diff --git a/include/swift/AST/Decl.h b/include/swift/AST/Decl.h
index 83d4d3a89d46c..0141a896c8ca2 100644
--- a/include/swift/AST/Decl.h
+++ b/include/swift/AST/Decl.h
@@ -4624,6 +4624,8 @@ class AbstractStorageDecl : public ValueDecl {
   /// True if any of the accessors to the storage is private or fileprivate.
   bool hasPrivateAccessor() const;
 
+  bool hasDidSetOrWillSetDynamicReplacement() const;
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) {
     return D->getKind() >= DeclKind::First_AbstractStorageDecl &&
diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index 0041138456f8b..2573469d17db6 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -4443,6 +4443,14 @@ bool AbstractStorageDecl::hasPrivateAccessor() const {
   return false;
 }
 
+bool AbstractStorageDecl::hasDidSetOrWillSetDynamicReplacement() const {
+  if (auto *func = getDidSetFunc())
+    return func->getAttrs().hasAttribute<DynamicReplacementAttr>();
+  if (auto *func = getWillSetFunc())
+    return func->getAttrs().hasAttribute<DynamicReplacementAttr>();
+  return false;
+}
+
 void AbstractStorageDecl::setAccessors(StorageImplInfo implInfo,
                                        SourceLoc lbraceLoc,
                                        ArrayRef<AccessorDecl *> accessors,
diff --git a/lib/SILGen/SILGenType.cpp b/lib/SILGen/SILGenType.cpp
index d81ad265d96e0..7e2886d6f3c55 100644
--- a/lib/SILGen/SILGenType.cpp
+++ b/lib/SILGen/SILGenType.cpp
@@ -1094,6 +1094,28 @@ class SILGenExtension : public TypeMemberVisitor<SILGenExtension> {
     SILGenType(SGM, ntd).emitType();
   }
   void visitFuncDecl(FuncDecl *fd) {
+    // Don't emit other accessors for a dynamic replacement of didSet inside of
+    // an extension. We only allow such a construct to allow definition of a
+    // didSet/willSet dynamic replacement. Emitting other accessors is
+    // problematic because there is no storage.
+    //
+    // extension SomeStruct {
+    //   @_dynamicReplacement(for: someProperty)
+    //   var replacement : Int {
+    //     didSet {
+    //     }
+    //   }
+    // }
+    if (auto *accessor = dyn_cast<AccessorDecl>(fd)) {
+      auto *storage = accessor->getStorage();
+      bool hasDidSetOrWillSetDynamicReplacement =
+          storage->hasDidSetOrWillSetDynamicReplacement();
+
+      if (hasDidSetOrWillSetDynamicReplacement &&
+          isa<ExtensionDecl>(storage->getDeclContext()) &&
+          fd != storage->getDidSetFunc() && fd != storage->getWillSetFunc())
+        return;
+    }
     SGM.emitFunction(fd);
     if (SGM.requiresObjCMethodEntryPoint(fd))
       SGM.emitObjCMethodThunk(fd);
@@ -1119,8 +1141,12 @@ class SILGenExtension : public TypeMemberVisitor<SILGenExtension> {
 
   void visitVarDecl(VarDecl *vd) {
     if (vd->hasStorage()) {
-      assert(vd->isStatic() && "stored property in extension?!");
-      return emitTypeMemberGlobalVariable(SGM, vd);
+      bool hasDidSetOrWillSetDynamicReplacement =
+          vd->hasDidSetOrWillSetDynamicReplacement();
+      assert((vd->isStatic() || hasDidSetOrWillSetDynamicReplacement) &&
+             "stored property in extension?!");
+      if (!hasDidSetOrWillSetDynamicReplacement)
+        return emitTypeMemberGlobalVariable(SGM, vd);
     }
     visitAbstractStorageDecl(vd);
   }
diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index 11e3440a33b61..5d45cb37b0851 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -2623,7 +2623,8 @@ void TypeChecker::addImplicitDynamicAttribute(Decl *D) {
   if (auto *VD = dyn_cast<VarDecl>(D)) {
     // Don't turn stored into computed properties. This could conflict with
     // exclusivity checking.
-    if (VD->hasStorage())
+    // If there is a didSet or willSet function we allow dynamic replacement.
