diff --git a/include/swift/ABI/MetadataValues.h b/include/swift/ABI/MetadataValues.h
index f258ffcc7f5..c9ef96b4e52 100644
--- a/include/swift/ABI/MetadataValues.h
+++ b/include/swift/ABI/MetadataValues.h
@@ -313,6 +313,11 @@ enum class DynamicCastFlags : size_t {
   /// True if the cast should destroy the source value on failure;
   /// false if the value should be left in place.
   DestroyOnFailure = 0x4,
+
+  /// True if the cast should prohibit the use of any isolated conformances,
+  /// for example because there is a Sendable constraint on the existential
+  /// type we're casting to.
+  ProhibitIsolatedConformances = 0x8,
 };
 inline bool operator&(DynamicCastFlags a, DynamicCastFlags b) {
   return (size_t(a) & size_t(b)) != 0;
diff --git a/lib/IRGen/GenCast.cpp b/lib/IRGen/GenCast.cpp
index 22fd39ef9aa..b15002ea7a1 100644
--- a/lib/IRGen/GenCast.cpp
+++ b/lib/IRGen/GenCast.cpp
@@ -42,8 +42,11 @@ using namespace swift;
 using namespace irgen;
 
 /// Compute the flags to pass to swift_dynamicCast.
-static DynamicCastFlags getDynamicCastFlags(CastConsumptionKind consumptionKind,
-                                            CheckedCastMode mode) {
+static DynamicCastFlags getDynamicCastFlags(
+    CastConsumptionKind consumptionKind,
+    CheckedCastMode mode,
+    CastingIsolatedConformances isolatedConformances
+) {
   DynamicCastFlags flags = DynamicCastFlags::Default;
 
   if (mode == CheckedCastMode::Unconditional)
@@ -53,6 +56,14 @@ static DynamicCastFlags getDynamicCastFlags(CastConsumptionKind consumptionKind,
   if (shouldTakeOnSuccess(consumptionKind))
     flags |= DynamicCastFlags::TakeOnSuccess;
 
+  switch (isolatedConformances) {
+  case CastingIsolatedConformances::Allow:
+    break;
+  case CastingIsolatedConformances::Prohibit:
+    flags |= DynamicCastFlags::ProhibitIsolatedConformances;
+    break;
+  }
+
   return flags;
 }
 
@@ -63,10 +74,12 @@ llvm::Value *irgen::emitCheckedCast(IRGenFunction &IGF,
                                     Address dest,
                                     CanType targetType,
                                     CastConsumptionKind consumptionKind,
-                                    CheckedCastMode mode) {
+                                    CheckedCastMode mode,
+                            CastingIsolatedConformances isolatedConformances) {
   // TODO: attempt to specialize this based on the known types.
 
-  DynamicCastFlags flags = getDynamicCastFlags(consumptionKind, mode);
+  DynamicCastFlags flags = getDynamicCastFlags(consumptionKind, mode,
+                                               isolatedConformances);
 
   // Cast both addresses to opaque pointer type.
   dest = IGF.Builder.CreateElementBitCast(dest, IGF.IGM.OpaqueTy);
@@ -847,6 +860,7 @@ void irgen::emitScalarCheckedCast(IRGenFunction &IGF,
                                   SILType targetLoweredType,
                                   CanType targetFormalType,
                                   CheckedCastMode mode,
+                              CastingIsolatedConformances isolatedConformances,
                                   Explosion &out) {
   assert(sourceLoweredType.isObject());
   assert(targetLoweredType.isObject());
@@ -976,7 +990,7 @@ void irgen::emitScalarCheckedCast(IRGenFunction &IGF,
                                                src, sourceFormalType,
                                                dest, targetFormalType,
                                                CastConsumptionKind::TakeAlways,
-                                               mode);
+                                               mode, isolatedConformances);
         llvm::Value *successResult = IGF.Builder.CreateLoad(dest);
         llvm::Value *failureResult = llvm::ConstantPointerNull::get(destPtrType);
         llvm::Value *result = IGF.Builder.CreateSelect(success, successResult, failureResult);
diff --git a/lib/IRGen/GenCast.h b/lib/IRGen/GenCast.h
index a9a8002fd25..3529b466709 100644
--- a/lib/IRGen/GenCast.h
+++ b/lib/IRGen/GenCast.h
@@ -28,6 +28,7 @@ namespace swift {
   class SILType;
   class ProtocolDecl;
   enum class CastConsumptionKind : unsigned char;
+  enum class CastingIsolatedConformances: uint8_t;
 
