[SE-0466] Extensions and members thereof can apply default isolation

Extensions of nonisolated types can get default isolation from context
(e.g., @mainactor) independently. This was previously true for members,
but not conformances, and recently regressed for members. Fix the logic
to consistently allow such extensions to be default main-actor
isolated, which affects both their members and conformances.

Fixes rdar://156644976.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -6109,6 +6109,23 @@ static bool sendableConformanceRequiresNonisolated(NominalTypeDecl *nominal) {
   return requiresNonisolated;
 }
 
+/// Determine the default isolation for the given declaration context.
+static DefaultIsolation getDefaultIsolationForContext(const DeclContext *dc) {
+  // Check whether there is a file-specific setting.
+  if (auto *sourceFile = dc->getParentSourceFile()) {
+    if (auto defaultIsolationInFile = sourceFile->getDefaultIsolation())
+      return defaultIsolationInFile.value();
+  }
+
+  // If we're in the main module, check the language option.
+  ASTContext &ctx = dc->getASTContext();
+  if (dc->getParentModule() == ctx.MainModule)
+    return ctx.LangOpts.DefaultIsolationBehavior;
+
+  // Otherwise, default to nonisolated.
+  return DefaultIsolation::Nonisolated;
+}
+
 /// Determine the default isolation and isolation source for this declaration,
 /// which may still be overridden by other inference rules.
 static std::tuple<InferredActorIsolation, ValueDecl *,
@@ -6152,12 +6169,27 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
             return {};
 
           if (dc != dyn_cast<DeclContext>(value)) {
+            // If the nominal type is global-actor-isolated, there's nothing
+            // more to look for.
             if (getActorIsolation(nominal).isMainActor())
               break;
 
-            return {};
+            // If this is an extension of a nonisolated type, its isolation
+            // is independent of the type.
+            if (auto ext = dyn_cast<ExtensionDecl>(dc)) {
+              // If there were isolation attributes on the extension, respect
+              // them.
+              if (getIsolationFromAttributes(ext).has_value())
+                return {};
+
+              // Keep looking.
+            } else {
+              // The type is nonisolated, so its members are nonisolated.
+              return {};
+            }
           }
         }
+
         dc = dc->getParent();
       }
 
@@ -6187,12 +6219,8 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
       return {};
     };
 
-    DefaultIsolation defaultIsolation = ctx.LangOpts.DefaultIsolationBehavior;
-    if (auto *SF = value->getDeclContext()->getParentSourceFile()) {
-      if (auto defaultIsolationInFile = SF->getDefaultIsolation())
-        defaultIsolation = defaultIsolationInFile.value();
-    }
-
+    DefaultIsolation defaultIsolation =
+        getDefaultIsolationForContext(value->getDeclContext());
     // If we are required to use main actor... just use that.
     if (defaultIsolation == DefaultIsolation::MainActor)
       if (auto result =
@@ -8439,6 +8467,19 @@ ActorIsolation swift::inferConformanceIsolation(
   // isolated to a global actor, we may use the conforming type's isolation.
   auto nominalIsolation = getActorIsolation(nominal);
   if (!nominalIsolation.isGlobalActor()) {
+    // If we are in an extension of the type, we might still infer an
+    // isolated conformance depending on default isolation and on the extension
+    // itself.
+    if (auto ext = dyn_cast<ExtensionDecl>(dc)) {
+      // If there's an isolation-related attribute on the extension, use it.
+      if (auto attrIsolation = getIsolationFromAttributes(ext))
+        return *attrIsolation;
+
+      // If we're defaulting to main-actor isolation, use that.
+      if (getDefaultIsolationForContext(dc) == DefaultIsolation::MainActor)
+        return ActorIsolation::forMainActor(ctx);
+    }
+
     return ActorIsolation::forNonisolated(false);
   }
 

test/Concurrency/assume_mainactor_typechecker_errors.swift

@@ -79,3 +79,24 @@ func testTaskDetached() async {
     await k.doSomething()
   }
 }
+
+// @MainActor
+extension Int {
+  func memberOfInt() { } // expected-note{{calls to instance method 'memberOfInt()' from outside of its actor context are implicitly asynchronous}}
+}
+
+nonisolated func testMemberOfInt(i: Int) {
+  i.memberOfInt() // expected-swift5-warning{{call to main actor-isolated instance method 'memberOfInt()' in a synchronous nonisolated context}}
+  // expected-swift6-error@-1{{call to main actor-isolated instance method 'memberOfInt()' in a synchronous nonisolated context}}
+}
+
+protocol SendableProto: Sendable { }
+
+struct MyStruct: SendableProto { }
+
+extension MyStruct: CustomStringConvertible {
+  var description: String {
+    17.memberOfInt() // okay, on main actor
+    return "hello"
+  }
+}