diff --git a/include/swift/AST/DiagnosticsFrontend.def b/include/swift/AST/DiagnosticsFrontend.def
index 361bd1cddbaf7..c8d7c58f00372 100644
--- a/include/swift/AST/DiagnosticsFrontend.def
+++ b/include/swift/AST/DiagnosticsFrontend.def
@@ -576,6 +576,9 @@ ERROR(no_allocations_without_embedded,none,
 ERROR(no_swift_sources_with_embedded,none,
       "embedded swift cannot be enabled in a compiler built without Swift sources", ())
 
+ERROR(package_cmo_requires_library_evolution, none,
+        "Library evolution must be enabled for Package CMO", ())
+
 ERROR(experimental_not_supported_in_production,none,
       "experimental feature '%0' cannot be enabled in production compiler",
       (StringRef))
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 6ad91ddd8e7f1..e2f3db7401207 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -2688,6 +2688,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       Diags.diagnose(SourceLoc(), diag::ignoring_option_requires_option,
                      "-package-cmo",
                      "-allow-non-resilient-access");
+    } else if (!FEOpts.EnableLibraryEvolution) {
+      Diags.diagnose(SourceLoc(), diag::package_cmo_requires_library_evolution);
     } else {
       Opts.EnableSerializePackage = true;
       Opts.CMOMode = CrossModuleOptimizationMode::Default;
diff --git a/lib/SILOptimizer/IPO/CrossModuleOptimization.cpp b/lib/SILOptimizer/IPO/CrossModuleOptimization.cpp
index ae320df503772..d825009ef293b 100644
--- a/lib/SILOptimizer/IPO/CrossModuleOptimization.cpp
+++ b/lib/SILOptimizer/IPO/CrossModuleOptimization.cpp
@@ -947,9 +947,15 @@ void CrossModuleOptimization::makeSubstUsableFromInline(
 class CrossModuleOptimizationPass: public SILModuleTransform {
   void run() override {
     auto &M = *getModule();
-    if (M.getSwiftModule()->isResilient() &&
-        !M.getSwiftModule()->serializePackageEnabled())
+    if (M.getSwiftModule()->serializePackageEnabled()) {
+      assert(M.getSwiftModule()->isResilient() &&
+             "Package CMO requires library-evolution");
+    } else if (M.getSwiftModule()->isResilient()) {
+      // If no Package CMO flags are passed and library
+      // evolution is enabled, just return.
       return;
+    }
+
     if (!M.isWholeModule())
       return;
 
@@ -974,6 +980,9 @@ class CrossModuleOptimizationPass: public SILModuleTransform {
       return;
     }
 
+    if (isPackageCMOEnabled(M.getSwiftModule()))
+      assert(conservative && "Package CMO requires conservative CMO mode");
+
     CrossModuleOptimization CMO(M, conservative, everything);
     CMO.serializeFunctionsInModule(PM);
 
diff --git a/test/SILOptimizer/package-cmo-non-resilient-mode.swift b/test/SILOptimizer/cmo-with-package-decls.swift
similarity index 90%
rename from test/SILOptimizer/package-cmo-non-resilient-mode.swift
rename to test/SILOptimizer/cmo-with-package-decls.swift
index d55cc354ef9bf..259025678ee67 100644
--- a/test/SILOptimizer/package-cmo-non-resilient-mode.swift
+++ b/test/SILOptimizer/cmo-with-package-decls.swift
@@ -1,7 +1,7 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-build-swift -O -wmo -Xfrontend -experimental-package-cmo -Xfrontend -experimental-allow-non-resilient-access -parse-as-library -emit-module -emit-module-path=%t/Submodule.swiftmodule -module-name=Submodule -package-name Pkg %S/Inputs/cross-module/default-submodule.swift -c -o %t/submodule.o
-// RUN: %target-build-swift -O -wmo -Xfrontend -experimental-package-cmo -Xfrontend -experimental-allow-non-resilient-access -parse-as-library -emit-module -emit-module-path=%t/Module.swiftmodule -module-name=Module -package-name Pkg -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/module.o
-// RUN: %target-build-swift -O -wmo -Xfrontend -experimental-package-cmo -Xfrontend -experimental-allow-non-resilient-access -parse-as-library -emit-tbd -emit-tbd-path %t/ModuleTBD.tbd -emit-module -emit-module-path=%t/ModuleTBD.swiftmodule -module-name=ModuleTBD -package-name Pkg -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/moduletbd.o -Xfrontend -tbd-install_name -Xfrontend module
+// RUN: %target-build-swift -O -wmo -Xfrontend -enable-default-cmo -parse-as-library -emit-module -emit-module-path=%t/Submodule.swiftmodule -module-name=Submodule -package-name Pkg %S/Inputs/cross-module/default-submodule.swift -c -o %t/submodule.o
+// RUN: %target-build-swift -O -wmo -Xfrontend -enable-default-cmo -parse-as-library -emit-module -emit-module-path=%t/Module.swiftmodule -module-name=Module -package-name Pkg -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/module.o
+// RUN: %target-build-swift -O -wmo -Xfrontend -enable-default-cmo -parse-as-library -emit-tbd -emit-tbd-path %t/ModuleTBD.tbd -emit-module -emit-module-path=%t/ModuleTBD.swiftmodule -module-name=ModuleTBD -package-name Pkg -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/moduletbd.o -Xfrontend -tbd-install_name -Xfrontend module
 