 namespace irgen {
   class Address;
@@ -46,7 +47,8 @@ namespace irgen {
                                Address dest,
                                CanType toType,
                                CastConsumptionKind consumptionKind,
-                               CheckedCastMode mode);
+                               CheckedCastMode mode,
+                              CastingIsolatedConformances isolatedConformances);
 
   void emitScalarCheckedCast(IRGenFunction &IGF, Explosion &value,
                              SILType sourceLoweredType,
@@ -54,6 +56,7 @@ namespace irgen {
                              SILType targetLoweredType,
                              CanType targetFormalType,
                              CheckedCastMode mode,
+                             CastingIsolatedConformances isolatedConformances,
                              Explosion &out);
 
   llvm::Value *emitFastClassCastIfPossible(
diff --git a/lib/IRGen/IRGenSIL.cpp b/lib/IRGen/IRGenSIL.cpp
index 086da5d307e..3aabe05c04b 100644
--- a/lib/IRGen/IRGenSIL.cpp
+++ b/lib/IRGen/IRGenSIL.cpp
@@ -7186,7 +7186,8 @@ visitUncheckedRefCastAddrInst(swift::UncheckedRefCastAddrInst *i) {
                   src, i->getSourceFormalType(),
                   dest, i->getTargetFormalType(),
                   CastConsumptionKind::TakeAlways,
-                  CheckedCastMode::Unconditional);
+                  CheckedCastMode::Unconditional,
+                  CastingIsolatedConformances::Allow);
 }
 
 void IRGenSILFunction::visitUncheckedAddrCastInst(
@@ -7416,6 +7417,7 @@ void IRGenSILFunction::visitUnconditionalCheckedCastInst(
                         i->getTargetLoweredType(),
                         i->getTargetFormalType(),
                         CheckedCastMode::Unconditional,
+                        i->getIsolatedConformances(),
                         ex);
   setLoweredExplosion(i, ex);
 }
@@ -7604,7 +7606,8 @@ void IRGenSILFunction::visitUnconditionalCheckedCastAddrInst(
                   src, i->getSourceFormalType(),
                   dest, i->getTargetFormalType(),
                   CastConsumptionKind::TakeAlways,
-                  CheckedCastMode::Unconditional);
+                  CheckedCastMode::Unconditional,
+                  i->getIsolatedConformances());
 }
 
 void IRGenSILFunction::visitCheckedCastBranchInst(
@@ -7625,6 +7628,7 @@ void IRGenSILFunction::visitCheckedCastBranchInst(
                           i->getTargetLoweredType(),
                           i->getTargetFormalType(),
                           CheckedCastMode::Conditional,
+                          i->getIsolatedConformances(),
                           ex);
     auto val = ex.claimNext();
     castResult.casted = val;
@@ -7662,7 +7666,8 @@ void IRGenSILFunction::visitCheckedCastAddrBranchInst(
     emitCheckedCast(*this,
                     src, i->getSourceFormalType(),
                     dest, i->getTargetFormalType(),
-                    i->getConsumptionKind(), CheckedCastMode::Conditional);
+                    i->getConsumptionKind(), CheckedCastMode::Conditional,
+                    i->getIsolatedConformances());
   Builder.CreateCondBr(castSucceeded,
                        getLoweredBB(i->getSuccessBB()).bb,
                        getLoweredBB(i->getFailureBB()).bb);
diff --git a/stdlib/public/runtime/Casting.cpp b/stdlib/public/runtime/Casting.cpp
index 8125662ebad..f495c03d0c0 100644
--- a/stdlib/public/runtime/Casting.cpp
+++ b/stdlib/public/runtime/Casting.cpp
@@ -614,12 +614,19 @@ bool swift::_conformsToProtocolInContext(
     const OpaqueValue *value,
     const Metadata *type,
     ProtocolDescriptorRef protocol,
-    const WitnessTable **conformance) {
+    const WitnessTable **conformance,
+    bool prohibitIsolatedConformances) {
 
   ConformanceExecutionContext context;
   if (!_conformsToProtocol(value, type, protocol, conformance, &context))
     return false;
 
