Sema: Relax associated conformance availability checking.

If an associated conformance comes from an associated type requirement, then
the associated conformance only needs to be as available as that associated
type.

Resolves rdar://133544161.

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -295,6 +295,13 @@ ExportContext ExportContext::withExported(bool exported) const {
   return copy;
 }
 
+ExportContext ExportContext::withRefinedAvailability(
+    const AvailabilityContext &availability) const {
+  auto copy = *this;
+  copy.RunningOSVersion.intersectWith(availability);
+  return copy;
+}
+
 std::optional<PlatformKind> ExportContext::getUnavailablePlatformKind() const {
   if (Unavailable)
     return PlatformKind(Platform);

lib/Sema/TypeCheckAvailability.h

@@ -153,6 +153,11 @@ class ExportContext {
   /// That is, this will perform a 'bitwise and' on the 'exported' bit.
   ExportContext withExported(bool exported) const;
 
+  /// Produce a new context with the same properties as this one, except the
+  /// availability context is constrained by \p availability if necessary.
+  ExportContext
+  withRefinedAvailability(const AvailabilityContext &availability) const;
+
   DeclContext *getDeclContext() const { return DC; }
 
   AvailabilityContext getAvailabilityContext() const {

lib/Sema/TypeCheckProtocol.cpp

@@ -4987,9 +4987,21 @@ static void ensureRequirementsAreSatisfied(ASTContext &ctx,
       if (assocConf.isConcrete()) {
         auto *concrete = assocConf.getConcrete();
         auto replacementTy = dc->mapTypeIntoContext(concrete->getType());
-        diagnoseConformanceAvailability(conformance->getLoc(),
-                                        assocConf, where,
-                                        depTy, replacementTy);
+
+        // If this requirement has a dependent member type, only require the
+        // associated conformance to be as available as the requirement's
+        // associated type.
+        auto availability = AvailabilityContext::alwaysAvailable();
+        if (auto depMemberType = depTy->getAs<DependentMemberType>()) {
+          auto assocType = depMemberType->getAssocType();
+          availability.intersectWith(
+              TypeChecker::overApproximateAvailabilityAtLocation(
+                  assocType->getLoc(), assocType->getDeclContext()));
+        }
+
+        diagnoseConformanceAvailability(
+            conformance->getLoc(), assocConf,
+            where.withRefinedAvailability(availability), depTy, replacementTy);
       }
 
       return false;

test/Sema/conformance_availability.swift

@@ -279,6 +279,15 @@ func passAvailableConformance1a(x: HasAvailableConformance1) {
   _ = UsesHorse<HasAvailableConformance1>.self
 }
 
+// Always available conformance
+struct HasAvailableConformance2: Horse {}
+
+// Conformance available in macOS 200
+struct HasAvailableConformance3 {}
+
+@available(macOS 200, *)
+extension HasAvailableConformance3: Horse {}
+
 // Associated conformance with unavailability
 protocol Rider {
   associatedtype H : Horse
@@ -326,6 +335,58 @@ extension AssocConformanceAvailable4 : Rider {
   typealias H = HasAvailableConformance1
 }
 
+@available(macOS 100, *)
+protocol Saddle {
+  associatedtype H : Horse = HasAvailableConformance1
+}
+
+struct ConformsToSaddle1 : Saddle {}
+
+struct ConformsToSaddle2 : Saddle {
+  typealias H = HasAvailableConformance2
+}
+
+struct ConformsToSaddle3 : Saddle {
+  // expected-error@-1 {{conformance of 'HasAvailableConformance3' to 'Horse' is only available in macOS 200 or newer}}
+  // expected-note@-2 {{add @available attribute to enclosing struct}}
+  // expected-note@-3 {{in associated type 'Self.H' (inferred as 'HasAvailableConformance3'}}
+  typealias H = HasAvailableConformance3
+}
+
+struct ConformsToSaddle4 : Saddle {
+  // expected-error@-1 {{conformance of 'HasAvailableConformance3' to 'Horse' is only available in macOS 200 or newer}}
+  // expected-note@-2 {{add @available attribute to enclosing struct}}
+  // expected-note@-3 {{in associated type 'Self.H' (inferred as 'HasAvailableConformance3'}}
+  @available(macOS 200, *)
+  typealias H = HasAvailableConformance3
+}
+
+protocol Barn {
+  @available(macOS 100, *)
+  associatedtype H : Horse = HasAvailableConformance1
+}
+
+struct ConformsToBarn1 : Barn {}
+
+struct ConformsToBarn2 : Barn {
+  typealias H = HasAvailableConformance2
+}
+
+struct ConformsToBarn3 : Barn {
+  // expected-error@-1 {{conformance of 'HasAvailableConformance3' to 'Horse' is only available in macOS 200 or newer}}
+  // expected-note@-2 {{add @available attribute to enclosing struct}}
+  // expected-note@-3 {{in associated type 'Self.H' (inferred as 'HasAvailableConformance3'}}
+  typealias H = HasAvailableConformance3
+}
+
+struct ConformsToBarn4 : Barn {
+  // expected-error@-1 {{conformance of 'HasAvailableConformance3' to 'Horse' is only available in macOS 200 or newer}}
+  // expected-note@-2 {{add @available attribute to enclosing struct}}
+  // expected-note@-3 {{in associated type 'Self.H' (inferred as 'HasAvailableConformance3'}}
+  @available(macOS 200, *)
+  typealias H = HasAvailableConformance3
+}
+
 // Solution ranking should down-rank solutions involving unavailable conformances
 protocol First {}
 extension First {