diff --git a/Runtimes/Overlay/CMakeLists.txt b/Runtimes/Overlay/CMakeLists.txt
index 9b8b0c600f1a2..38bb4ae54b7a6 100644
--- a/Runtimes/Overlay/CMakeLists.txt
+++ b/Runtimes/Overlay/CMakeLists.txt
@@ -57,6 +57,7 @@ include(ResourceEmbedding)
 include("${${PROJECT_NAME}_VENDOR_MODULE_DIR}/Settings.cmake" OPTIONAL)
 
 defaulted_option(SwiftOverlay_ENABLE_REFLECTION "Enable runtime support for mirrors and reflection support")
+defaulted_option(SwiftOverlay_ENABLE_CXX_INTEROP "Enable C++ Interop support overlays")
 
 option(SwiftOverlay_INSTALL_NESTED_SUBDIR "Install libraries under a platform and architecture subdirectory" ON)
 set(SwiftOverlay_INSTALL_LIBDIR "${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>$<$<BOOL:${SwiftOverlay_INSTALL_NESTED_SUBDIR}>:/${SwiftOverlay_PLATFORM_SUBDIR}/${SwiftOverlay_ARCH_SUBDIR}>")
@@ -67,16 +68,23 @@ option(SwiftOverlay_ENABLE_LIBRARY_EVOLUTION "Generate ABI resilient runtime lib
 option(SwiftOverlay_ENABLE_BACKDEPLOYMENT_SUPPORT "Add symbols for runtime backdeployment"
   ${SwiftCore_ENABLE_BACKDEPLOYMENT_SUPPORT})
 
+add_compile_definitions(
+  $<$<BOOL:${SwiftOverlay_ENABLE_BACKDEPLOYMENT_SUPPORT}>:SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT>)
+
 add_compile_options(
   $<$<COMPILE_LANGUAGE:Swift>:-explicit-module-build>
   $<$<COMPILE_LANGUAGE:Swift>:-nostdlibimport>
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enforce-exclusivity=unchecked>"
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -target-min-inlining-version -Xfrontend min>"
+  $<$<COMPILE_LANGUAGE:Swift>:-strict-memory-safety>
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enable-lexical-lifetimes=false>"
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enable-ossa-modules>"
-  "$<$<AND:$<BOOL:${SwiftOverlay_ENABLE_LIBRARY_EVOLUTION}>,$<COMPILE_LANGUAGE:Swift>>:-enable-library-evolution>"
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -disable-implicit-concurrency-module-import>"
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -disable-implicit-string-processing-module-import>")
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -disable-implicit-string-processing-module-import>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enforce-exclusivity=unchecked>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -enable-ossa-modules>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -target-min-inlining-version -Xfrontend min>"
+  "$<$<AND:$<BOOL:${${PROJECT_NAME}_ENABLE_LIBRARY_EVOLUTION}>,$<COMPILE_LANGUAGE:Swift>>:-enable-library-evolution>"
+  "$<$<AND:$<BOOL:${${PROJECT_NAME}_ENABLE_PRESPECIALIZATION}>,$<COMPILE_LANGUAGE:Swift>>:SHELL:-Xfrontend -prespecialize-generic-metadata>")
+
+include(ExperimentalFeatures)
 
 # LNK4049: symbol 'symbol' defined in 'filename.obj' is imported
 # LNK4286: symbol 'symbol' defined in 'filename_1.obj' is imported by 'filename_2.obj'
@@ -87,9 +95,6 @@ add_compile_options(
 # a compromise, treat all linker warnings as errors.
 add_link_options($<$<PLATFORM_ID:Windows>:LINKER:/WX>)
 
-add_compile_definitions(
-  $<$<BOOL:${SwiftOverlay_ENABLE_BACKDEPLOYMENT_SUPPORT}>:SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT>)
-
 include(ExperimentalFeatures)
 
 add_subdirectory(clang)
@@ -100,6 +105,10 @@ if(WIN32)
   add_subdirectory(Windows)
 endif()
 
