[SE-0470] Prohibit inference of isolated conformances with nonisolated witnesses

If all of the witnesses to a conformance are nonisolated, then infer that
conformance as nonisolated rather than global-actor-isolated. This is
only relevant when InferIsolatedConformances is enabled, and prevents
that inference to help maintain source compatibility.

lib/Sema/TypeCheckConcurrency.cpp

@@ -7991,17 +7991,51 @@ ConformanceIsolationRequest::evaluate(Evaluator &evaluator, ProtocolConformance
   if (getActorIsolation(rootNormal->getProtocol()).isActorIsolated())
     return ActorIsolation::forNonisolated(false);
 
-  // If we are inferring isolated conformances and the conforming type is
-  // isolated to a global actor, use the conforming type's isolation.
+  // Isolation inference rules follow. If we aren't inferring isolated conformances,
+  // we're done.
+  if (!ctx.LangOpts.hasFeature(Feature::InferIsolatedConformances))
+    return ActorIsolation::forNonisolated(false);
+
   auto nominal = dc->getSelfNominalTypeDecl();
-  if (ctx.LangOpts.hasFeature(Feature::InferIsolatedConformances) &&
-      nominal) {
-    auto nominalIsolation = getActorIsolation(nominal);
-    if (nominalIsolation.isGlobalActor())
-      return nominalIsolation;
+  if (!nominal) {
+    return ActorIsolation::forNonisolated(false);
   }
 
-  return ActorIsolation::forNonisolated(false);
+  // If we are inferring isolated conformances and the conforming type is
+  // isolated to a global actor, we may use the conforming type's isolation.
+  auto nominalIsolation = getActorIsolation(nominal);
+  if (!nominalIsolation.isGlobalActor()) {
+    return ActorIsolation::forNonisolated(false);
+  }
+
+  // If all of the value witnesses are nonisolated, then we should not infer
+  // global actor isolation.
+  bool anyIsolatedWitness = false;
+  auto protocol = conformance->getProtocol();
+  for (auto requirement : protocol->getMembers()) {
+    if (isa<TypeDecl>(requirement))
+      continue;
+
+    auto valueReq = dyn_cast<ValueDecl>(requirement);
+    if (!valueReq)
+      continue;
+
+    auto witness = conformance->getWitnessDecl(valueReq);
+    if (!witness)
+      continue;
+
+    auto witnessIsolation = getActorIsolation(witness);
+    if (witnessIsolation.isActorIsolated()) {
+      anyIsolatedWitness = true;
+      break;
+    }
+  }
+
+  if (!anyIsolatedWitness) {
+    return ActorIsolation::forNonisolated(false);
+  }
+
+  return nominalIsolation;
 }
 
 namespace {

test/Concurrency/isolated_conformance_default_actor.swift

@@ -46,8 +46,8 @@ func acceptSendablePMeta<T: Sendable & P>(_: T.Type) { }
 func acceptSendableQMeta<T: Sendable & Q>(_: T.Type) { }
 
 nonisolated func testConformancesFromNonisolated() {
-  let _: any P = CExplicitMainActor() // expected-error{{main actor-isolated conformance of 'CExplicitMainActor' to 'P' cannot be used in nonisolated context}}
-  let _: any P = CImplicitMainActor() // expected-error{{main actor-isolated conformance of 'CImplicitMainActor' to 'P' cannot be used in nonisolated context}}
+  let _: any P = CExplicitMainActor() // okay
+  let _: any P = CImplicitMainActor() // okay
 
   let _: any P = CNonIsolated()
   let _: any P = CImplicitMainActorNonisolatedConformance()

test/Concurrency/isolated_conformance_inference.swift

@@ -32,6 +32,11 @@ extension CExplicit: Q {
   func g() { }
 }
 
+@SomeGlobalActor
+class CViaNonisolatedWitness: P {
+  nonisolated func f() { } // okay! conformance above is nonisolated via this witness
+}
+
 // expected-error@+3{{conformance of 'CNonIsolated' to protocol 'P' crosses into global actor 'SomeGlobalActor'-isolated code and can cause data races}}
 // expected-note@+2{{turn data races into runtime errors with '@preconcurrency'}}
 // expected-note@+1{{isolate this conformance to the global actor 'SomeGlobalActor' with '@SomeGlobalActor'}}{{33-33=@SomeGlobalActor }}
@@ -49,4 +54,6 @@ nonisolated func testConformancesFromNonisolated() {
 
   // Okay, these are nonisolated conformances.
   let _: any Q = CExplicit()
+
+  let _: any P = CViaNonisolatedWitness()
 }

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -2881,6 +2881,58 @@ public struct HangingMacro: PeerMacro {
   }
 }
 
+public struct PWithNonisolatedFuncMacro: ExtensionMacro {
+  public static var inferNonisolatedConformances: Bool { false }
+
+  public static func expansion(
+    of node: AttributeSyntax,
+    attachedTo decl: some DeclGroupSyntax,
+    providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [ExtensionDeclSyntax] {
+    if (protocols.isEmpty) {
+      return []
+    }
+
+    let decl: DeclSyntax =
+      """
+      extension \(raw: type.trimmedDescription): P {
+        nonisolated static func requirement() { }
+      }
+      """
+
+    return [
+      decl.cast(ExtensionDeclSyntax.self)
+    ]
+  }
+}
+
+public struct NonisolatedPWithNonisolatedFuncMacro: ExtensionMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    attachedTo decl: some DeclGroupSyntax,
+    providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [ExtensionDeclSyntax] {
+    if (protocols.isEmpty) {
+      return []
+    }
+
+    let decl: DeclSyntax =
+      """
+      extension \(raw: type.trimmedDescription): P {
+        nonisolated static func requirement() { }
+      }
+      """
+
+    return [
+      decl.cast(ExtensionDeclSyntax.self)
+    ]
+  }
+}
+
 public struct BigEndianAccessorMacro: AccessorMacro {
     public static func expansion(
         of node: AttributeSyntax,

test/Macros/macro_expand_extensions.swift

@@ -283,3 +283,15 @@ struct HasNestedType {
 // extensions of nested types when the outer type has an
 // attached macro that can add other nested types.
 extension HasNestedType.Inner {}
+
+@attached(extension, conformances: P, names: named(requirement))
+macro AddPWithNonisolated() = #externalMacro(module: "MacroDefinition", type: "PWithNonisolatedFuncMacro")
+
+@attached(extension, conformances: P, names: named(requirement))
+macro AddNonisolatedPWithNonisolated() = #externalMacro(module: "MacroDefinition", type: "NonisolatedPWithNonisolatedFuncMacro")
+
+@AddNonisolatedPWithNonisolated
+struct MakeMeNonisolated { }
+
+@AddPWithNonisolated
+struct KeepMeIsolated { }