Merge pull request #76067 from tshortli/associated-type-inference-availability-6.0

[6.0] Sema: Relax availability of typealiases for inferred type witness

Author: tshortli

lib/Sema/AssociatedTypeInference.cpp

@@ -331,7 +331,18 @@ static void recordTypeWitness(NormalProtocolConformance *conformance,
 
     // 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 };
+    llvm::SmallVector<Decl *, 2> availabilitySources = {dc->getAsDecl()};
+
+    // Only constrain the availability of the typealias by the availability of
+    // the associated type if the associated type is less available than its
+    // protocol. This is required for source compatibility.
+    auto protoAvailability = AvailabilityInference::availableRange(proto, ctx);
+    auto assocTypeAvailability =
+        AvailabilityInference::availableRange(assocType, ctx);
+    if (protoAvailability.isSupersetOf(assocTypeAvailability)) {
+      availabilitySources.push_back(assocType);
+    }
+
     AvailabilityInference::applyInferredAvailableAttrs(
         aliasDecl, availabilitySources, ctx);
 

test/decl/protocol/associated_type_availability.swift

@@ -53,3 +53,34 @@ func testPrimaryExistentialBad<U>(_: any P1<Int, U>) {}
 
 @available(macOS 13, *)
 func testPrimaryExistentialGood<U>(_: any P1<Int, U>) {}
+
+
+@available(macOS 13, *)
+protocol P2 {
+  associatedtype A
+
+  @available(macOS 14, *)
+  associatedtype B
+
+  var a: A { get }
+
+  @available(macOS 14, *)
+  var b: B { get }
+}
+
+struct ModelP2: P2 {
+  var a: Int
+  var b: Double
+}
+
+extension ModelP2 {
+  // expected-note@-1{{add @available attribute to enclosing extension}}
+
+  // Ok, the inferred typealias for A is always available.
+  func takesA(_ a: A) {}
+
+  // Bad, the inferred typealias for B is introduced with associated type B.
+  func takesB(_ b: B) {}
+  // expected-error@-1{{'B' is only available in macOS 14 or newer}}
+  // expected-note@-2{{add @available attribute to enclosing instance method}}
+}

test/stdlib/Identifiable.swift

@@ -4,4 +4,10 @@ struct IdentifiableValue: Identifiable {
   let id = 42
 }
 
-class IdentifiableClass: Identifiable {}
+class IdentifiableClass: Identifiable {}
+
+extension IdentifiableValue {
+  var nextID: ID {
+    return id + 1
+  }
+}