+if(SwiftOverlay_ENABLE_CXX_INTEROP)
+  add_subdirectory(Cxx)
+endif()
+
 # Inter-project install info
 export(EXPORT SwiftOverlayTargets
   FILE "cmake/SwiftOverlay/SwiftOverlayTargets.cmake")
diff --git a/Runtimes/Overlay/Cxx/CMakeLists.txt b/Runtimes/Overlay/Cxx/CMakeLists.txt
new file mode 100644
index 0000000000000..c9de5b997b0ca
--- /dev/null
+++ b/Runtimes/Overlay/Cxx/CMakeLists.txt
@@ -0,0 +1,45 @@
+
+if(NOT APPLE)
+  add_subdirectory(cxxshim)
+endif()
+if(LINUX)
+  add_subdirectory(libstdcxx)
+endif()
+add_subdirectory(std)
+
+add_library(swiftCxx STATIC
+  CxxConvertibleToBool.swift
+  CxxConvertibleToCollection.swift
+  CxxDictionary.swift
+  CxxOptional.swift
+  CxxPair.swift
+  CxxRandomAccessCollection.swift
+  CxxSequence.swift
+  CxxSet.swift
+  CxxSpan.swift
+  CxxVector.swift
+  UnsafeCxxIterators.swift)
+set_target_properties(swiftCxx PROPERTIES
+  Swift_MODULE_NAME Cxx)
+target_compile_options(swiftCxx PRIVATE
+  "$<$<COMPILE_LANGUAGE:Swift>:-cxx-interoperability-mode=default>"
+  "$<$<COMPILE_LANGUAGE:Swift>:-warn-implicit-overrides>"
+  # This module should not pull in the C++ standard library, so we disable it
+  # explicitly.  For functionality that depends on the C++ stdlib, use C++
+  # stdlib overlay (`swiftstd` module).
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -nostdinc++>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-enable-experimental-feature AllowUnsafeAttribute>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-enable-experimental-feature BuiltinModule>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-enable-experimental-feature Span>")
+target_link_libraries(swiftCxx PRIVATE
+  swiftCore)
+
+install(TARGETS swiftCxx
+  EXPORT SwiftOverlayTargets
+  ARCHIVE DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  LIBRARY DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")
+emit_swift_interface(swiftCxx)
+install_swift_interface(swiftCxx)
+
+embed_manifest(swiftCxx)
diff --git a/Runtimes/Overlay/Cxx/cxxshim/CMakeLists.txt b/Runtimes/Overlay/Cxx/cxxshim/CMakeLists.txt
new file mode 100644
index 0000000000000..de0e8b1fa5b7c
--- /dev/null
+++ b/Runtimes/Overlay/Cxx/cxxshim/CMakeLists.txt
@@ -0,0 +1,17 @@
+
+add_library(cxxshim INTERFACE)
+target_compile_options(cxxshim INTERFACE
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/libcxxshim.modulemap>")
+target_include_directories(cxxshim INTERFACE
+  $<$<COMPILE_LANGUAGE:Swift>:$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>>)
+
+install(TARGETS cxxshim
+  EXPORT SwiftOverlayTargets
+  ARCHIVE DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  LIBRARY DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")
+install(FILES
+  libcxxshim.h
+  libcxxshim.modulemap
+  libcxxstdlibshim.h
+  DESTINATION "${CMAKE_INSTALL_LIBDIR}/swift/${SwiftOverlay_PLATFORM_SUBDIR}")
diff --git a/Runtimes/Overlay/Cxx/libstdcxx/CMakeLists.txt b/Runtimes/Overlay/Cxx/libstdcxx/CMakeLists.txt
new file mode 100644
index 0000000000000..4050cd5be0efc
--- /dev/null
+++ b/Runtimes/Overlay/Cxx/libstdcxx/CMakeLists.txt
@@ -0,0 +1,16 @@
+
+add_library(libstdcxx INTERFACE)
+target_compile_options(libstdcxx INTERFACE
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/libstdcxx.modulemap>")
+target_include_directories(libstdcxx INTERFACE
+  $<$<COMPILE_LANGUAGE:Swift>:$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>>)
+
+install(TARGETS libstdcxx
+  EXPORT SwiftOverlayTargets
+  ARCHIVE DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  LIBRARY DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")
+install(FILES
+  libstdcxx.h
+  libstdcxx.modulemap
+  DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}")
diff --git a/Runtimes/Overlay/Cxx/std/CMakeLists.txt b/Runtimes/Overlay/Cxx/std/CMakeLists.txt
new file mode 100644
index 0000000000000..1b9797b6da2ac
--- /dev/null
+++ b/Runtimes/Overlay/Cxx/std/CMakeLists.txt
@@ -0,0 +1,43 @@
+
+add_library(swiftCxxStdlib STATIC
+  std.swift
+  Chrono.swift
+  String.swift)
+set_target_properties(swiftCxxStdlib PROPERTIES
+  Swift_MODULE_NAME CxxStdlib)
+target_compile_options(swiftCxxStdlib PRIVATE
+  "-strict-memory-safety"
+  "-cxx-interoperability-mode=default"
+  "SHELL:-enable-experimental-feature AllowUnsafeAttribute"
+  # This flag is unnecessary when building with newer compilers that allow using
+  # C++ symbols in resilient overlays (see f4204568).
+  "SHELL:-enable-experimental-feature AssumeResilientCxxTypes"
+  # The varying modularization of the C++ standard library on different
+  # platforms makes it difficult to enable MemberImportVisibility for this
+  # module
+  "SHELL:-disable-upcoming-feature MemberImportVisibility"
+  "SHELL:-Xfrontend -module-interface-preserve-types-as-written")
+# NOTE: We need to setup the sysroot here as we need to ensure that we pick up
+# the module.map from the C++ runtime for the `std` (spelt `CxxStdlib`) import.
+target_compile_options(swiftCxxStdlib PRIVATE
+  "$<$<PLATFORM_ID:Android>:SHELL:-Xcc --sysroot -Xcc ${CMAKE_ANDROID_NDK_TOOLCHAIN_UNIFIED}/sysroot>")
+target_link_libraries(swiftCxxStdlib PRIVATE
+  $<$<PLATFORM_ID:Linux>:libstdcxx>
+  $<$<NOT:$<PLATFORM_ID:Darwin>>:cxxshim>
+  swiftCxx
+  swiftCore
+  swift_Builtin_float
+  $<$<PLATFORM_ID:Android>:SwiftAndroid>
+  $<$<PLATFORM_ID:Windows>:ClangModules>)
+
+install(FILES std.apinotes
+  DESTINATION ${CMAKE_INSTALL_LIBDIR}/swift/apinotes)
+install(TARGETS swiftCxxStdlib
+  EXPORT SwiftOverlayTargets
+  ARCHIVE DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  LIBRARY DESTINATION "${SwiftOverlay_INSTALL_LIBDIR}"
+  RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")
+emit_swift_interface(swiftCxxStdlib)
+install_swift_interface(swiftCxxStdlib)
+
+embed_manifest(swiftCxxStdlib)
diff --git a/Runtimes/Overlay/cmake/modules/DefaultSettings.cmake b/Runtimes/Overlay/cmake/modules/DefaultSettings.cmake
index abdde72cc85f2..259f24aa20a18 100644
--- a/Runtimes/Overlay/cmake/modules/DefaultSettings.cmake
+++ b/Runtimes/Overlay/cmake/modules/DefaultSettings.cmake
@@ -26,12 +26,16 @@ endmacro()
 
