diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index 4dc428ca62ae1..8ed7b8c89a3eb 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -73,6 +73,7 @@
 #include "llvm/Support/Path.h"
 #include "llvm/Support/YAMLParser.h"
 #include "llvm/Support/YAMLTraits.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include <algorithm>
 #include <memory>
 
@@ -693,14 +694,6 @@ importer::getNormalInvocationArguments(
     } else {
       // FIXME: Emit a warning of some kind.
     }
-
-    if (EnableCXXInterop) {
-      if (auto path =
-              getLibStdCxxModuleMapPath(searchPathOpts, triple, buffer)) {
-        invocationArgStrs.push_back(
-            (Twine("-fmodule-map-file=") + *path).str());
-      }
-    }
   }
 
   if (searchPathOpts.getSDKPath().empty()) {
@@ -870,6 +863,92 @@ importer::addCommonInvocationArguments(
   }
 }
 
+/// On Linux, some platform libraries (glibc, libstdc++) are not modularized.
+/// We inject modulemaps for those libraries into their include directories
+/// to allow using them from Swift.
+static SmallVector<std::pair<std::string, std::string>>
+getClangInvocationFileMapping(ASTContext &ctx) {
+  using Path = SmallString<128>;
+
+  const llvm::Triple &triple = ctx.LangOpts.Target;
+  // We currently only need this when building for Linux.
+  if (!triple.isOSLinux())
+    return {};
+
+  SearchPathOptions &searchPathOpts = ctx.SearchPathOpts;
+
+  Path sdkPath(searchPathOpts.getSDKPath());
+  if (sdkPath.empty())
+    sdkPath = "/";
+
+  // Currently only a modulemap for libstdc++ is injected.
+  if (!ctx.LangOpts.EnableCXXInterop)
+    return {};
+
+  Path actualModuleMapPath;
+  Path buffer;
+  if (auto path = getLibStdCxxModuleMapPath(searchPathOpts, triple, buffer))
+    actualModuleMapPath = path.getValue();
+  else
+    return {};
+
+  // Only inject the module map if it actually exists. It may not, for example
+  // if `swiftc -target x86_64-unknown-linux-gnu -emit-ir` is invoked using
+  // a Swift compiler not built for Linux targets.
+  if (!llvm::sys::fs::exists(actualModuleMapPath))
+    // FIXME: emit a warning of some kind.
+    return {};
+
+  // TODO: remove the libstdcxx.h header and reference all libstdc++ headers
+  // directly from the modulemap.
+  Path actualHeaderPath = actualModuleMapPath;
+  llvm::sys::path::remove_filename(actualHeaderPath);
+  llvm::sys::path::append(actualHeaderPath, "libstdcxx.h");
+
+  Path cxxStdlibsRoot(sdkPath);
+  llvm::sys::path::append(cxxStdlibsRoot, "usr", "include", "c++");
+  if (!llvm::sys::fs::exists(cxxStdlibsRoot))
+    return {};
+
+  // Collect all installed versions of libstdc++. We currently have no way to
+  // know which libstdc++ version will be used for this Clang invocation.
+  // TODO: extract this information from the Clang driver.
+  SmallVector<Path, 1> cxxStdlibDirs;
+  std::error_code errorCode;
+  for (llvm::vfs::directory_iterator
+           iter = ctx.SourceMgr.getFileSystem()->dir_begin(cxxStdlibsRoot,
+                                                           errorCode),
+           endIter;
+       !errorCode && iter != endIter; iter = iter.increment(errorCode)) {
+    cxxStdlibDirs.push_back(Path(iter->path()));
+  }
+
+  SmallVector<std::pair<std::string, std::string>> result;
+  // Inject a modulemap into the VFS for each of the libstdc++ versions.
+  for (const Path &cxxStdlibDir : cxxStdlibDirs) {
+    // Only inject the module map if the module does not already exist at
+    // {sysroot}/usr/include/module.{map,modulemap}.
+    Path injectedModuleMapLegacyPath(cxxStdlibDir);
+    llvm::sys::path::append(injectedModuleMapLegacyPath, "module.map");
+    if (llvm::sys::fs::exists(injectedModuleMapLegacyPath))
+      continue;
+
+    Path injectedModuleMapPath = cxxStdlibDir;
+    llvm::sys::path::append(injectedModuleMapPath, "module.modulemap");
+    if (llvm::sys::fs::exists(injectedModuleMapPath))
+      continue;
+
+    Path injectedHeaderPath = cxxStdlibDir;
+    llvm::sys::path::append(injectedHeaderPath, "libstdcxx.h");
+
+    result.push_back(
+        {std::string(injectedModuleMapPath), std::string(actualModuleMapPath)});
+    result.push_back(
+        {std::string(injectedHeaderPath), std::string(actualHeaderPath)});
+  }
+  return result;
+}
+
 bool ClangImporter::canReadPCH(StringRef PCHFilename) {
   if (!llvm::sys::fs::exists(PCHFilename))
     return false;
@@ -1122,9 +1201,10 @@ ClangImporter::create(ASTContext &ctx,
     }
   }
 
