[6.0] Fix function subtyping rules for sending

include/swift/AST/DiagnosticsSema.def

@@ -7952,6 +7952,19 @@ ERROR(sending_only_on_parameters_and_results, none,
       "'sending' may only be used on parameters and results", ())
 ERROR(sending_cannot_be_applied_to_tuple_elt, none,
       "'sending' cannot be applied to tuple elements", ())
+ERROR(sending_function_wrong_sending,none,
+      "converting a value of type %0 to type %1 risks causing data races",
+      (Type, Type))
+NOTE(sending_function_param_with_sending_param_note, none,
+     "converting a function typed value with a sending parameter to one "
+     "without risks allowing actor-isolated values to escape their isolation "
+     "domain as an argument to an invocation of value",
+     ())
+NOTE(sending_function_result_with_sending_param_note, none,
+     "converting a function typed value without a sending result as one with "
+     "risks allowing actor-isolated values to escape their "
+     "isolation domain through a result of an invocation of value",
+     ())
 
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

include/swift/AST/Types.h

@@ -3237,6 +3237,9 @@ class AnyFunctionType : public TypeBase {
     /// Whether the parameter is 'isolated'.
     bool isIsolated() const { return Flags.isIsolated(); }
 
+    /// Whether or not the parameter is 'sending'.
+    bool isSending() const { return Flags.isSending(); }
+
     /// Whether the parameter is 'isCompileTimeConst'.
     bool isCompileTimeConst() const { return Flags.isCompileTimeConst(); }
 

include/swift/Sema/CSFix.h

@@ -471,6 +471,17 @@ enum class FixKind : uint8_t {
   /// Ignore situations when key path subscript index gets passed an invalid
   /// type as an argument (something that is not a key path).
   IgnoreKeyPathSubscriptIndexMismatch,
+
+  /// Ignore the following situations:
+  ///
+  /// 1. Where we have a function that expects a function typed parameter
+  /// without a sendable parameter but is passed a function type with a sending
+  /// parameter.
+  ///
+  /// 2. Where we have a function that expects a function typed parameter with a
+  /// sending result, but is passed a function typeed parameter without a
+  /// sending result.
+  AllowSendingMismatch,
 };
 
 class ConstraintFix {
@@ -2619,6 +2630,49 @@ class TreatEphemeralAsNonEphemeral final : public AllowArgumentMismatch {
   }
 };
 
+/// Error if a user passes let f: (sending T) -> () as a (T) -> ().
+///
+/// This prevents data races since f assumes its parameter if the parameter is
+/// non-Sendable is safe to transfer onto other situations. The caller though
+/// that this is being sent to does not enforce that invariants within its body.
+class AllowSendingMismatch final : public ContextualMismatch {
+public:
+  enum class Kind {
+    Parameter,
+    Result,
+  };
+
+private:
+  Kind kind;
+
+  AllowSendingMismatch(ConstraintSystem &cs, Type argType, Type paramType,
+                       ConstraintLocator *locator, Kind kind,
+                       FixBehavior fixBehavior)
+      : ContextualMismatch(cs, FixKind::AllowSendingMismatch, argType,
+                           paramType, locator, fixBehavior),
+        kind(kind) {}
+
+public:
+  std::string getName() const override {
+    return "treat a function argument with sending parameter as a function "
+           "argument without sending parameters";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
+  static AllowSendingMismatch *create(ConstraintSystem &cs,
+                                      ConstraintLocator *locator, Type srcType,
+                                      Type dstType, Kind kind);
+
+  static bool classof(const ConstraintFix *fix) {
+    return fix->getKind() == FixKind::AllowSendingMismatch;
+  }
+};
+
 class SpecifyBaseTypeForContextualMember final : public ConstraintFix {
   DeclNameRef MemberName;
 

lib/Parse/ParseType.cpp

@@ -1555,6 +1555,8 @@ bool Parser::canParseType() {
     consumeToken();
   } else if (Tok.isContextualKeyword("each")) {
     consumeToken();
+  } else if (Tok.isContextualKeyword("sending")) {
+    consumeToken();
   }
 