+    if (VD->hasStorage() && !VD->getDidSetFunc() && !VD->getWillSetFunc())
       return;
     // Don't add dynamic to local variables.
     if (VD->getDeclContext()->isLocalContext())
diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index 685ed8cbce1dd..c44f840fd73c7 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -2161,7 +2161,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         TC.diagnose(VD->getLoc(), diag::enum_stored_property);
         VD->markInvalid();
       } else if (isa<ExtensionDecl>(VD->getDeclContext()) &&
-                 !VD->isStatic()) {
+                 !VD->isStatic() &&
+                 !VD->getAttrs().getAttribute<DynamicReplacementAttr>()) {
         TC.diagnose(VD->getLoc(), diag::extension_stored_property);
         VD->markInvalid();
       }
diff --git a/test/Interpreter/Inputs/dynamic_replacement_property_observer_orig.swift b/test/Interpreter/Inputs/dynamic_replacement_property_observer_orig.swift
new file mode 100644
index 0000000000000..21f6364b5263f
--- /dev/null
+++ b/test/Interpreter/Inputs/dynamic_replacement_property_observer_orig.swift
@@ -0,0 +1,54 @@
+struct Stored {
+  var i: Int {
+    didSet {
+      print("Stored.i.didSet from \(oldValue) to \(i) original")
+    }
+  }
+
+  var y: Int {
+    willSet {
+      print("Stored.y.willSet from \(y) to \(newValue) original")
+    }
+  }
+
+  var z: Int {
+    willSet {
+      print("Stored.z.willSet from \(z) to \(newValue) original")
+    }
+    didSet {
+      print("Stored.z.didSet from \(oldValue) to \(z) original")
+    }
+  }
+}
+
+var myglobal : Int = 1 {
+  didSet {
+    print("myglobal.didSet from \(oldValue) to \(myglobal) original")
+  }
+}
+
+var myglobal2 : Int = 1 {
+  willSet {
+    print("myglobal2.willSet from \(myglobal2) to \(newValue) original")
+  }
+}
+
+var myglobal3 : Int = 1 {
+  willSet {
+    print("myglobal3.willSet from \(myglobal3) to \(newValue) original")
+  }
+  didSet {
+    print("myglobal3.didSet from \(oldValue) to \(myglobal3) original")
+  }
+}
+
+class HeapStored {
+  var z: Int = 5{
+    willSet {
+      print("HeapStored.z.willSet from \(z) to \(newValue) original")
+    }
+    didSet {
+      print("HeapStored.z.didSet from \(oldValue) to \(z) original")
+    }
+  }
+}
diff --git a/test/Interpreter/Inputs/dynamic_replacement_property_observer_repl.swift b/test/Interpreter/Inputs/dynamic_replacement_property_observer_repl.swift
new file mode 100644
index 0000000000000..5b9ebafb4495c
--- /dev/null
+++ b/test/Interpreter/Inputs/dynamic_replacement_property_observer_repl.swift
@@ -0,0 +1,62 @@
+@_private(sourceFile: "dynamic_replacement_property_observer_orig.swift") import TestDidWillSet
+
+extension Stored {
+  @_dynamicReplacement(for: i)
+  var _replacement_i: Int {
+    didSet {
+      print("Stored.i.didSet from \(oldValue) to \(i) replacement")
+    }
+  }
+  @_dynamicReplacement(for: y)
+  var _replacement_y: Int {
+    willSet {
+      print("Stored.y.willSet from \(y) to \(newValue) replacement")
+    }
+  }
+
+  @_dynamicReplacement(for: z)
+  var _replacement_z: Int {
+    willSet {
+      print("Stored.z.willSet from \(z) to \(newValue) replacement")
+    }
+    didSet {
+      print("Stored.z.didSet from \(oldValue) to \(z) replacement")
+    }
+  }
+}
+
+@_dynamicReplacement(for: myglobal)
+public var _replacement_myglobal : Int = 1 {
+  didSet {