 if(APPLE)
   set(SwiftOverlay_ENABLE_REFLECTION_default ON)
+  set(SwiftOverlay_ENABLE_CXX_INTEROP_default OFF)
 elseif(CMAKE_SYSTEM_NAME STREQUAL "WASM")
   set(SwiftOverlay_ENABLE_REFLECTION_default OFF)
+  set(SwiftOverlay_ENABLE_CXX_INTEROP_default OFF)
 elseif(LINUX OR ANDROID OR BSD)
   set(SwiftOverlay_ENABLE_REFLECTION_default OFF)
+  set(SwiftOverlay_ENABLE_CXX_INTEROP_default OFF)
 elseif(WIN32)
   set(SwiftOverlay_ENABLE_REFLECTION_default ON)
+  set(SwiftOverlay_ENABLE_CXX_INTEROP_default OFF)
 endif()
 
 include("${SwiftOverlay_VENDOR_MODULE_DIR}/DefaultSettings.cmake" OPTIONAL)
diff --git a/Runtimes/Resync.cmake b/Runtimes/Resync.cmake
index 098d006ba14bd..7f355944d8c1e 100644
--- a/Runtimes/Resync.cmake
+++ b/Runtimes/Resync.cmake
@@ -122,6 +122,8 @@ copy_library_sources("linker-support" "public/ClangOverlays" "Overlay")
 message(STATUS "Clang[${StdlibSources}/public/ClangOverlays] -> ${CMAKE_CURRENT_LIST_DIR}/Overlay/clang")
 copy_files(public/ClangOverlays Overlay/clang FILES float.swift.gyb)
 
+copy_library_sources("Cxx" "public" "Overlay")
+
 # Android Overlay
 message(STATUS "Android modulemaps[${StdlibSources}/Platform] -> ${CMAKE_CURRENT_LIST_DIR}/Overlay/Android/clang")
 copy_files(public/Platform Overlay/Android/clang