[ConformanceLookup] Don't allow skipping inherited unavailable conformances

in favor of explicit available ones.

The type checker does not support the notion of multiple protocol
conformances; there can only be one conformance, and if that conformance
is unavailable, you cannot specify your own available conformance. This
is important for Sendable checking; if a framework specifies that a type
is explicitly not Sendable with an unavailable Sendable conformance,
clients cannot ignore Sendable violations involving that type. If a
superclass wants to allow subclasses to add a Sendable conformance, it
should not declare an unavailable Sendable conformance.

Author: hborla

lib/AST/ConformanceLookupTable.cpp

@@ -259,14 +259,6 @@ void ConformanceLookupTable::inheritConformances(ClassDecl *classDecl,
   auto addInheritedConformance = [&](ConformanceEntry *entry) {
     auto protocol = entry->getProtocol();
 
-    // Don't add unavailable conformances.
-    if (auto dc = entry->Source.getDeclContext()) {
-      if (auto ext = dyn_cast<ExtensionDecl>(dc)) {
-        if (AvailableAttr::isUnavailable(ext))
-          return;
-      }
-    }
-
     // Don't add redundant conformances here. This is merely an
     // optimization; resolveConformances() would zap the duplicates
     // anyway.
@@ -628,6 +620,14 @@ ConformanceLookupTable::Ordering ConformanceLookupTable::compareConformances(
 
       // Allow replacement of an explicit conformance to a marker protocol.
       // (This permits redundant explicit declarations of `Sendable`.)
+      //
+      // FIXME: We need to warn on attempts to make an unavailable Sendable
+      // conformance available, which does not work.
+      //
+      // We probably also want to warn if there is an existing, explicit
+      // conformance, so clients are prompted to remove retroactive unchecked
+      // Sendable conformances when the proper Sendable conformance is added
+      // in the original module.
       return (kind == ConformanceEntryKind::Explicit
               && entry->getProtocol()->isMarkerProtocol());
     };
@@ -879,8 +879,6 @@ DeclContext *ConformanceLookupTable::getConformingContext(
           return nullptr;
         auto inheritedConformance = ModuleDecl::lookupConformance(
             superclassTy, protocol, /*allowMissing=*/false);
-        if (inheritedConformance.hasUnavailableConformance())
-          inheritedConformance = ProtocolConformanceRef::forInvalid();
         if (inheritedConformance)
           return superclassDecl;
       } while ((superclassDecl = superclassDecl->getSuperclassDecl()));

test/Concurrency/sendable_checking.swift

@@ -102,7 +102,10 @@ public actor MyActor: MyProto {
 }
 
 // Make sure the generic signature doesn't minimize away Sendable requirements.
-@_nonSendable class NSClass { }
+class NSClass { }
+
+@available(*, unavailable)
+extension NSClass: @unchecked Sendable {} // expected-note {{conformance of 'NSClass' to 'Sendable' has been explicitly marked unavailable here}}
 
 struct WrapClass<T: NSClass> {
   var t: T
@@ -116,7 +119,7 @@ class SendableSubclass: NSClass, @unchecked Sendable { }
 
 @available(SwiftStdlib 5.1, *)
 func testSubclassing(obj: SendableSubclass) async {
-  acceptCV(obj) // okay!
+  acceptCV(obj) // expected-warning {{conformance of 'NSClass' to 'Sendable' is unavailable; this is an error in the Swift 6 language mode}}
 }