+    print("myglobal.didSet from \(oldValue) to \(myglobal) replacement")
+  }
+}
+
+@_dynamicReplacement(for: myglobal2)
+var _replacement_myglobal2 : Int = 1 {
+  willSet {
+    print("myglobal2.willSet from \(myglobal2) to \(newValue) replacement")
+  }
+}
+
+@_dynamicReplacement(for: myglobal3)
+var _replacement_myglobal3 : Int = 1 {
+  willSet {
+    print("myglobal3.willSet from \(myglobal3) to \(newValue) replacement")
+  }
+  didSet {
+    print("myglobal3.didSet from \(oldValue) to \(myglobal3) replacement")
+  }
+}
+
+extension HeapStored {
+  @_dynamicReplacement(for: z)
+  var _replacement_z: Int {
+    willSet {
+      print("HeapStored.z.willSet from \(z) to \(newValue) replacement")
+    }
+    didSet {
+      print("HeapStored.z.didSet from \(oldValue) to \(z) replacement")
+    }
+  }
+}
diff --git a/test/Interpreter/dynamicReplacement_property_observer.swift b/test/Interpreter/dynamicReplacement_property_observer.swift
new file mode 100644
index 0000000000000..f042bc65a4ec9
--- /dev/null
+++ b/test/Interpreter/dynamicReplacement_property_observer.swift
@@ -0,0 +1,87 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift-dylib(%t/%target-library-name(TestDidWillSet)) -module-name TestDidWillSet -emit-module -emit-module-path %t/TestDidWillSet.swiftmodule -swift-version 5 %S/Inputs/dynamic_replacement_property_observer_orig.swift -Xfrontend -enable-private-imports -Xfrontend -enable-implicit-dynamic
+// RUN: %target-build-swift-dylib(%t/%target-library-name(TestDidWillSet2)) -I%t -L%t -lTestDidWillSet %target-rpath(%t) -module-name TestDidWillSet2 -swift-version 5 %S/Inputs/dynamic_replacement_property_observer_repl.swift
+// RUN: %target-build-swift -I%t -L%t -lTestDidWillSet -o %t/main %target-rpath(%t) %s -swift-version 5
+// RUN: %target-codesign %t/main %t/%target-library-name(TestDidWillSet) %t/%target-library-name(TestDidWillSet2)
+// RUN: %target-run %t/main %t/%target-library-name(TestDidWillSet) %t/%target-library-name(TestDidWillSet)
+
+// REQUIRES: executable_test
+
+@_private(sourceFile: "dynamic_replacement_property_observer_orig.swift") import TestDidWillSet
+
+#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
+  import Darwin
+#elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
+  import Glibc
+#elseif os(Windows)
+  import MSVCRT
+  import WinSDK
+#else
+#error("Unsupported platform")
+#endif
+
+private func target_library_name(_ name: String) -> String {
+#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
+  return "lib\(name).dylib"
+#elseif os(Windows)
+  return "\(name).dll"
+#else
+  return "lib\(name).so"
+#endif
+}
+
+var s = Stored(i: 5,  y: 5, z: 5)
+var h = HeapStored()
+
+// CHECK: Stored.i.didSet from 5 to 10 original
+s.i = 10
+// CHECK: Stored.y.willSet from 5 to 11 original
+s.y = 11
+// CHECK: Stored.z.willSet from 5 to 12 original
+// CHECK: Stored.z.didSet from 5 to 12 original
+s.z = 12
+
+// CHECK: HeapStored.z.willSet from 5 to 16 original
+// CHECK: HeapStored.z.didSet from 5 to 16 original
+h.z = 16
+
+// CHECK: myglobal.didSet from 1 to 13 original
+myglobal = 13
+// CHECK: myglobal2.willSet from 1 to 14 original
+myglobal2 = 14
+// CHECK: myglobal3.willSet from 1 to 15 original
+// CHECK: myglobal3.didSet from 1 to 15 original
+myglobal3 = 15
+
+var executablePath = CommandLine.arguments[0]
+executablePath.removeLast(4)
+
+// Now, test with the module containing the replacements.
