,
+                                     SILDeclRef witness,
+                                     SubstitutionMap subs, ManagedValue value,
+                                     SILParameterInfo destParameter) {
+  auto typeAndConf = getSelfTypeAndConformanceForWitness(witness, subs);
+  auto archetype = typeAndConf.first->castTo<ArchetypeType>();
+  assert(archetype->isOpenedExistential());
+
+  auto openedTy = destParameter.getSILStorageType();
+  auto state = SGF.emitOpenExistential(loc, value, archetype, openedTy,
+                                       destParameter.isIndirectMutating()
+                                         ? AccessKind::ReadWrite
+                                         : AccessKind::Read);
+
+  return state.Value;
+}
+
 void SILGenFunction::emitProtocolWitness(AbstractionPattern reqtOrigTy,
                                          CanAnyFunctionType reqtSubstTy,
                                          SILDeclRef requirement,
                                          SubstitutionMap reqtSubs,
                                          SILDeclRef witness,
                                          SubstitutionMap witnessSubs,
-                                         IsFreeFunctionWitness_t isFree) {
+                                         IsFreeFunctionWitness_t isFree,
+                                         bool isSelfConformance) {
   // FIXME: Disable checks that the protocol witness carries debug info.
   // Should we carry debug info for witnesses?
   F.setBare(IsBare);
@@ -3679,7 +3725,7 @@ void SILGenFunction::emitProtocolWitness(AbstractionPattern reqtOrigTy,
   FullExpr scope(Cleanups, cleanupLoc);
   FormalEvaluationScope formalEvalScope(*this);
 
-  auto witnessKind = getWitnessDispatchKind(witness);
+  auto witnessKind = getWitnessDispatchKind(witness, isSelfConformance);
   auto thunkTy = F.getLoweredFunctionType();
 
   SmallVector<ManagedValue, 8> origParams;
@@ -3704,9 +3750,24 @@ void SILGenFunction::emitProtocolWitness(AbstractionPattern reqtOrigTy,
                                           ->getCanonicalType());
   }
 
+  // Get the lowered type of the witness.
+  auto origWitnessFTy = getWitnessFunctionType(SGM, witness, witnessKind);
+  auto witnessFTy = origWitnessFTy;
+  if (!witnessSubs.empty())
+    witnessFTy = origWitnessFTy->substGenericArgs(SGM.M, witnessSubs);
+
   auto reqtSubstParams = reqtSubstTy.getParams();
   auto witnessSubstParams = witnessSubstTy.getParams();
 
+  // For a self-conformance, open the self parameter.
+  if (isSelfConformance) {
+    assert(!isFree && "shouldn't have a free witness for a self-conformance");
+    origParams.back() =
+      emitOpenExistentialInSelfConformance(*this, loc, witness, witnessSubs,
+                                           origParams.back(),
+                                           witnessFTy->getSelfParameter());
+  }
+
   // For a free function witness, discard the 'self' parameter of the
   // requirement.
   if (isFree) {
@@ -3716,11 +3777,6 @@ void SILGenFunction::emitProtocolWitness(AbstractionPattern reqtOrigTy,
 
   // Translate the argument values from the requirement abstraction level to
   // the substituted signature of the witness.
-  auto origWitnessFTy = getWitnessFunctionType(SGM, witness, witnessKind);
-  auto witnessFTy = origWitnessFTy;
-  if (!witnessSubs.empty())
-    witnessFTy = origWitnessFTy->substGenericArgs(SGM.M, witnessSubs);
-
   SmallVector<ManagedValue, 8> witnessParams;
   AbstractionPattern witnessOrigTy(witnessInfo.LoweredType);
   TranslateArguments(*this, loc,
@@ -3733,7 +3789,7 @@ void SILGenFunction::emitProtocolWitness(AbstractionPattern reqtOrigTy,
 
   SILValue witnessFnRef = getWitnessFunctionRef(*this, witness,
                                                 origWitnessFTy,
-                                                witnessKind,
+                                                witnessKind, witnessSubs,
                                                 witnessParams, loc);
 
   auto coroutineKind =