[Concurrency] Allow default arguments to require actor isolation.

include/swift/AST/Decl.h

@@ -2273,7 +2273,9 @@ class PatternBindingDecl final : public Decl,
   void setInitializerSubsumed(unsigned i) {
     getMutablePatternList()[i].setInitializerSubsumed();
   }
-
+
+  ActorIsolation getInitializerIsolation(unsigned i) const;
+
   /// Does this binding declare something that requires storage?
   bool hasStorage() const;
 
@@ -5973,6 +5975,19 @@ class VarDecl : public AbstractStorageDecl {
     return getParentExecutableInitializer() != nullptr;
   }
 
+  /// Get the required actor isolation for evaluating the initializer
+  /// expression synchronously (if there is one).
+  ///
+  /// If this VarDecl is a stored instance property, the initializer
+  /// can only be used in an `init` that meets the required isolation.
+  /// Otherwise, the property must be explicitly initialized in the `init`.
+  ///
+  /// If this is a ParamDecl, the initializer isolation is required at
+  /// the call-site in order to use the default argument for this parameter.
+  /// If the required isolation is not met, an argument must be written
+  /// explicitly at the call-site.
+  ActorIsolation getInitializerIsolation() const;
+
   // Return whether this VarDecl has an initial value, either by checking
   // if it has an initializer in its parent pattern binding or if it has
   // the @_hasInitialValue attribute.

include/swift/AST/DiagnosticsSema.def

@@ -5236,6 +5236,13 @@ ERROR(distributed_actor_isolated_non_self_reference,none,
       "distributed actor-isolated %kind0 can not be accessed from a "
       "non-isolated context",
       (const ValueDecl *))
+ERROR(isolated_default_argument,none,
+      "%0 default argument cannot be synchronously evaluated from a "
+      "%1 context",
+      (ActorIsolation, ActorIsolation))
+ERROR(conflicting_default_argument_isolation,none,
+      "default argument cannot be both %0 and %1",
+      (ActorIsolation, ActorIsolation))
 ERROR(distributed_actor_needs_explicit_distributed_import,none,
       "'Distributed' module not imported, required for 'distributed actor'",
       ())

include/swift/AST/Expr.h

@@ -4569,6 +4569,11 @@ class DefaultArgumentExpr final : public Expr {
   /// expression within the context of the call site.
   Expr *getCallerSideDefaultExpr() const;
 
+  /// Get the required actor isolation for evaluating this default argument
+  /// synchronously. If the caller does not meet the required isolation, the
+  /// argument must be written explicitly at the call-site.
+  ActorIsolation getRequiredIsolation() const;
+
   static bool classof(const Expr *E) {
     return E->getKind() == ExprKind::DefaultArgument;
   }

include/swift/AST/TypeCheckRequests.h

@@ -2934,6 +2934,27 @@ class DefaultArgumentTypeRequest
   void cacheResult(Type type) const;
 };
 
+/// Compute the actor isolation needed to synchronously evaluate the
+/// default initializer expression.
+class DefaultInitializerIsolation
+    : public SimpleRequest<DefaultInitializerIsolation,
+                           ActorIsolation(VarDecl *),
+                           RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  ActorIsolation evaluate(Evaluator &evaluator, 
+                          VarDecl *) const;
+
+public:
+  bool isCached() const { return true; }
+};
+
+void simple_display(llvm::raw_ostream &out, Initializer *init);
+
 /// Computes the fully type-checked caller-side default argument within the
 /// context of the call site that it will be inserted into.
 class CallerSideDefaultArgExprRequest

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -62,6 +62,9 @@ SWIFT_REQUEST(TypeChecker, DefaultArgumentExprRequest,
               Expr *(ParamDecl *), SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, DefaultArgumentTypeRequest,
               Type(ParamDecl *), SeparatelyCached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, DefaultInitializerIsolation,
+              ActorIsolation(VarDecl *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, DefaultArgumentInitContextRequest,
               Initializer *(ParamDecl *), SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, DefaultDefinitionTypeRequest,

include/swift/Basic/Features.def

@@ -222,6 +222,9 @@ EXPERIMENTAL_FEATURE(SendNonSendable, false)
 /// Within strict concurrency, narrow global variable isolation requirements.
 EXPERIMENTAL_FEATURE(GlobalConcurrency, false)
 
+/// Allow default arguments to require isolation at the call-site.
+EXPERIMENTAL_FEATURE(IsolatedDefaultArguments, false)
+
 /// Enable extended callbacks (with additional parameters) to be used when the
 /// "playground transform" is enabled.
 EXPERIMENTAL_FEATURE(PlaygroundExtendedCallbacks, true)

lib/AST/ASTPrinter.cpp

@@ -3551,6 +3551,10 @@ static bool usesFeatureSendNonSendable(Decl *decl) {
 
 static bool usesFeatureGlobalConcurrency(Decl *decl) { return false; }
 
+static bool usesFeatureIsolatedDefaultArguments(Decl *decl) {
+  return false;
+}
+
 static bool usesFeaturePlaygroundExtendedCallbacks(Decl *decl) {
   return false;
 }

lib/AST/Decl.cpp

@@ -2088,6 +2088,14 @@ bool PatternBindingDecl::isAsyncLet() const {
   return false;
 }
 
