diff --git a/include/swift/AST/Module.h b/include/swift/AST/Module.h
index 7b3eb44587b8e..499f24c52960f 100644
--- a/include/swift/AST/Module.h
+++ b/include/swift/AST/Module.h
@@ -676,9 +676,10 @@ class ModuleDecl : public DeclContext, public TypeDecl {
   /// This assumes that \p module was imported.
   bool isImportedImplementationOnly(const ModuleDecl *module) const;
 
-  /// Returns true if a function, which is using \p nominal, can be serialized
-  /// by cross-module-optimization.
-  bool canBeUsedForCrossModuleOptimization(NominalTypeDecl *nominal) const;
+  /// Returns true if decl context or its content can be serialized by
+  /// cross-module-optimization.
+  /// The \p ctxt can e.g. be a NominalType or the context of a function.
+  bool canBeUsedForCrossModuleOptimization(DeclContext *ctxt) const;
 
   /// Finds all top-level decls of this module.
   ///
diff --git a/lib/AST/Module.cpp b/lib/AST/Module.cpp
index 205f5fec6c74d..3bc88ab72adec 100644
--- a/lib/AST/Module.cpp
+++ b/lib/AST/Module.cpp
@@ -2353,14 +2353,15 @@ bool ModuleDecl::isImportedImplementationOnly(const ModuleDecl *module) const {
 }
 
 bool ModuleDecl::
-canBeUsedForCrossModuleOptimization(NominalTypeDecl *nominal) const {
-  ModuleDecl *moduleOfNominal = nominal->getParentModule();
+canBeUsedForCrossModuleOptimization(DeclContext *ctxt) const {
+  ModuleDecl *moduleOfCtxt = ctxt->getParentModule();
 
-  // If the nominal is defined in the same module, it's fine.
-  if (moduleOfNominal == this)
+  // If the context defined in the same module - or is the same module, it's
+  // fine.
+  if (moduleOfCtxt == this)
     return true;
 
-  // See if nominal is imported in a "regular" way, i.e. not with
+  // See if context is imported in a "regular" way, i.e. not with
   // @_implementationOnly or @_spi.
   ModuleDecl::ImportFilter filter = {
     ModuleDecl::ImportFilterKind::Exported,
@@ -2370,7 +2371,7 @@ canBeUsedForCrossModuleOptimization(NominalTypeDecl *nominal) const {
 
   auto &imports = getASTContext().getImportCache();
   for (auto &desc : results) {
-    if (imports.isImportedBy(moduleOfNominal, desc.importedModule))
+    if (imports.isImportedBy(moduleOfCtxt, desc.importedModule))
       return true;
   }
   return false;
diff --git a/lib/SILOptimizer/IPO/CrossModuleSerializationSetup.cpp b/lib/SILOptimizer/IPO/CrossModuleSerializationSetup.cpp
index 90e6895fba47e..fde588c301a3f 100644
--- a/lib/SILOptimizer/IPO/CrossModuleSerializationSetup.cpp
+++ b/lib/SILOptimizer/IPO/CrossModuleSerializationSetup.cpp
@@ -386,6 +386,11 @@ bool CrossModuleSerializationSetup::canUseFromInline(SILFunction *func,
   if (!func)
     return false;
 