+
+#if os(Linux)
+	_ = dlopen(target_library_name("Module2"), RTLD_NOW)
+#elseif os(Windows)
+        _ = LoadLibraryA(target_library_name("Module2"))
+#else
+	_ = dlopen(executablePath+target_library_name("Module2"), RTLD_NOW)
+#endif
+
+// CHECK: Stored.i.didSet from 5 to 10 replacement
+s.i = 10
+// CHECK: Stored.y.willSet from 5 to 11 replacement
+s.y = 11
+// CHECK: Stored.z.willSet from 5 to 12 replacement
+// CHECK: Stored.z.didSet from 5 to 12 replacement
+s.z = 12
+
+// CHECK: HeapStored.z.willSet from 5 to 16 replacement
+// CHECK: HeapStored.z.didSet from 5 to 16 replacement
+h.z = 16
+
+// CHECK: myglobal.didSet from 1 to 13 replacement
+myglobal = 13
+// CHECK: myglobal2.willSet from 1 to 14 replacement
+myglobal2 = 14
+// CHECK: myglobal3.willSet from 1 to 15 replacement
+// CHECK: myglobal3.didSet from 1 to 15 replacement
+myglobal3 = 15
diff --git a/test/SILGen/dynamically_replaceable.swift b/test/SILGen/dynamically_replaceable.swift
index 2d4fa3b6c7774..c234e3fe1b23e 100644
--- a/test/SILGen/dynamically_replaceable.swift
+++ b/test/SILGen/dynamically_replaceable.swift
@@ -16,6 +16,8 @@ dynamic func dynamic_replaceable() {
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable6StruktV08dynamic_B4_varSivs
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable6StruktVyS2icig : $@convention(method) (Int, Strukt) -> Int
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable6StruktVyS2icis : $@convention(method) (Int, Int, @inout Strukt) -> ()
+// CHECK-LABEL: sil private [dynamically_replacable] [ossa] @$s23dynamically_replaceable6StruktV22property_with_observerSivW
+// CHECK-LABEL: sil private [dynamically_replacable] [ossa] @$s23dynamically_replaceable6StruktV22property_with_observerSivw
 struct Strukt {
   dynamic init(x: Int) {
   }
@@ -37,13 +39,23 @@ struct Strukt {
     set {
     }
   }
+
+  dynamic var property_with_observer : Int {
+    didSet {
+    }
+    willSet {
+    }
+  }
 }
+
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassC1xACSi_tcfc : $@convention(method) (Int, @owned Klass) -> @owned Klass
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassC08dynamic_B0yyF : $@convention(method) (@guaranteed Klass) -> () {
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassC08dynamic_B4_varSivg
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassC08dynamic_B4_varSivs
 // CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassCyS2icig : $@convention(method) (Int, @guaranteed Klass) -> Int
-// CHECK_LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassCyS2icis : $@convention(method) (Int, Int, @guaranteed Klass) -> ()
+// CHECK-LABEL: sil hidden [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassCyS2icis : $@convention(method) (Int, Int, @guaranteed Klass) -> ()
+// CHECK-LABEL: sil private [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassC22property_with_observerSivW
+// CHECK-LABEL: sil private [dynamically_replacable] [ossa] @$s23dynamically_replaceable5KlassC22property_with_observerSivw
 class Klass {
   dynamic init(x: Int) {
   }
@@ -72,6 +84,13 @@ class Klass {
     set {
     }
   }
+
+  dynamic var property_with_observer : Int {
+    didSet {
+    }
+    willSet {
+    }
+  }
 }
 
 class SubKlass : Klass {
@@ -253,6 +272,16 @@ struct GenericS<T> {
     set {
     }
   }
+
+// CHECK-LABEL: sil private [dynamically_replacable] [ossa] @$s23dynamically_replaceable8GenericSV22property_with_observerSivW
+// CHECK-LABEL: sil private [dynamically_replacable] [ossa] @$s23dynamically_replaceable8GenericSV22property_with_observerSivw
+  dynamic var property_with_observer : Int {
+    didSet {
+    }
+    willSet {
+    }
+  }
+
 }
 
 extension GenericS {
@@ -299,6 +328,16 @@ extension GenericS {
       self[y] = newValue
     }
   }
+
+// CHECK-LABEL: sil private [dynamic_replacement_for "$s23dynamically_replaceable8GenericSV22property_with_observerSivW"] [ossa] @$s23dynamically_replaceable8GenericSV34replacement_property_with_observerSivW
+// CHECK-LABEL: sil private [dynamic_replacement_for "$s23dynamically_replaceable8GenericSV22property_with_observerSivw"] [ossa] @$s23dynamically_replaceable8GenericSV34replacement_property_with_observerSivw
+  @_dynamicReplacement(for: property_with_observer)
+  var replacement_property_with_observer : Int {
+    didSet {
+    }
+    willSet {
+    }
+  }
 }
 
 dynamic var globalX = 0