Support missing Sendable conformances everywhere in the requirement machine

Author: DougGregor

lib/AST/RequirementMachine/ConcreteTypeWitness.cpp

@@ -144,7 +144,8 @@ void PropertyMap::concretizeNestedTypesFromConcreteParent(
     auto *module = proto->getParentModule();
 
     auto conformance = module->lookupConformance(concreteType,
-                                                 const_cast<ProtocolDecl *>(proto));
+                                                 const_cast<ProtocolDecl *>(proto),
+                                                 /*allowMissing=*/true);
     if (conformance.isInvalid()) {
       // For superclass rules, it is totally fine to have a signature like:
       //

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -292,7 +292,8 @@ static void desugarConformanceRequirement(Type subjectType, Type constraintType,
       // Check if the subject type actually conforms.
       auto *protoDecl = constraintType->castTo<ProtocolType>()->getDecl();
       auto *module = protoDecl->getParentModule();
-      auto conformance = module->lookupConformance(subjectType, protoDecl);
+      auto conformance = module->lookupConformance(
+          subjectType, protoDecl, /*allowMissing=*/true);
       if (conformance.isInvalid()) {
         errors.push_back(RequirementError::forInvalidRequirementSubject(
             {RequirementKind::Conformance, subjectType, constraintType}, loc));

test/Concurrency/sendable_conformance_checking.swift

@@ -168,8 +168,26 @@ extension MultiConformance: @unchecked Sendable {} // expected-error {{redundant
 
 // rdar://91174106 - allow missing Sendable conformances when extending a
 // type generically.
-// FIXME: Should warn because of missing Sendable, but currently is silent.
-// We don't want a hard error except in Swift 6.
+// FIXME: Should warn because of missing Sendable, but currently is silent
+// because we aren't checking conformance availability here yet.
 struct X<T: Sendable> { }
 enum Y {}
 extension X where T == Y {}
+
+protocol P2 {
+  associatedtype A: Sendable
+}
+
+enum Y2: P2, P3 {
+  typealias A = Y
+}
+
+struct X2<T: P2> { }
+extension X2 where T == Y2 { }
+
+protocol P3 {
+  associatedtype A
+}
+
+struct X3<T: P3> where T.A: Sendable { }
+extension X3 where T == Y2 { }