 // RUN: %target-build-swift -O -wmo -module-name=Main -package-name Pkg -I%t -I%S/Inputs/cross-module %s -emit-sil -o %t/Main.sil
 // RUN: %FileCheck %s < %t/Main.sil
diff --git a/test/SILOptimizer/package-cmo-require-library-evolution.swift b/test/SILOptimizer/package-cmo-require-library-evolution.swift
new file mode 100644
index 0000000000000..a9b1ae8a61e60
--- /dev/null
+++ b/test/SILOptimizer/package-cmo-require-library-evolution.swift
@@ -0,0 +1,33 @@
+// RUN: %empty-directory(%t)
+
+// RUN: not %target-build-swift %s \
+// RUN: -module-name=Lib -package-name Pkg \
+// RUN: -emit-module -o %t/Lib.swiftmodule -I%t \
+// RUN: -Xfrontend -package-cmo \
+// RUN: -Xfrontend -allow-non-resilient-access \
+// RUN: -O -wmo 2>&1 | %FileCheck %s --check-prefix CHECK-DIAGS
+// CHECK-DIAGS: error: Library evolution must be enabled for Package CMO
+
+// RUN: %target-build-swift %s \
+// RUN: -module-name=Lib -package-name Pkg \
+// RUN: -emit-module -o %t/Lib.swiftmodule -I%t \
+// RUN: -Xfrontend -package-cmo \
+// RUN: -Xfrontend -allow-non-resilient-access \
+// RUN: -enable-library-evolution \
+// RUN: -O -wmo
+
+// RUN: llvm-bcanalyzer %t/Lib.swiftmodule | %FileCheck %s -check-prefix=CHECK-BC
+// CHECK-BC: SERIALIZE_PACKAGE_ENABLED
+
+// REQUIRES: swift_in_compiler
+
+public struct S {
+  public init() {}
+  package var x: Int {
+    get { return 0 }
+    set {}
+  }
+  package func f() -> Int {
+    return 1
+  }
+}
diff --git a/test/SILOptimizer/package-cmo-resilient-mode.swift b/test/SILOptimizer/package-cmo-resilient-mode.swift
index bd0dfd9acaba4..808429e6d2124 100644
--- a/test/SILOptimizer/package-cmo-resilient-mode.swift
+++ b/test/SILOptimizer/package-cmo-resilient-mode.swift
@@ -1,6 +1,7 @@
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t
 
+/// Enable Package CMO; conservative mode on resilient module.
 // RUN: %target-build-swift %t/Lib.swift \
 // RUN: -module-name=Lib -package-name Pkg \
 // RUN: -parse-as-library -emit-module -emit-module-path %t/Lib.swiftmodule -I%t \
@@ -18,10 +19,12 @@
 
 // RUN: rm -rf %t/Lib.swiftmodule
 
+/// Enable non-package CMO; conservative mode on non-resilient module,
+/// and compare results with Package CMO.
 // RUN: %target-build-swift %t/Lib.swift \
 // RUN: -module-name=Lib -package-name Pkg \
 // RUN: -parse-as-library -emit-module -emit-module-path %t/Lib.swiftmodule -I%t \
-// RUN: -Xfrontend -experimental-package-cmo -Xfrontend -experimental-allow-non-resilient-access \
+// RUN: -Xfrontend -enable-default-cmo \
 // RUN: -O -wmo
 
 // RUN: %target-sil-opt %t/Lib.swiftmodule -sil-verify-all -o %t/Lib-non-res.sil