+  // If we aren't allowed to use isolated conformances and we ended up with
+  // one, fail.
+  if (prohibitIsolatedConformances &&
+      context.globalActorIsolationType)
+    return false;
+
   if (!swift_isInConformanceExecutionContext(type, &context))
     return false;
 
@@ -631,7 +638,8 @@ bool swift::_conformsToProtocolInContext(
 static bool _conformsToProtocols(const OpaqueValue *value,
                                  const Metadata *type,
                                  const ExistentialTypeMetadata *existentialType,
-                                 const WitnessTable **conformances) {
+                                 const WitnessTable **conformances,
+                                 bool prohibitIsolatedConformances) {
   if (auto *superclass = existentialType->getSuperclassConstraint()) {
     if (!swift_dynamicCastMetatype(type, superclass))
       return false;
@@ -644,7 +652,7 @@ static bool _conformsToProtocols(const OpaqueValue *value,
 
   for (auto protocol : existentialType->getProtocols()) {
     if (!_conformsToProtocolInContext(
-            value, type, protocol, conformances))
+            value, type, protocol, conformances, prohibitIsolatedConformances))
       return false;
     if (conformances != nullptr && protocol.needsWitnessTable()) {
       assert(*conformances != nullptr);
@@ -1050,9 +1058,10 @@ swift_dynamicCastMetatypeImpl(const Metadata *sourceType,
 }
 
 static const Metadata *
-swift_dynamicCastMetatypeUnconditionalImpl(const Metadata *sourceType,
-                                           const Metadata *targetType,
-                                           const char *file, unsigned line, unsigned column) {
+swift_dynamicCastMetatypeUnconditionalImpl(
+    const Metadata *sourceType,
+    const Metadata *targetType,
+    const char *file, unsigned line, unsigned column) {
   auto origSourceType = sourceType;
 
   // Identical types always succeed
@@ -1138,7 +1147,8 @@ swift_dynamicCastMetatypeUnconditionalImpl(const Metadata *sourceType,
 
   case MetadataKind::Existential: {
     auto targetTypeAsExistential = static_cast<const ExistentialTypeMetadata *>(targetType);
-    if (_conformsToProtocols(nullptr, sourceType, targetTypeAsExistential, nullptr))
+    if (_conformsToProtocols(nullptr, sourceType, targetTypeAsExistential,
+                             nullptr, /*prohibitIsolatedConformances=*/false))
       return origSourceType;
     swift_dynamicCastFailure(sourceType, targetType);
   }
diff --git a/stdlib/public/runtime/DynamicCast.cpp b/stdlib/public/runtime/DynamicCast.cpp
index 237fc7e006d..f9aefd9489a 100644
--- a/stdlib/public/runtime/DynamicCast.cpp
+++ b/stdlib/public/runtime/DynamicCast.cpp
@@ -87,7 +87,7 @@ typedef DynamicCastResult (tryCastFunctionType)(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances
 );
 
 // Forward-declare the main top-level `tryCast()` function
@@ -401,7 +401,7 @@ tryCastUnwrappingObjCSwiftValueSource(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
     id srcObject;
     memcpy(&srcObject, srcValue, sizeof(id));
@@ -422,7 +422,7 @@ tryCastUnwrappingObjCSwiftValueSource(
       destLocation, destType,
       const_cast<OpaqueValue *>(srcInnerValue), srcInnerType,
       destFailureType, srcFailureType,
-      /*takeOnSuccess=*/ false, mayDeferChecks);
+      /*takeOnSuccess=*/ false, mayDeferChecks, prohibitIsolatedConformances);
 }
 #else
 static DynamicCastResult
@@ -430,7 +430,7 @@ tryCastUnwrappingSwiftValueSource(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType->getKind() == MetadataKind::Class);
 
