[Concurrency] Implement global actor isolation checking for conformances.

Witnesses and requirements need to agree on their global actor
annotations. However, this is not true for 'async' or '@asyncHandler'
witnesses, for which it does not matter what the actor annotation is
because part of the contract is that the function will execute on the
appropriate actor.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4194,6 +4194,18 @@ ERROR(actor_isolated_witness_could_be_async_handler,none,
      "actor-isolated %0 %1 cannot be used to satisfy a protocol requirement; "
      "did you mean to make it an asychronous handler?",
      (DescriptiveDeclKind, DeclName))
+ERROR(global_actor_isolated_requirement,none,
+      "%0 %1 must be isolated to the global actor %2 to satisfy corresponding "
+      "requirement from protocol %3",
+      (DescriptiveDeclKind, DeclName, Type, Identifier))
+ERROR(global_actor_isolated_witness,none,
+      "%0 %1 isolated to global actor %2 can not satisfy corresponding "
+      "requirement from protocol %3",
+      (DescriptiveDeclKind, DeclName, Type, Identifier))
+ERROR(global_actor_isolated_requirement_witness_conflict,none,
+      "%0 %1 isolated to global actor %2 can not satisfy corresponding "
+      "requirement from protocol %3 isolated to global actor %4",
+      (DescriptiveDeclKind, DeclName, Type, Identifier, Type))
 
 ERROR(actorisolated_let,none,
       "'@actorIsolated' is meaningless on 'let' declarations because "

lib/Sema/TypeCheckProtocol.cpp

@@ -2656,6 +2656,114 @@ static void emitDeclaredHereIfNeeded(DiagnosticEngine &diags,
   diags.diagnose(value, diag::decl_declared_here, value->getName());
 }
 
+bool ConformanceChecker::checkActorIsolation(
+    ValueDecl *requirement, ValueDecl *witness) {
+  // Ensure that the witness is not actor-isolated in a manner that makes it
+  // unsuitable as a witness.
+  Type witnessGlobalActor;
+  switch (auto witnessRestriction =
+              ActorIsolationRestriction::forDeclaration(witness)) {
+  case ActorIsolationRestriction::ActorSelf: {
+    // Actor-isolated witnesses cannot conform to protocol requirements.
+    bool canBeAsyncHandler = false;
+    if (auto witnessFunc = dyn_cast<FuncDecl>(witness)) {
+      canBeAsyncHandler = !witnessFunc->isAsyncHandler() &&
+      witnessFunc->canBeAsyncHandler();
+    }
+    auto diag = witness->diagnose(
+        canBeAsyncHandler
+            ? diag::actor_isolated_witness_could_be_async_handler
+            : diag::actor_isolated_witness,
+        witness->getDescriptiveKind(), witness->getName());
+
+    if (canBeAsyncHandler) {
+      diag.fixItInsert(
+         witness->getAttributeInsertionLoc(false), "@asyncHandler ");
+    }
+
+    return true;
+  }
+
+  case ActorIsolationRestriction::GlobalActor: {
+    // Hang on to the global actor that's used for the witness. It will need
+    // to match that of the requirement.
+    witnessGlobalActor = witness->getDeclContext()->mapTypeIntoContext(
+        witnessRestriction.getGlobalActor());
+    break;
+  }
+
+  case ActorIsolationRestriction::Unsafe:
+  case ActorIsolationRestriction::LocalCapture:
+    break;
+
+  case ActorIsolationRestriction::Unrestricted:
+    // The witness is completely unrestricted, so ignore any annotations on
+    // the requirement.
+    return false;
+  }
+
+  // Check whether the requirement requires some particular actor isolation.
+  Type requirementGlobalActor;
+  switch (auto requirementIsolation = getActorIsolation(requirement)) {
+  case ActorIsolation::ActorInstance:
+    llvm_unreachable("There are not actor protocols");
+
+  case ActorIsolation::GlobalActor: {
+    auto requirementSubs = SubstitutionMap::getProtocolSubstitutions(
+        Proto, Adoptee, ProtocolConformanceRef(Conformance));
+    requirementGlobalActor = requirementIsolation.getGlobalActor()
+        .subst(requirementSubs);
+    break;
+  }
+
+  case ActorIsolation::Independent:
+  case ActorIsolation::Unspecified:
+    break;
+  }
+
+  // If neither has a global actor, we're done.
+  if (!witnessGlobalActor && !requirementGlobalActor)
+    return false;
+
+  // If the witness has a global actor but the requirement does not, we have
+  // an isolation error.
+  if (witnessGlobalActor && !requirementGlobalActor) {
+    witness->diagnose(
+        diag::global_actor_isolated_witness, witness->getDescriptiveKind(),
+        witness->getName(), witnessGlobalActor, Proto->getName());
+    requirement->diagnose(diag::decl_declared_here, requirement->getName());
+    return true;
+  }
+
+  // If the requirement has a global actor but the witness does not, we have
+  // an isolation error.
+  //
+  // FIXME: Within a module, this will be an inference rule.
+  if (requirementGlobalActor && !witnessGlobalActor) {
+    witness->diagnose(
+        diag::global_actor_isolated_requirement, witness->getDescriptiveKind(),
+        witness->getName(), requirementGlobalActor, Proto->getName())
+      .fixItInsert(
+        witness->getAttributeInsertionLoc(/*forModifier=*/false),
+      "@" + requirementGlobalActor.getString());
+    requirement->diagnose(diag::decl_declared_here, requirement->getName());
+    return true;
+  }
+
+  // If both have global actors but they differ, this is an isolation error.
+  if (!witnessGlobalActor->isEqual(requirementGlobalActor)) {
+    witness->diagnose(
+        diag::global_actor_isolated_requirement_witness_conflict,
+        witness->getDescriptiveKind(), witness->getName(), witnessGlobalActor,
+        Proto->getName(), requirementGlobalActor);
+    requirement->diagnose(diag::decl_declared_here, requirement->getName());
+    return true;
+  }
+
+  // Everything is okay.
+  return false;
+}
+
 bool ConformanceChecker::checkObjCTypeErasedGenerics(
                                                  AssociatedTypeDecl *assocType,
                                                  Type type,
@@ -4337,39 +4445,8 @@ void ConformanceChecker::resolveValueWitnesses() {
         return;
       }
 