   switch (Tok.getKind()) {

lib/Sema/AssociatedTypeInference.cpp

@@ -2425,7 +2425,9 @@ AssociatedTypeInference::computeFailureTypeWitness(
   // it.
   for (const auto &witness : valueWitnesses) {
     if (isAsyncIteratorProtocolNext(witness.first)) {
-      if (auto witnessFunc = dyn_cast<AbstractFunctionDecl>(witness.second)) {
+      // We use a dyn_cast_or_null since we can get a nullptr here if we fail to
+      // match a witness. In such a case, we should just fail here.
+      if (auto witnessFunc = dyn_cast_or_null<AbstractFunctionDecl>(witness.second)) {
         auto thrownError = witnessFunc->getEffectiveThrownErrorType();
 
         // If it doesn't throw, Failure == Never.

lib/Sema/CSDiagnostics.cpp

@@ -7918,6 +7918,24 @@ bool NonEphemeralConversionFailure::diagnoseAsError() {
   return true;
 }
 
+bool SendingOnFunctionParameterMismatchFail::diagnoseAsError() {
+  emitDiagnosticAt(getLoc(), diag::sending_function_wrong_sending,
+                   getFromType(), getToType())
+      .warnUntilSwiftVersion(6);
+  emitDiagnosticAt(getLoc(),
+                   diag::sending_function_param_with_sending_param_note);
+  return true;
+}
+
+bool SendingOnFunctionResultMismatchFailure::diagnoseAsError() {
+  emitDiagnosticAt(getLoc(), diag::sending_function_wrong_sending,
+                   getFromType(), getToType())
+      .warnUntilSwiftVersion(6);
+  emitDiagnosticAt(getLoc(),
+                   diag::sending_function_result_with_sending_param_note);
+  return true;
+}
+
 bool AssignmentTypeMismatchFailure::diagnoseMissingConformance() const {
   auto srcType = getFromType();
   auto dstType = getToType()->lookThroughAllOptionalTypes();

lib/Sema/CSDiagnostics.h

@@ -2291,6 +2291,28 @@ class NonEphemeralConversionFailure final : public ArgumentMismatchFailure {
   void emitSuggestionNotes() const;
 };
 
+class SendingOnFunctionParameterMismatchFail final : public ContextualFailure {
+public:
+  SendingOnFunctionParameterMismatchFail(const Solution &solution, Type srcType,
+                                         Type dstType,
+                                         ConstraintLocator *locator,
+                                         FixBehavior fixBehavior)
+      : ContextualFailure(solution, srcType, dstType, locator, fixBehavior) {}
+
+  bool diagnoseAsError() override;
+};
+
+class SendingOnFunctionResultMismatchFailure final : public ContextualFailure {
+public:
+  SendingOnFunctionResultMismatchFailure(const Solution &solution, Type srcType,
+                                         Type dstType,
+                                         ConstraintLocator *locator,
+                                         FixBehavior fixBehavior)
+      : ContextualFailure(solution, srcType, dstType, locator, fixBehavior) {}
+
+  bool diagnoseAsError() override;
+};
+
 class AssignmentTypeMismatchFailure final : public ContextualFailure {
 public:
   AssignmentTypeMismatchFailure(const Solution &solution,

lib/Sema/CSFix.cpp

@@ -1845,6 +1845,34 @@ std::string TreatEphemeralAsNonEphemeral::getName() const {
   return name;
 }
 