+  if (DeclContext *funcCtxt = func->getDeclContext()) {
+    if (!M.getSwiftModule()->canBeUsedForCrossModuleOptimization(funcCtxt))
+      return false;
+  }
+
   switch (func->getLinkage()) {
   case SILLinkage::PublicNonABI:
     return func->isSerialized() != IsNotSerialized;
diff --git a/test/SILOptimizer/Inputs/cross-module/c-module.c b/test/SILOptimizer/Inputs/cross-module/c-module.c
new file mode 100644
index 0000000000000..808c26c020781
--- /dev/null
+++ b/test/SILOptimizer/Inputs/cross-module/c-module.c
@@ -0,0 +1,7 @@
+
+#include "c-module.h"
+
+long privateCFunc() {
+  return 123;
+}
+
diff --git a/test/SILOptimizer/Inputs/cross-module/c-module.h b/test/SILOptimizer/Inputs/cross-module/c-module.h
new file mode 100644
index 0000000000000..ef27e8d9c2114
--- /dev/null
+++ b/test/SILOptimizer/Inputs/cross-module/c-module.h
@@ -0,0 +1,3 @@
+
+long privateCFunc();
+
diff --git a/test/SILOptimizer/Inputs/cross-module-objc.swift b/test/SILOptimizer/Inputs/cross-module/cross-module-objc.swift
similarity index 100%
rename from test/SILOptimizer/Inputs/cross-module-objc.swift
rename to test/SILOptimizer/Inputs/cross-module/cross-module-objc.swift
diff --git a/test/SILOptimizer/Inputs/cross-module.swift b/test/SILOptimizer/Inputs/cross-module/cross-module.swift
similarity index 94%
rename from test/SILOptimizer/Inputs/cross-module.swift
rename to test/SILOptimizer/Inputs/cross-module/cross-module.swift
index 68d5c0b8aba72..09a38be6e1cee 100644
--- a/test/SILOptimizer/Inputs/cross-module.swift
+++ b/test/SILOptimizer/Inputs/cross-module/cross-module.swift
@@ -1,5 +1,6 @@
 import Submodule
 @_implementationOnly import PrivateSubmodule
+@_implementationOnly import PrivateCModule
 
 private enum PE<T> {
   case A
@@ -268,11 +269,19 @@ public func callUnrelated<T>(_ t: T) -> T {
   return t
 }
 
-public func callImplementationOnly<T>(_ t: T) -> T {
+public func callImplementationOnlyType<T>(_ t: T) -> T {
   let p = PrivateStr(i: 27)
   print(p.test())
   return t
 }
 
+public func callImplementationOnlyFunc<T>(_ t: T) -> Int {
+  return privateFunc()
+}
+
+public func callCImplementationOnly<T>(_ t: T) -> Int {
+  return Int(privateCFunc())
+}
+
 
 public let globalLet = 529387
diff --git a/test/SILOptimizer/Inputs/cross-private-submodule.swift b/test/SILOptimizer/Inputs/cross-module/cross-private-submodule.swift
similarity index 75%
rename from test/SILOptimizer/Inputs/cross-private-submodule.swift
rename to test/SILOptimizer/Inputs/cross-module/cross-private-submodule.swift
index f43a8e65e2ee8..2de51eafa7664 100644
--- a/test/SILOptimizer/Inputs/cross-private-submodule.swift
+++ b/test/SILOptimizer/Inputs/cross-module/cross-private-submodule.swift
@@ -12,3 +12,6 @@ public struct PrivateStr {
   }
 }
 
+public func privateFunc() -> Int {
+  return 40
+}
diff --git a/test/SILOptimizer/Inputs/cross-submodule.swift b/test/SILOptimizer/Inputs/cross-module/cross-submodule.swift
similarity index 100%
rename from test/SILOptimizer/Inputs/cross-submodule.swift
rename to test/SILOptimizer/Inputs/cross-module/cross-submodule.swift
diff --git a/test/SILOptimizer/Inputs/cross-module/module.modulemap b/test/SILOptimizer/Inputs/cross-module/module.modulemap
new file mode 100644
index 0000000000000..ef50f56508135
--- /dev/null
+++ b/test/SILOptimizer/Inputs/cross-module/module.modulemap
@@ -0,0 +1,3 @@
+module PrivateCModule {
+  header "c-module.h"
+}
diff --git a/test/SILOptimizer/cross-module-optimization-objc.swift b/test/SILOptimizer/cross-module-optimization-objc.swift
index 8d3129a3c1735..bf692cf641501 100644
--- a/test/SILOptimizer/cross-module-optimization-objc.swift
+++ b/test/SILOptimizer/cross-module-optimization-objc.swift
@@ -1,7 +1,7 @@
 // First test: functional correctness
 