@@ -452,7 +452,7 @@ tryCastToSwiftClass(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Class);
@@ -496,7 +496,7 @@ tryCastToObjectiveCClass(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::ObjCClassWrapper);
@@ -546,7 +546,7 @@ tryCastToForeignClass(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
 #if SWIFT_OBJC_INTEROP
   assert(srcType != destType);
@@ -594,7 +594,8 @@ tryCastToForeignClass(
 static DynamicCastResult tryCastToForeignReferenceType(
     OpaqueValue *destLocation, const Metadata *destType, OpaqueValue *srcValue,
     const Metadata *srcType, const Metadata *&destFailureType,
-    const Metadata *&srcFailureType, bool takeOnSuccess, bool mayDeferChecks) {
+    const Metadata *&srcFailureType, bool takeOnSuccess, bool mayDeferChecks,
+    bool prohibitIsolatedConformances) {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::ForeignReferenceType);
 
@@ -610,7 +611,7 @@ tryCastToEnum(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   // Note: Optional is handled elsewhere
@@ -783,7 +784,7 @@ tryCastToAnyHashable(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Struct);
@@ -859,7 +860,7 @@ tryCastToArray(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Struct);
@@ -899,7 +900,7 @@ tryCastToDictionary(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Struct);
@@ -940,7 +941,7 @@ tryCastToSet(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Struct);
@@ -982,7 +983,7 @@ tryCastToString(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Struct);
@@ -1013,7 +1014,7 @@ tryCastToStruct(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Struct);
@@ -1036,7 +1037,7 @@ tryCastToOptional(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Optional);
@@ -1110,7 +1111,7 @@ tryCastUnwrappingOptionalBoth(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(destType->getKind() == MetadataKind::Optional);
   assert(srcType->getKind() == MetadataKind::Optional);
@@ -1134,7 +1135,8 @@ tryCastUnwrappingOptionalBoth(
     auto destInnerLocation = destLocation; // Single-payload enum layout
     auto subcastResult = tryCast(
       destInnerLocation, destInnerType, srcValue, srcInnerType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       destInnerType->vw_storeEnumTagSinglePayload(
         destLocation, /*case*/ 0, /*emptyCases*/ 1);
@@ -1153,7 +1155,7 @@ tryCastUnwrappingOptionalDestination(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(destType->getKind() == MetadataKind::Optional);
 
@@ -1162,7 +1164,8 @@ tryCastUnwrappingOptionalDestination(
   auto destInnerLocation = destLocation; // Single-payload enum layout
   auto subcastResult = tryCast(
     destInnerLocation, destInnerType, srcValue, srcType,
-    destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+    destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+    prohibitIsolatedConformances);
   if (isSuccess(subcastResult)) {
     destInnerType->vw_storeEnumTagSinglePayload(
       destLocation, /*case*/ 0, /*emptyCases*/ 1);
@@ -1179,7 +1182,7 @@ tryCastUnwrappingOptionalSource(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType->getKind() == MetadataKind::Optional);
 
@@ -1190,7 +1193,8 @@ tryCastUnwrappingOptionalSource(
   if (nonNil) {
     // Recurse with unwrapped source
     return tryCast(destLocation, destType, srcValue, srcInnerType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
   }
   return DynamicCastResult::Failure;
 }
@@ -1208,7 +1212,7 @@ tryCastToTuple(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Tuple);
@@ -1284,7 +1288,8 @@ tryCastToTuple(
       auto subcastResult = tryCast(destElt.findIn(destLocation), destElt.Type,
                                    srcElt.findIn(srcValue), srcElt.Type,
                                    destFailureType, srcFailureType,
-                                   false, mayDeferChecks);
+                                   false, mayDeferChecks,
+                                   prohibitIsolatedConformances);
       if (subcastResult == DynamicCastResult::Failure) {
         for (unsigned k = 0; k != j; ++k) {
           const auto &elt = destTupleType->getElement(k);
@@ -1310,7 +1315,7 @@ tryCastToFunction(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Function);
@@ -1372,7 +1377,8 @@ tryCastToFunction(
 static bool _conformsToProtocols(const OpaqueValue *value,
                                  const Metadata *type,
                                  const ExistentialTypeMetadata *existentialType,
-                                 const WitnessTable **conformances) {
+                                 const WitnessTable **conformances,
+                                 bool prohibitIsolatedConformances) {
   if (auto *superclass = existentialType->getSuperclassConstraint()) {
     if (!swift_dynamicCastMetatype(type, superclass))
       return false;
@@ -1385,7 +1391,7 @@ static bool _conformsToProtocols(const OpaqueValue *value,
 