+bool AllowSendingMismatch::diagnose(const Solution &solution,
+                                    bool asNote) const {
+  switch (kind) {
+  case Kind::Parameter: {
+    SendingOnFunctionParameterMismatchFail failure(
+        solution, getFromType(), getToType(), getLocator(), fixBehavior);
+    return failure.diagnose(asNote);
+  }
+  case Kind::Result: {
+    SendingOnFunctionResultMismatchFailure failure(
+        solution, getFromType(), getToType(), getLocator(), fixBehavior);
+    return failure.diagnose(asNote);
+  }
+  }
+  llvm_unreachable("Covered switch isn't covered?!");
+}
+
+AllowSendingMismatch *AllowSendingMismatch::create(ConstraintSystem &cs,
+                                                   ConstraintLocator *locator,
+                                                   Type srcType, Type dstType,
+                                                   Kind kind) {
+  auto fixBehavior = cs.getASTContext().LangOpts.isSwiftVersionAtLeast(6)
+                         ? FixBehavior::Error
+                         : FixBehavior::DowngradeToWarning;
+  return new (cs.getAllocator())
+      AllowSendingMismatch(cs, srcType, dstType, locator, kind, fixBehavior);
+}
+
 bool SpecifyBaseTypeForContextualMember::diagnose(const Solution &solution,
                                                   bool asNote) const {
   MissingContextualBaseInMemberRefFailure failure(solution, MemberName,

lib/Sema/CSGen.cpp

@@ -1636,10 +1636,9 @@ namespace {
     }
 
     Type
-    resolveTypeReferenceInExpression(TypeRepr *repr, TypeResolverContext resCtx,
+    resolveTypeReferenceInExpression(TypeRepr *repr,
+                                     TypeResolutionOptions options,
                                      const ConstraintLocatorBuilder &locator) {
-      TypeResolutionOptions options(resCtx);
-
       // Introduce type variables for unbound generics.
       const auto genericOpener = OpenUnboundGenericType(CS, locator);
       const auto placeholderHandler = HandlePlaceholderType(CS, locator);
@@ -2528,9 +2527,11 @@ namespace {
             return declaredTy;
           }
 
+          auto options =
+              TypeResolutionOptions(TypeResolverContext::InExpression);
+          options.setContext(TypeResolverContext::ClosureExpr);
           const auto resolvedTy = resolveTypeReferenceInExpression(
-              closure->getExplicitResultTypeRepr(),
-              TypeResolverContext::InExpression, resultLocator);
+              closure->getExplicitResultTypeRepr(), options, resultLocator);
           if (resolvedTy)
             return resolvedTy;
         }

lib/Sema/CSSimplify.cpp

@@ -3236,6 +3236,16 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
       return getTypeMatchFailure(locator);
   }
 
+  // () -> sending T can be a subtype of () -> T... but not vis-a-versa.
+  if (func1->hasSendingResult() != func2->hasSendingResult() &&
+      (!func1->hasSendingResult() || kind < ConstraintKind::Subtype)) {
+    auto *fix = AllowSendingMismatch::create(
+        *this, getConstraintLocator(locator), func1, func2,
+        AllowSendingMismatch::Kind::Result);
+    if (recordFix(fix))
+      return getTypeMatchFailure(locator);
+  }
+
   if (!matchFunctionIsolations(func1, func2, kind, flags, locator))
     return getTypeMatchFailure(locator);
 
@@ -3666,6 +3676,17 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
         return getTypeMatchFailure(argumentLocator);
       }
 
+      // Do not allow for functions that expect a sending parameter to match
+      // with a function that expects a non-sending parameter.
+      if (func1Param.getParameterFlags().isSending() &&
+          !func2Param.getParameterFlags().isSending()) {
+        auto *fix = AllowSendingMismatch::create(
+            *this, getConstraintLocator(argumentLocator), func1, func2,
+            AllowSendingMismatch::Kind::Parameter);
+        if (recordFix(fix))
+          return getTypeMatchFailure(argumentLocator);
+      }
+
       // FIXME: We should check value ownership too, but it's not completely
       // trivial because of inout-to-pointer conversions.
 