+ActorIsolation
+PatternBindingDecl::getInitializerIsolation(unsigned i) const {
+  auto *var = getPatternList()[i].getAnchoringVarDecl();
+  if (!var)
+    return ActorIsolation::forUnspecified();
+
+  return var->getInitializerIsolation();
+}
 
 bool PatternBindingDecl::hasStorage() const {
   // Walk the pattern, to check to see if any of the VarDecls included in it
@@ -7127,6 +7135,14 @@ Expr *VarDecl::getParentExecutableInitializer() const {
   return nullptr;
 }
 
+ActorIsolation VarDecl::getInitializerIsolation() const {
+  auto *mutableThis = const_cast<VarDecl *>(this);
+  return evaluateOrDefault(
+      getASTContext().evaluator,
+      DefaultInitializerIsolation{mutableThis},
+      ActorIsolation::forUnspecified());
+}
+
 SourceRange VarDecl::getSourceRange() const {
   if (auto Param = dyn_cast<ParamDecl>(this))
     return Param->getSourceRange();
@@ -10445,8 +10461,25 @@ ActorIsolation swift::getActorIsolationOfContext(
   if (auto *vd = dyn_cast_or_null<ValueDecl>(dcToUse->getAsDecl()))
     return getActorIsolation(vd);
 
-  if (auto *var = dcToUse->getNonLocalVarDecl())
+  // In the context of the initializing or default-value expression of a
+  // stored property, the isolation varies between instance and type members:
+  //   - For a static stored property, the isolation matches the VarDecl.
+  //     Static properties are initialized upon first use, so the isolation
+  //     of the initializer must match the isolation required to access the
+  //     property.
+  //   - For a field of a nominal type, the expression can require a specific
+  //     actor isolation. That default expression may only be used from inits
+  //     that meet the required isolation.
+  if (auto *var = dcToUse->getNonLocalVarDecl()) {
+    auto &ctx = dc->getASTContext();
+    if (ctx.LangOpts.hasFeature(Feature::IsolatedDefaultArguments) &&
+        var->isInstanceMember() &&
+        !var->getAttrs().hasAttribute<LazyAttr>()) {
+      return ActorIsolation::forNonisolated();
+    }
+
     return getActorIsolation(var);
+  }
 
   if (auto *closure = dyn_cast<AbstractClosureExpr>(dcToUse)) {
     return getClosureActorIsolation(closure);

lib/AST/Expr.cpp

@@ -1606,6 +1606,11 @@ Expr *DefaultArgumentExpr::getCallerSideDefaultExpr() const {
                            new (ctx) ErrorExpr(getSourceRange(), getType()));
 }
 
+ActorIsolation
+DefaultArgumentExpr::getRequiredIsolation() const {
+  return getParamDecl()->getInitializerIsolation();
+}
+
 ValueDecl *ApplyExpr::getCalledValue(bool skipFunctionConversions) const {
   return ::getCalledValue(Fn, skipFunctionConversions);
 }

lib/AST/TypeCheckRequests.cpp

@@ -1302,6 +1302,24 @@ void DefaultArgumentTypeRequest::cacheResult(Type type) const {
   param->setDefaultExprType(type);
 }
 
+//----------------------------------------------------------------------------//
+// DefaultInitializerIsolation computation.
+//----------------------------------------------------------------------------//
+
+void swift::simple_display(llvm::raw_ostream &out, Initializer *init) {
+  switch (init->getInitializerKind()) {
+  case InitializerKind::PatternBinding:
+    out << "pattern binding initializer";
+    break;
+  case InitializerKind::DefaultArgument:
+    out << "default argument initializer";
+    break;
+  case InitializerKind::PropertyWrapper:
+    out << "property wrapper initializer";
+    break;
+  }
+}
+
 //----------------------------------------------------------------------------//
 // CallerSideDefaultArgExprRequest computation.
 //----------------------------------------------------------------------------//

lib/SILGen/SILGenConstructor.cpp

@@ -1542,6 +1542,27 @@ void SILGenFunction::emitMemberInitializer(DeclContext *dc, VarDecl *selfDecl,
     if (!init)
       continue;
 
+    // Member initializer expressions are only used in a constructor with
+    // matching actor isolation. If the isolation prohibits the member
+    // initializer from being evaluated synchronously (or propagating required
+    // isolation through closure bodies), then the default value cannot be used
+    // and the member must be explicitly initialized in the constructor.
+    auto *var = field->getAnchoringVarDecl(i);
+    auto requiredIsolation = var->getInitializerIsolation();
+    auto contextIsolation = getActorIsolationOfContext(dc);
+    switch (requiredIsolation) {
+    // 'nonisolated' expressions can be evaluated from anywhere
+    case ActorIsolation::Unspecified:
+    case ActorIsolation::Nonisolated:
+      break;
+
+    case ActorIsolation::GlobalActor:
+    case ActorIsolation::GlobalActorUnsafe:
+    case ActorIsolation::ActorInstance:
+      if (requiredIsolation != contextIsolation)
+        continue;
+    }
+
     auto *varPattern = field->getPattern(i);
 
     // Cleanup after this initialization.