Merge pull request #71496 from DougGregor/infer-assoc-type-witness-availability

Author: DougGregor

lib/Sema/AssociatedTypeInference.cpp

@@ -298,6 +298,12 @@ static void recordTypeWitness(NormalProtocolConformance *conformance,
       aliasDecl->getAttrs().add(attr);
     }
 
+    // Construct the availability of the type witnesses based on the
+    // availability of the enclosing type and the associated type.
+    const Decl * availabilitySources[2] = {dc->getAsDecl(), assocType };
+    AvailabilityInference::applyInferredAvailableAttrs(
+        aliasDecl, availabilitySources, ctx);
+
     if (nominal == dc) {
       nominal->addMember(aliasDecl);
     } else {

test/ModuleInterface/async_sequence_conformance.swift

@@ -0,0 +1,27 @@
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -module-name conformances
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name conformances
+// RUN: %FileCheck %s < %t.swiftinterface
+
+// REQUIRES: concurrency, OS=macosx
+
+// CHECK: @available(
+// CHECK-NEXT: public struct SequenceAdapte
+@available(SwiftStdlib 5.1, *)
+public struct SequenceAdapter<Base: AsyncSequence>: AsyncSequence {
+  // CHECK-LABEL: public struct AsyncIterator
+  // CHECK: @available{{.*}}macOS 10.15
+  // CHECK-NEXT: public typealias Element = Base.Element
+  // CHECK: @available(
+  // CHECK-NEXT: public typealias Failure = Base.Failure
+  public typealias Element = Base.Element
+
+  public struct AsyncIterator: AsyncIteratorProtocol {
+    public mutating func next() async rethrows -> Base.Element? { nil }
+  }
+
+  // CHECK-LABEL: public func makeAsyncIterator
+  public func makeAsyncIterator() -> AsyncIterator { AsyncIterator() }
+
+  // CHECK: @available(
+  // CHECK-NEXT: public typealias Failure = Base.Failure
+}