@@ -11770,10 +11791,10 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
         if (contextualParam->isIsolated() && !flags.isIsolated() && paramDecl)
           isolatedParams.insert(paramDecl);
 
-        param =
-            param.withFlags(flags.withInOut(contextualParam->isInOut())
-                                 .withVariadic(contextualParam->isVariadic())
-                                 .withIsolated(contextualParam->isIsolated()));
+        param = param.withFlags(flags.withInOut(contextualParam->isInOut())
+                                    .withVariadic(contextualParam->isVariadic())
+                                    .withIsolated(contextualParam->isIsolated())
+                                    .withSending(contextualParam->isSending()));
       }
     }
 
@@ -11900,6 +11921,12 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
       closureExtInfo = closureExtInfo.withSendable();
   }
 
+  // Propagate sending result from the contextual type to the closure.
+  if (auto contextualFnType = contextualType->getAs<FunctionType>()) {
+    if (contextualFnType->hasExtInfo() && contextualFnType->hasSendingResult())
+      closureExtInfo = closureExtInfo.withSendingResult();
+  }
+
   // Isolated parameters override any other kind of isolation we might infer.
   if (hasIsolatedParam) {
     closureExtInfo = closureExtInfo.withIsolation(
@@ -15098,6 +15125,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
     }
   }
 
+  case FixKind::AllowSendingMismatch:
   case FixKind::InsertCall:
   case FixKind::RemoveReturn:
   case FixKind::RemoveAddressOf:

lib/Sema/TypeCheckProtocol.cpp

@@ -743,6 +743,15 @@ RequirementMatch swift::matchWitness(
           reqTypeIsIUO != witnessTypeIsIUO)
         return RequirementMatch(witness, MatchKind::TypeConflict, witnessType);
 
+      // If our requirement says that it has a sending result, then our witness
+      // must also have a sending result since otherwise, in generic contexts,
+      // we would be returning non-disconnected values as disconnected.
+      if (dc->getASTContext().LangOpts.isSwiftVersionAtLeast(6)) {
+        if (reqFnType->hasExtInfo() && reqFnType->hasSendingResult() &&
+            (!witnessFnType->hasExtInfo() || !witnessFnType->hasSendingResult()))
+          return RequirementMatch(witness, MatchKind::TypeConflict, witnessType);
+      }
+
       if (auto result = matchTypes(std::get<0>(types), std::get<1>(types))) {
         return std::move(result.value());
       }
@@ -775,6 +784,14 @@ RequirementMatch swift::matchWitness(
       if (reqParams[i].isInOut() != witnessParams[i].isInOut())
         return RequirementMatch(witness, MatchKind::TypeConflict, witnessType);
 
+      // If we have a requirement without sending and our witness expects a
+      // sending parameter, error.
+      if (dc->getASTContext().isSwiftVersionAtLeast(6)) {
+        if (!reqParams[i].getParameterFlags().isSending() &&
+            witnessParams[i].getParameterFlags().isSending())
+          return RequirementMatch(witness, MatchKind::TypeConflict, witnessType);
+      }
+
       auto reqParamDecl = reqParamList->get(i);
       auto witnessParamDecl = witnessParamList->get(i);
 

lib/Sema/TypeCheckType.cpp

@@ -4971,7 +4971,8 @@ TypeResolver::resolveSendingTypeRepr(SendingTypeRepr *repr,
     return ErrorType::get(getASTContext());
   }
 
-  if (!options.is(TypeResolverContext::FunctionResult) &&
+  if (!options.is(TypeResolverContext::ClosureExpr) &&
+      !options.is(TypeResolverContext::FunctionResult) &&
       (!options.is(TypeResolverContext::FunctionInput) ||
        options.hasBase(TypeResolverContext::EnumElementDecl))) {
     diagnoseInvalid(repr, repr->getSpecifierLoc(),

lib/Sema/TypeCheckType.h

@@ -118,7 +118,7 @@ enum class TypeResolverContext : uint8_t {
   /// Whether we are checking the parameter list of a subscript.
   SubscriptDecl,
 
-  /// Whether we are checking the parameter list of a closure.
+  /// Whether we are checking the parameter list or result of a closure.
   ClosureExpr,
 
   /// Whether we are in the input type of a function, or under one level of