Always infer AsyncSequence.Failure from AsyncIteratorProtocol.Failure

The newly-introduced associated type `AsyncSequence.Failure` must
always be equivalent to the `Failure` type of the
`AsyncIteratorProtocol`. If the `AsyncSequence` type itself defines a
nested `Failure` type (say, for another purpose), associated type inference
would pick it and reject the `AsyncSequence`, causing a source compatibility
problem.

Work around the issue in two ways. First, always infer the type
witness for `AsyncSequence.Failure` from the type witness for
`AsyncIteratorProtocol.Failure`, so they can't be out of sync. This
means that we'll never even consider a nested `Failure` type in the
`AsyncSequence`-conforming type. This hack only applies prior to Swift 6.

Second, when we have inferred a `Failure` type and there is already
something else called `Failure` within that same nominal type, don't
print the inferred typelias into a module interface because it will
cause a conflict.

Fixes rdar://123543633.

lib/AST/ASTPrinter.cpp

@@ -6374,6 +6374,32 @@ void Decl::printInherited(ASTPrinter &Printer, const PrintOptions &Opts) const {
   printer.printInherited(this);
 }
 
+/// Determine whether this typealias is an inferred typealias "Failure" that
+/// would conflict with another entity named failure in the same type.
+static bool isConflictingFailureTypeWitness(
+    const TypeAliasDecl *typealias) {
+  if (!typealias->isImplicit())
+    return false;
+
+  ASTContext &ctx = typealias->getASTContext();
+  if (typealias->getName() != ctx.Id_Failure)
+    return false;
+
+  auto nominal = typealias->getDeclContext()->getSelfNominalTypeDecl();
+  if (!nominal)
+    return false;
+
+  // Look for another entity with the same name.
+  auto lookupResults = nominal->lookupDirect(
+      typealias->getName(), typealias->getLoc());
+  for (auto found : lookupResults) {
+    if (found != typealias)
+      return true;
+  }
+
+  return false;
+}
+
 bool Decl::shouldPrintInContext(const PrintOptions &PO) const {
   // Skip getters/setters. They are part of the variable or subscript.
   if (isa<AccessorDecl>(this))
@@ -6413,6 +6439,14 @@ bool Decl::shouldPrintInContext(const PrintOptions &PO) const {
     return PO.PrintIfConfig;
   }
 
+  // Prior to Swift 6, we shouldn't print the inferred associated type
+  // witness for AsyncSequence.Failure. It is always determined from the
+  // AsyncIteratorProtocol witness.
+  if (auto typealias = dyn_cast<TypeAliasDecl>(this)) {
+    if (isConflictingFailureTypeWitness(typealias))
+      return false;
+  }
+
   // Print everything else.
   return true;
 }

lib/Sema/AssociatedTypeInference.cpp

@@ -389,6 +389,35 @@ static void recordTypeWitness(NormalProtocolConformance *conformance,
   }
 }
 
+/// Determine whether this is the AsyncIteratorProtocol.Failure associated type.
+static bool isAsyncIteratorProtocolFailure(AssociatedTypeDecl *assocType) {
+  auto proto = assocType->getProtocol();
+  if (!proto->isSpecificProtocol(KnownProtocolKind::AsyncIteratorProtocol))
+    return false;
+
+  return assocType->getName() == assocType->getASTContext().Id_Failure;
+}
+
+/// Determine whether this is the AsyncSequence.Failure associated type.
+static bool isAsyncSequenceFailure(AssociatedTypeDecl *assocType) {
+  auto proto = assocType->getProtocol();
+  if (!proto->isSpecificProtocol(KnownProtocolKind::AsyncSequence))
+    return false;
+
+  return assocType->getName() == assocType->getASTContext().Id_Failure;
+}
+
+/// Determine whether this is the AsyncIteratorProtocol.Failure or
+/// AsyncSequence.Failure associated type.
+static bool isAsyncIteratorOrSequenceFailure(AssociatedTypeDecl *assocType) {
+  auto proto = assocType->getProtocol();
+  if (!proto->isSpecificProtocol(KnownProtocolKind::AsyncIteratorProtocol) &&
+      !proto->isSpecificProtocol(KnownProtocolKind::AsyncSequence))
+    return false;
+
+  return assocType->getName() == assocType->getASTContext().Id_Failure;
+}
+
 /// Attempt to resolve a type witness via member name lookup.
 static ResolveWitnessResult resolveTypeWitnessViaLookup(
                        NormalProtocolConformance *conformance,
@@ -396,6 +425,13 @@ static ResolveWitnessResult resolveTypeWitnessViaLookup(
   auto *dc = conformance->getDeclContext();
   auto &ctx = dc->getASTContext();
 
+  // Prior to Swift 6, don't look for a named type witness for
+  // AsyncSequence.Failure. We'll always infer it from
+  // AsyncIteratorProtocol.Failure.
+  if (isAsyncSequenceFailure(assocType) &&
+      !ctx.LangOpts.isSwiftVersionAtLeast(6))
+    return ResolveWitnessResult::Missing;
+
   // Conformances constructed by the ClangImporter should have explicit type
   // witnesses already.
   if (isa<ClangModuleUnit>(dc->getModuleScopeContext())) {
@@ -1816,17 +1852,6 @@ next_witness:;
   return result;
 }
 