-      // Check for actor-isolation consistency.
-      switch (auto restriction =
-                  ActorIsolationRestriction::forDeclaration(witness)) {
-      case ActorIsolationRestriction::ActorSelf: {
-        // Actor-isolated witnesses cannot conform to protocol requirements.
-        bool canBeAsyncHandler = false;
-        if (auto witnessFunc = dyn_cast<FuncDecl>(witness)) {
-          canBeAsyncHandler = !witnessFunc->isAsyncHandler() &&
-          witnessFunc->canBeAsyncHandler();
-        }
-        auto diag = witness->diagnose(
-            canBeAsyncHandler
-                ? diag::actor_isolated_witness_could_be_async_handler
-                : diag::actor_isolated_witness,
-            witness->getDescriptiveKind(), witness->getName());
-
-        if (canBeAsyncHandler) {
-          diag.fixItInsert(
-             witness->getAttributeInsertionLoc(false), "@asyncHandler ");
-        }
+      if (checkActorIsolation(requirement, witness))
         return;
-      }
-
-      case ActorIsolationRestriction::GlobalActor: {
-        // FIXME: Check against the requirement. This needs serious refactoring.
-        break;
-      }
-
-      case ActorIsolationRestriction::Unrestricted:
-      case ActorIsolationRestriction::Unsafe:
-      case ActorIsolationRestriction::LocalCapture:
-        break;
-      }
 
       // Objective-C checking for @objc requirements.
       if (requirement->isObjC() &&

lib/Sema/TypeCheckProtocol.h

@@ -739,6 +739,11 @@ class ConformanceChecker : public WitnessChecker {
                                    Type type,
                                    TypeDecl *typeDecl);
 
+  /// Check that the witness and requirement have compatible actor contexts.
+  ///
+  /// \returns true if an error occurred, false otherwise.
+  bool checkActorIsolation(ValueDecl *requirement, ValueDecl *witness);
+
   /// Record a type witness.
   ///
   /// \param assocType The associated type whose witness is being recorded.

test/decl/class/actor/global_actor_conformance.swift

@@ -0,0 +1,59 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency
+// REQUIRES: concurrency
+
+import _Concurrency
+
+actor class SomeActor { }
+
+@globalActor
+struct GlobalActor {
+  static var shared: SomeActor { SomeActor() }
+}
+
+@globalActor
+struct GenericGlobalActor<T> {
+  static var shared: SomeActor { SomeActor() }
+}
+
+protocol P1 {
+  associatedtype Assoc
+
+  @GlobalActor func method1() // expected-note{{declared here}}
+  @GenericGlobalActor<Int> func method2()  // expected-note{{declared here}}
+  @GenericGlobalActor<Assoc> func method3()
+  func method4() // expected-note{{declared here}}
+}
+
+protocol P2 {
+  @GlobalActor func asyncMethod1() async
+  @GenericGlobalActor<Int> func asyncMethod2() async
+  func asyncMethod3() async
+}
+
+class C1 : P1, P2 {
+  typealias Assoc = String
+
+  // FIXME: This will be inferred
+  func method1() { } // expected-error{{instance method 'method1()' must be isolated to the global actor 'GlobalActor' to satisfy corresponding requirement from protocol 'P1'}}{{3-3=@GlobalActor}}
+
+  @GenericGlobalActor<String> func method2() { } // expected-error{{instance method 'method2()' isolated to global actor 'GenericGlobalActor<String>' can not satisfy corresponding requirement from protocol 'P1' isolated to global actor 'GenericGlobalActor<Int>'}}
+  @GenericGlobalActor<String >func method3() { }
+  @GlobalActor func method4() { } // expected-error{{instance method 'method4()' isolated to global actor 'GlobalActor' can not satisfy corresponding requirement from protocol 'P1'}}
+
+  // Okay: we can ignore the mismatch in global actor types for 'async' methods.
+  func asyncMethod1() async { }
+  @GenericGlobalActor<String> func asyncMethod2() async { }
+  @GlobalActor func asyncMethod3() async { }
+}
+
+
+class C2: P1 {
+  typealias Assoc = Int
+
+  // Okay: we can ignore the mismatch in global actor types for 'asyncHandler'
+  // methods.
+  @asyncHandler func method1() { }
+  @asyncHandler func method2() { }
+  @asyncHandler func method3() { }
+  @asyncHandler func method4() { }
+}