Merge pull request #72552 from DougGregor/implements-on-asyncsequence-failure-6.0

[Associated type inference] Resolve `@_implements` on AsyncSequence.Failure

Author: DougGregor

lib/Sema/AssociatedTypeInference.cpp

@@ -430,13 +430,6 @@ 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())) {
@@ -479,10 +472,18 @@ static ResolveWitnessResult resolveTypeWitnessViaLookup(
     // Also skip candidates in protocol extensions, because they tend to cause
     // request cycles. We'll look at those during associated type inference.
     if (assocType->getName() != typeDecl->getName() &&
-        !(witnessHasImplementsAttrForRequiredName(typeDecl, assocType) &&
+        !(witnessHasImplementsAttrForExactRequirement(typeDecl, assocType) &&
           !typeDecl->getDeclContext()->getSelfProtocolDecl()))
       continue;
 
+    // Prior to Swift 6, ignore a member named Failure when matching
+    // AsyncSequence.Failure. We'll infer it from the AsyncIterator.Failure
+    // instead.
+    if (isAsyncSequenceFailure(assocType) &&
+        !ctx.LangOpts.isSwiftVersionAtLeast(6) &&
+        assocType->getName() == typeDecl->getName())
+      continue;;
+
     auto *genericDecl = cast<GenericTypeDecl>(typeDecl);
 
     // If the declaration has generic parameters, it cannot witness an

test/Concurrency/async_sequence_syntax.swift

@@ -119,3 +119,15 @@ public struct ReaderSeq: AsyncSequence, Sendable {
 func test1() -> Error {
   return ReaderSeq.Failure.x
 }
+
+@available(SwiftStdlib 5.1, *)
+public struct MineOwnIterator<Element>: AsyncSequence, AsyncIteratorProtocol {
+  public mutating func next() async -> Element? { nil }
+  public func makeAsyncIterator() -> Self { self }
+
+  @_implements(AsyncIteratorProtocol, Failure)
+  public typealias __AsyncIteratorProtocol_Failure = Never
+
+  @_implements(AsyncSequence, Failure)
+  public typealias __AsyncSequence_Failure = Never
+}