-/// Determine whether this is AsyncIteratorProtocol.Failure or
-/// AsyncSequenceProtoco.Failure associated type.
-static bool isAsyncIteratorProtocolFailure(AssociatedTypeDecl *assocType) {
-  auto proto = assocType->getProtocol();
-  if (!proto->isSpecificProtocol(KnownProtocolKind::AsyncIteratorProtocol) &&
-      !proto->isSpecificProtocol(KnownProtocolKind::AsyncSequence))
-    return false;
-
-  return assocType->getName() == assocType->getASTContext().Id_Failure;
-}
-
 /// Determine whether this is AsyncIteratorProtocol.next() function.
 static bool isAsyncIteratorProtocolNext(ValueDecl *req) {
   auto proto = dyn_cast<ProtocolDecl>(req->getDeclContext());
@@ -1834,7 +1859,8 @@ static bool isAsyncIteratorProtocolNext(ValueDecl *req) {
       !proto->isSpecificProtocol(KnownProtocolKind::AsyncIteratorProtocol))
     return false;
 
-  return req->getName().getBaseName() == req->getASTContext().Id_next;
+  return req->getName().getBaseName() == req->getASTContext().Id_next &&
+         req->getName().getArgumentNames().empty();
 }
 
 InferredAssociatedTypes
@@ -2537,8 +2563,7 @@ std::optional<AbstractTypeWitness>
 AssociatedTypeInference::computeFailureTypeWitness(
     AssociatedTypeDecl *assocType,
     ArrayRef<std::pair<ValueDecl *, ValueDecl *>> valueWitnesses) const {
-  // Inference only applies to AsyncIteratorProtocol.Failure and
+  // Inference only applies to AsyncIteratorProtocol.Failure.
-  // AsyncSequence.Failure.
   if (!isAsyncIteratorProtocolFailure(assocType))
     return std::nullopt;
 
@@ -2582,7 +2607,7 @@ AssociatedTypeInference::computeDefaultTypeWitness(
     AssociatedTypeDecl *assocType) const {
   // Ignore the default for AsyncIteratorProtocol.Failure and
   // AsyncSequence.Failure.
-  if (isAsyncIteratorProtocolFailure(assocType))
+  if (isAsyncIteratorOrSequenceFailure(assocType))
     return std::nullopt;
 
   // Go find a default definition.
@@ -2683,8 +2708,8 @@ AssociatedTypeInference::computeAbstractTypeWitness(
 
   // Don't consider the generic parameter names for AsyncSequence.Failure or
   // AsyncIteratorProtocol.Failure; we always rely on inference from next() or
-  // next(_:).
+  // next(isolation:).
-  if (isAsyncIteratorProtocolFailure(assocType)) {
+  if (isAsyncIteratorOrSequenceFailure(assocType)) {
     // If this is specifically AsyncSequence.Failure with the older associated
     // type inference implementation, our abstract witness is
     // "AsyncIterator.Failure". The new implementation is smart enough to do
@@ -2727,7 +2752,7 @@ Type AssociatedTypeInference::computeGenericParamWitness(
   if (auto genericSig = dc->getGenericSignatureOfContext()) {
     // Ignore the generic parameters for AsyncIteratorProtocol.Failure and
     // AsyncSequence.Failure.
-    if (!isAsyncIteratorProtocolFailure(assocType)) {
+    if (!isAsyncIteratorOrSequenceFailure(assocType)) {
       for (auto *gp : genericSig.getInnermostGenericParams()) {
         // Packs cannot witness associated type requirements.
         if (gp->isParameterPack())

test/Concurrency/async_sequence_syntax.swift

@@ -99,3 +99,23 @@ func forTryAwaitReturningExistentialType() async throws {
   for try await _ in S().seq() { // Ok
   }
 }
+
+@available(SwiftStdlib 5.1, *)
+public struct ReaderSeq: AsyncSequence, Sendable {
+  public enum Failure: Error {
+    case x
+  }
+
+  public typealias Element = Int
+
+  public func makeAsyncIterator() -> Reader {}
+
+  public actor Reader: AsyncIteratorProtocol {
+    public func next() async throws -> Element? {}
+  }
+}
+
+@available(SwiftStdlib 5.1, *)
+func test1() -> Error {
+  return ReaderSeq.Failure.x
+}

test/ModuleInterface/async_sequence_conformance.swift

@@ -25,3 +25,30 @@ public struct SequenceAdapter<Base: AsyncSequence>: AsyncSequence {
   // CHECK: @available(
   // CHECK-NEXT: public typealias Failure = Base.Failure
 }
+
+// CHECK: @available(
+// CHECK-NEXT: public struct OtherSequenceAdapte
+@available(SwiftStdlib 5.1, *)
+public struct OtherSequenceAdapter<Base: AsyncSequence>: AsyncSequence {
+  // CHECK: public typealias Element = Base.Element
+  // CHECK-NOT: public typealias Failure
+  // CHECK: public struct Failure
+
+  // CHECK-LABEL: public struct AsyncIterator
+  // CHECK: @available{{.*}}macOS 10.15
+  // CHECK: @available(
+  // CHECK-NEXT: public typealias Failure = Base.Failure
+  public typealias Element = Base.Element
+
+  public struct Failure: Error { }
+
+  // CHECK-NOT: public typealias Failure
+  public struct AsyncIterator: AsyncIteratorProtocol {
+    public mutating func next() async rethrows -> Base.Element? { nil }
+  }
+
+  // CHECK: public func makeAsyncIterator
+  public func makeAsyncIterator() -> AsyncIterator { AsyncIterator() }
+
+  // CHECK-NOT: public typealias Failure
+}