   for (auto protocol : existentialType->getProtocols()) {
     if (!swift::_conformsToProtocolInContext(
-             value, type, protocol, conformances))
+            value, type, protocol, conformances, prohibitIsolatedConformances))
       return false;
     if (conformances != nullptr && protocol.needsWitnessTable()) {
       assert(*conformances != nullptr);
@@ -1402,7 +1408,7 @@ tryCastToUnconstrainedOpaqueExistential(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Existential);
@@ -1429,7 +1435,7 @@ tryCastToConstrainedOpaqueExistential(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Existential);
@@ -1443,10 +1449,12 @@ tryCastToConstrainedOpaqueExistential(
   // TODO (rdar://17033499) If the source is an existential, we should
   // be able to compare the protocol constraints more efficiently than this.
   if (_conformsToProtocols(srcValue, srcType, destExistentialType,
-                           destExistential->getWitnessTables())) {
+                           destExistential->getWitnessTables(),
+                           prohibitIsolatedConformances)) {
     return tryCastToUnconstrainedOpaqueExistential(
       destLocation, destType, srcValue, srcType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
   } else {
     return DynamicCastResult::Failure;
   }
@@ -1457,7 +1465,7 @@ tryCastToClassExistential(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Existential);
@@ -1480,7 +1488,8 @@ tryCastToClassExistential(
       auto value = reinterpret_cast<OpaqueValue *>(&tmp);
       auto type = reinterpret_cast<const Metadata *>(tmp);
       if (_conformsToProtocols(value, type, destExistentialType,
-                               destExistentialLocation->getWitnessTables())) {
+                               destExistentialLocation->getWitnessTables(),
+                               prohibitIsolatedConformances)) {
         auto object = *(reinterpret_cast<HeapObject **>(value));
         destExistentialLocation->Value = object;
         if (takeOnSuccess) {
@@ -1529,7 +1538,8 @@ tryCastToClassExistential(
     }
     if (_conformsToProtocols(srcValue, srcType,
                              destExistentialType,
-                             destExistentialLocation->getWitnessTables())) {
+                             destExistentialLocation->getWitnessTables(),
+                             prohibitIsolatedConformances)) {
       destExistentialLocation->Value = srcObject;
       if (takeOnSuccess) {
         return DynamicCastResult::SuccessViaTake;
@@ -1646,7 +1656,7 @@ tryCastToErrorExistential(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Existential);
@@ -1665,7 +1675,8 @@ tryCastToErrorExistential(
     assert(destExistentialType->NumProtocols == 1);
     const WitnessTable *errorWitness;
     if (_conformsToProtocols(
-          srcValue, srcType, destExistentialType, &errorWitness)) {
+            srcValue, srcType, destExistentialType, &errorWitness,
+            prohibitIsolatedConformances)) {
 #if SWIFT_OBJC_INTEROP
       // If it already holds an NSError, just use that.
       if (auto embedded = getErrorEmbeddedNSErrorIndirect(
@@ -1697,7 +1708,7 @@ tryCastUnwrappingExistentialSource(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(srcType->getKind() == MetadataKind::Existential);
@@ -1738,13 +1749,14 @@ tryCastUnwrappingExistentialSource(
                  srcInnerValue, srcInnerType,
                  destFailureType, srcFailureType,
                  takeOnSuccess && (srcInnerValue == srcValue),
-                 mayDeferChecks);
+                 mayDeferChecks, prohibitIsolatedConformances);
 }
 
 static DynamicCastResult tryCastUnwrappingExtendedExistentialSource(
     OpaqueValue *destLocation, const Metadata *destType, OpaqueValue *srcValue,
     const Metadata *srcType, const Metadata *&destFailureType,
-    const Metadata *&srcFailureType, bool takeOnSuccess, bool mayDeferChecks) {
+    const Metadata *&srcFailureType, bool takeOnSuccess, bool mayDeferChecks,
+    bool prohibitIsolatedConformances) {
   assert(srcType != destType);
   assert(srcType->getKind() == MetadataKind::ExtendedExistential);
 
