Perform generic substitutions for Sendable checking in protocol conformances

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -2864,6 +2864,17 @@ static void emitDeclaredHereIfNeeded(DiagnosticEngine &diags,
 
 bool ConformanceChecker::checkActorIsolation(
     ValueDecl *requirement, ValueDecl *witness) {
+  /// Retrieve a concrete witness for Sendable checking.
+  auto getConcreteWitness = [&] {
+    if (auto genericEnv = witness->getInnermostDeclContext()
+            ->getGenericEnvironmentOfContext()) {
+      return ConcreteDeclRef(
+          witness, genericEnv->getForwardingSubstitutionMap());
+    }
+
+    return ConcreteDeclRef(witness);
+  };
+
   // Ensure that the witness is not actor-isolated in a manner that makes it
   // unsuitable as a witness.
   bool isCrossActor = false;
@@ -3014,8 +3025,8 @@ bool ConformanceChecker::checkActorIsolation(
 
   case ActorIsolationRestriction::CrossActorSelf: {
     if (diagnoseNonSendableTypesInReference(
-        witness, DC, witness->getLoc(),
-        ConcurrentReferenceKind::CrossActor)) {
+            getConcreteWitness(), DC, witness->getLoc(),
+            ConcurrentReferenceKind::CrossActor)) {
       return true;
     }
 
@@ -3145,8 +3156,8 @@ bool ConformanceChecker::checkActorIsolation(
       return false;
 
     return diagnoseNonSendableTypesInReference(
-      witness, DC, witness->getLoc(),
-      ConcurrentReferenceKind::CrossActor);
+        getConcreteWitness(), DC, witness->getLoc(),
+        ConcurrentReferenceKind::CrossActor);
   }
 
   // If the witness has a global actor but the requirement does not, we have

test/Concurrency/sendable_checking.swift

@@ -53,3 +53,15 @@ func testCV(
   // expected-note@-1{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   acceptSendableFn(fn) // expected-error{{passing non-sendable parameter 'fn' to function expecting a @Sendable closure}}
 }
+
+// rdar://83942484 - spurious Sendable diagnostics
+@available(SwiftStdlib 5.1, *)
+public protocol MyProto {
+    func foo<F>(aFoo: F) async where F: Sendable
+}
+
+@available(SwiftStdlib 5.1, *)
+public actor MyActor: MyProto {
+  public func foo<F>(aFoo: F) async where F: Sendable { }
+  public func bar<B>(aBar: B) async where B: Sendable { }
+}