 // RUN: %empty-directory(%t) 
-// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/Test.swiftmodule -module-name=Test -I%t %S/Inputs/cross-module-objc.swift -c -o %t/test.o
+// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/Test.swiftmodule -module-name=Test -I%t %S/Inputs/cross-module/cross-module-objc.swift -c -o %t/test.o
 // RUN: %target-build-swift -O -wmo -module-name=Main -I%t %s -c -o %t/main.o
 // RUN: %target-swiftc_driver %t/main.o %t/test.o -o %t/a.out
 // RUN: %target-codesign %t/a.out
diff --git a/test/SILOptimizer/cross-module-optimization.swift b/test/SILOptimizer/cross-module-optimization.swift
index 7ccf14386faa3..e8934eafcb059 100644
--- a/test/SILOptimizer/cross-module-optimization.swift
+++ b/test/SILOptimizer/cross-module-optimization.swift
@@ -1,18 +1,19 @@
 // First test: functional correctness
 
 // RUN: %empty-directory(%t) 
-// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/Submodule.swiftmodule -module-name=Submodule %S/Inputs/cross-submodule.swift -c -o %t/submodule.o
-// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/PrivateSubmodule.swiftmodule -module-name=PrivateSubmodule %S/Inputs/cross-private-submodule.swift -c -o %t/privatesubmodule.o
-// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/Test.swiftmodule -module-name=Test -I%t %S/Inputs/cross-module.swift -c -o %t/test.o
+// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/Submodule.swiftmodule -module-name=Submodule %S/Inputs/cross-module/cross-submodule.swift -c -o %t/submodule.o
+// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/PrivateSubmodule.swiftmodule -module-name=PrivateSubmodule %S/Inputs/cross-module/cross-private-submodule.swift -c -o %t/privatesubmodule.o
+// RUN: %target-clang -c --language=c %S/Inputs/cross-module/c-module.c -o %t/c-module.o
+// RUN: %target-build-swift -O -wmo -parse-as-library -cross-module-optimization -emit-module -emit-module-path=%t/Test.swiftmodule -module-name=Test -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/cross-module.swift -c -o %t/test.o
 // RUN: %target-build-swift -O -wmo -module-name=Main -I%t %s -c -o %t/main.o
-// RUN: %target-swiftc_driver %t/main.o %t/test.o %t/submodule.o %t/privatesubmodule.o -o %t/a.out
+// RUN: %target-swiftc_driver %t/main.o %t/test.o %t/submodule.o %t/privatesubmodule.o %t/c-module.o -o %t/a.out
 // RUN: %target-codesign %t/a.out
 // RUN: %target-run %t/a.out | %FileCheck %s -check-prefix=CHECK-OUTPUT
 
 // Check if it also works if the main module is compiled with -Onone:
 
 // RUN: %target-build-swift -Onone -wmo -module-name=Main -I%t %s -c -o %t/main-onone.o
-// RUN: %target-swiftc_driver %t/main-onone.o %t/test.o %t/submodule.o %t/privatesubmodule.o -o %t/a.out
+// RUN: %target-swiftc_driver %t/main-onone.o %t/test.o %t/submodule.o %t/privatesubmodule.o %t/c-module.o -o %t/a.out
 // RUN: %target-codesign %t/a.out
 // RUN: %target-run %t/a.out | %FileCheck %s -check-prefix=CHECK-OUTPUT
 
@@ -146,8 +147,14 @@ func testGlobal() {
 @inline(never)
 func testImplementationOnly() {
   // CHECK-OUTPUT: 27
-  // CHECK-SIL2: function_ref @$s4Test22callImplementationOnlyyxxlF
-  print(callImplementationOnly(27))
+  // CHECK-SIL2: function_ref @$s4Test26callImplementationOnlyTypeyxxlF
+  print(callImplementationOnlyType(27))
+  // CHECK-OUTPUT: 40
+  // CHECK-SIL2: function_ref @$s4Test26callImplementationOnlyFuncySixlF
+  print(callImplementationOnlyFunc(0))
+  // CHECK-OUTPUT: 123
+  // CHECK-SIL2: function_ref @$s4Test23callCImplementationOnlyySixlF
+  print(callCImplementationOnly(0))
   // CHECK-SIL2: } // end sil function '$s4Main22testImplementationOnlyyyF'
 }