@@ -1784,7 +1796,8 @@ static DynamicCastResult tryCastUnwrappingExtendedExistentialSource(
   srcFailureType = srcInnerType;
   return tryCast(destLocation, destType, srcInnerValue, srcInnerType,
                  destFailureType, srcFailureType,
-                 takeOnSuccess && (srcInnerValue == srcValue), mayDeferChecks);
+                 takeOnSuccess && (srcInnerValue == srcValue), mayDeferChecks,
+                 prohibitIsolatedConformances);
 }
 
 static DynamicCastResult
@@ -1792,7 +1805,7 @@ tryCastUnwrappingExistentialMetatypeSource(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(srcType->getKind() == MetadataKind::ExistentialMetatype);
@@ -1807,14 +1820,15 @@ tryCastUnwrappingExistentialMetatypeSource(
                  srcInnerValue, srcInnerType,
                  destFailureType, srcFailureType,
                  takeOnSuccess && (srcInnerValue == srcValue),
-                 mayDeferChecks);
+                 mayDeferChecks, prohibitIsolatedConformances);
 }
 
 
 static DynamicCastResult tryCastToExtendedExistential(
     OpaqueValue *destLocation, const Metadata *destType, OpaqueValue *srcValue,
     const Metadata *srcType, const Metadata *&destFailureType,
-    const Metadata *&srcFailureType, bool takeOnSuccess, bool mayDeferChecks) {
+    const Metadata *&srcFailureType, bool takeOnSuccess, bool mayDeferChecks,
+    bool prohibitIsolatedConformances) {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::ExtendedExistential);
 
@@ -1886,6 +1900,10 @@ static DynamicCastResult tryCastToExtendedExistential(
     if (error)
       return DynamicCastResult::Failure;
 
+    if (prohibitIsolatedConformances &&
+        context.globalActorIsolationType)
+      return DynamicCastResult::Failure;
+
     if (!swift_isInConformanceExecutionContext(selfType, &context))
       return DynamicCastResult::Failure;
   }
@@ -1951,7 +1969,7 @@ tryCastToOpaque(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Opaque);
@@ -1990,7 +2008,7 @@ tryCastToMetatype(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::Metatype);
@@ -2023,7 +2041,8 @@ tryCastToMetatype(
       auto srcInnerValue = reinterpret_cast<OpaqueValue *>(&metatype);
       auto srcInnerType = swift_getMetatypeMetadata(metatype);
       return tryCast(destLocation, destType, srcInnerValue, srcInnerType,
-        destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+        destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+        prohibitIsolatedConformances);
     }
 #endif
     return DynamicCastResult::Failure;
@@ -2040,7 +2059,7 @@ _dynamicCastMetatypeToExistentialMetatype(
   OpaqueValue *destLocation,  const ExistentialMetatypeMetadata *destType,
   const Metadata *srcMetatype,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   // The instance type of an existential metatype must be either an
   // existential or an existential metatype.
@@ -2057,7 +2076,7 @@ _dynamicCastMetatypeToExistentialMetatype(
       = destMetatype ? destMetatype->getWitnessTables() : nullptr;
     if (!_conformsToProtocols(nullptr, srcMetatype,
                               targetInstanceTypeAsExistential,
-                              conformance)) {
+                              conformance, prohibitIsolatedConformances)) {
       return DynamicCastResult::Failure;
     }
 
@@ -2096,7 +2115,7 @@ _dynamicCastMetatypeToExistentialMetatype(
     srcInstanceType,
     destFailureType,
     srcFailureType,
-    takeOnSuccess, mayDeferChecks);
+    takeOnSuccess, mayDeferChecks, prohibitIsolatedConformances);
 }
 
 // "ExistentialMetatype" is the metatype for an existential type.
@@ -2105,7 +2124,7 @@ tryCastToExistentialMetatype(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   assert(srcType != destType);
   assert(destType->getKind() == MetadataKind::ExistentialMetatype);
@@ -2122,7 +2141,7 @@ tryCastToExistentialMetatype(
       srcMetatype,
       destFailureType,
       srcFailureType,
-      takeOnSuccess, mayDeferChecks);
+      takeOnSuccess, mayDeferChecks, prohibitIsolatedConformances);
   }
 
   case MetadataKind::ObjCClassWrapper: {
@@ -2142,7 +2161,7 @@ tryCastToExistentialMetatype(
         metatype,
         destFailureType,
         srcFailureType,
-        takeOnSuccess, mayDeferChecks);
+        takeOnSuccess, mayDeferChecks, prohibitIsolatedConformances);
     }
 #endif
     return DynamicCastResult::Failure;
@@ -2262,7 +2281,7 @@ tryCast(
   OpaqueValue *destLocation, const Metadata *destType,
   OpaqueValue *srcValue, const Metadata *srcType,
   const Metadata *&destFailureType, const Metadata *&srcFailureType,
-  bool takeOnSuccess, bool mayDeferChecks)
+  bool takeOnSuccess, bool mayDeferChecks, bool prohibitIsolatedConformances)
 {
   destFailureType = destType;
   srcFailureType = srcType;
@@ -2295,7 +2314,8 @@ tryCast(
     return DynamicCastResult::Failure;
   }
   auto castResult = tryCastToDestType(destLocation, destType, srcValue,
-    srcType, destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+    srcType, destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+    prohibitIsolatedConformances);
   if (isSuccess(castResult)) {
     return castResult;
   }
@@ -2312,7 +2332,8 @@ tryCast(
         srcFailureType = srcDynamicType;
         auto castResult = tryCastToDestType(
           destLocation, destType, srcValue, srcDynamicType,
-          destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+          destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+          prohibitIsolatedConformances);
         if (isSuccess(castResult)) {
           return castResult;
         }
@@ -2345,7 +2366,8 @@ tryCast(
     // Try unwrapping Obj-C __SwiftValue implementation
     auto subcastResult = tryCastUnwrappingObjCSwiftValueSource(
       destLocation, destType, srcValue, srcType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       return subcastResult;
     }
@@ -2379,7 +2401,8 @@ tryCast(
   case MetadataKind::Existential: {
     auto subcastResult = tryCastUnwrappingExistentialSource(
       destLocation, destType, srcValue, srcType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       return subcastResult;
     }
@@ -2389,7 +2412,8 @@ tryCast(
   case MetadataKind::ExistentialMetatype: {
     auto subcastResult = tryCastUnwrappingExistentialMetatypeSource(
       destLocation, destType, srcValue, srcType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       return subcastResult;
     }
@@ -2399,7 +2423,8 @@ tryCast(
   case MetadataKind::ExtendedExistential: {
     auto subcastResult = tryCastUnwrappingExtendedExistentialSource(
         destLocation, destType, srcValue, srcType, destFailureType,
-        srcFailureType, takeOnSuccess, mayDeferChecks);
+        srcFailureType, takeOnSuccess, mayDeferChecks,
+        prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       return subcastResult;
     }
@@ -2423,14 +2448,16 @@ tryCast(
     if (srcKind == MetadataKind::Optional) {
       auto subcastResult = tryCastUnwrappingOptionalBoth(
         destLocation, destType, srcValue, srcType,
-        destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+        destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+        prohibitIsolatedConformances);
       if (isSuccess(subcastResult)) {
         return subcastResult;
       }
     }
     auto subcastResult = tryCastUnwrappingOptionalDestination(
       destLocation, destType, srcValue, srcType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       return subcastResult;
     }
@@ -2439,7 +2466,8 @@ tryCast(
   if (srcKind == MetadataKind::Optional) {
     auto subcastResult = tryCastUnwrappingOptionalSource(
       destLocation, destType, srcValue, srcType,
-      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks,
+      prohibitIsolatedConformances);
     if (isSuccess(subcastResult)) {
       return subcastResult;
     }
@@ -2583,6 +2611,11 @@ swift_dynamicCastImpl(OpaqueValue *destLocation,
   // actually accessed.
   bool mayDeferChecks = flags & DynamicCastFlags::Unconditional;
 
+  // Whether the compiler told us that we aren't allowed to use *any* isolated
+  // conformances, regardless of whether we are in that isolation domain.
+  bool prohibitIsolatedConformances =
+      flags & DynamicCastFlags::ProhibitIsolatedConformances;
+
   // Attempt the cast...
   const Metadata *destFailureType = destType;
   const Metadata *srcFailureType = srcType;
@@ -2590,7 +2623,7 @@ swift_dynamicCastImpl(OpaqueValue *destLocation,
     destLocation, destType,
     srcValue, srcType,
     destFailureType, srcFailureType,
-    takeOnSuccess, mayDeferChecks);
+    takeOnSuccess, mayDeferChecks, prohibitIsolatedConformances);
 
   switch (result) {
   case DynamicCastResult::Failure:
diff --git a/stdlib/public/runtime/Private.h b/stdlib/public/runtime/Private.h
index 2ef15f450a2..58f4783e9b2 100644
--- a/stdlib/public/runtime/Private.h
+++ b/stdlib/public/runtime/Private.h
@@ -721,7 +721,8 @@ public:
       const OpaqueValue *value,
       const Metadata *type,
       ProtocolDescriptorRef protocol,
-      const WitnessTable **conformance);
+      const WitnessTable **conformance,
+      bool prohibitIsolatedConformances);
 
   /// Construct type metadata for the given protocol.
   const Metadata *
diff --git a/test/Concurrency/Runtime/isolated_conformance.swift b/test/Concurrency/Runtime/isolated_conformance.swift
index ecda8b34b3b..ca3a3534404 100644
--- a/test/Concurrency/Runtime/isolated_conformance.swift
+++ b/test/Concurrency/Runtime/isolated_conformance.swift
@@ -20,6 +20,10 @@ protocol Q {
   func g()
 }
 
+protocol R: Sendable {
+  func h()
+}
+
 nonisolated class MyClass: @MainActor P {
   func f() {
     print("MyClass.f()")
@@ -45,6 +49,12 @@ extension MyClass: @SomeGlobalActor Q {
   }
 }
 
+extension MyClass: nonisolated R {
+  nonisolated func h() {
+    print("MyClass.h()")
+  }
+}
+
 struct Wrapper<T> {
   var wrapped: T
 }
@@ -63,6 +73,13 @@ extension Wrapper: Q where T: Q {
   }
 }
 
+extension Wrapper: R where T: R {
+  func h() {
+    print("Wrapper for ", terminator: "")
+    wrapped.h()
+  }
+}
+
 @available(SwiftStdlib 5.9, *)
 struct WrapMany<each T> {
   var wrapped: (repeat each T)
@@ -82,14 +99,23 @@ extension WrapMany: Q where repeat each T: Q {
   }
 }
 
-extension Int: P, Q {
-  func f() { }
-  func g() { }
+@available(SwiftStdlib 5.9, *)
+extension WrapMany: R where repeat each T: R {
+  func h() {
+    print("Wrapper for many")
+  }
 }
 
-extension String: P, Q {
+extension Int: P, Q, R {
   func f() { }
   func g() { }
+  func h() { }
+}
+
+extension String: P, Q, R {
+  func f() { }
+  func g() { }
+  func h() { }
 }
 
 func tryCastToP(_ value: any Sendable) -> Bool {
@@ -102,6 +128,16 @@ func tryCastToP(_ value: any Sendable) -> Bool {
   return false
 }
 
+func tryCastToPAndR(_ value: any Sendable) -> Bool {
+  if let p = value as? any P & R {
+    p.f()
+    return true
+  }
+
+  print("Conformance did not match")
+  return false
+}
+
 func tryCastToQ(_ value: any Sendable) -> Bool {
   if let q = value as? any Q {
     q.g()
@@ -134,6 +170,11 @@ await Task.detached { @MainActor in
   precondition(tryCastToP(mc))
   precondition(tryCastToP(wrappedMC))
 
+  // Cannot cast to P & R because the conformance to P is isolated, but R
+  // is Sendable.
+  precondition(!tryCastToPAndR(mc))
+  precondition(!tryCastToPAndR(wrappedMC))
+
   if #available(SwiftStdlib 5.9, *) {
     let wrappedMany = WrapMany(wrapped: (17, mc, "Pack"))
     precondition(tryCastToP(wrappedMany))
@@ -149,6 +190,11 @@ await Task.detached { @SomeGlobalActor in
   precondition(tryCastToQ(mc))
   precondition(tryCastToQ(wrappedMC))
 
+  // Cannot cast to P & R because the conformance to P is isolated, but R
+  // is Sendable.
+  precondition(!tryCastToPAndR(mc))
+  precondition(!tryCastToPAndR(wrappedMC))
+
   if #available(SwiftStdlib 5.9, *) {
     let wrappedMany = WrapMany(wrapped: (17, mc, "Pack"))
     precondition(tryCastToQ(wrappedMany))
@@ -176,5 +222,6 @@ await Task.detached {
   }
 }.value
 
+
 // Ensure that we access mc later
 print(mc)