+  auto fileMapping = getClangInvocationFileMapping(ctx);
   // Wrap Swift's FS to allow Clang to override the working directory
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
-      llvm::vfs::RedirectingFileSystem::create({}, true,
+      llvm::vfs::RedirectingFileSystem::create(fileMapping, true,
                                                *ctx.SourceMgr.getFileSystem());
 
   // Create a new Clang compiler invocation.
diff --git a/stdlib/public/Cxx/libstdcxx.h b/stdlib/public/Cxx/libstdcxx.h
index 5083a9c356b32..388398095da36 100644
--- a/stdlib/public/Cxx/libstdcxx.h
+++ b/stdlib/public/Cxx/libstdcxx.h
@@ -1,79 +1,79 @@
-#include <algorithm>
-#include <bitset>
-#include <complex>
-#include <deque>
-#include <exception>
-#include <fstream>
-#include <functional>
-#include <iomanip>
-#include <ios>
-#include <iosfwd>
-#include <iostream>
-#include <istream>
-#include <iterator>
-#include <limits>
-#include <list>
-#include <locale>
-#include <map>
-#include <memory>
-#include <new>
-#include <numeric>
-#include <ostream>
-#include <queue>
-#include <set>
-#include <sstream>
-#include <stack>
-#include <stdexcept>
-#include <streambuf>
-#include <string>
-#include <utility>
-#include <typeinfo>
-#include <valarray>
-#include <vector>
-#include <array>
-#include <atomic>
-#include <chrono>
-#include <codecvt>
-#include <condition_variable>
-#include <forward_list>
-#include <future>
-#include <initializer_list>
-#include <mutex>
-#include <random>
-#include <ratio>
-#include <regex>
-#include <scoped_allocator>
-#include <system_error>
-#include <thread>
-#include <tuple>
-#include <typeindex>
-#include <type_traits>
-#include <unordered_map>
-#include <unordered_set>
+#include "algorithm"
+#include "bitset"
+#include "complex"
+#include "deque"
+#include "exception"
+#include "fstream"
+#include "functional"
+#include "iomanip"
+#include "ios"
+#include "iosfwd"
+#include "iostream"
+#include "istream"
+#include "iterator"
+#include "limits"
+#include "list"
+#include "locale"
+#include "map"
+#include "memory"
+#include "new"
+#include "numeric"
+#include "ostream"
+#include "queue"
+#include "set"
+#include "sstream"
+#include "stack"
+#include "stdexcept"
+#include "streambuf"
+#include "string"
+#include "utility"
+#include "typeinfo"
+#include "valarray"
+#include "vector"
+#include "array"
+#include "atomic"
+#include "chrono"
+#include "codecvt"
+#include "condition_variable"
+#include "forward_list"
+#include "future"
+#include "initializer_list"
+#include "mutex"
+#include "random"
+#include "ratio"
+#include "regex"
+#include "scoped_allocator"
+#include "system_error"
+#include "thread"
+#include "tuple"
+#include "typeindex"
+#include "type_traits"
+#include "unordered_map"
+#include "unordered_set"
 
 // C++17 and newer:
 
-#if __has_include(<any>)
-#include <any>
+#if __has_include("any")
+#include "any"
 #endif
-#if __has_include(<charconv>)
-#include <charconv>
+#if __has_include("charconv")
+#include "charconv"
 #endif
-#if __has_include(<execution>)
-#include <execution>
+#if __has_include("execution")
+#include "execution"
 #endif
-#if __has_include(<filesystem>)
-#include <filesystem>
+#if __has_include("filesystem")
+#include "filesystem"
 #endif
-#if __has_include(<memory_resource>)
-#include <memory_resource>
+#if __has_include("memory_resource")
+#include "memory_resource"
 #endif
-#if __has_include(<optional>)
-#include <optional>
+#if __has_include("optional")
+#include "optional"
 #endif
-#if __has_include(<string_view>)
-#include <string_view>
+#if __has_include("string_view")
+#include "string_view"
 #endif
-#if __has_include(<variant>)
-#include <variant>
+#if __has_include("variant")
+#include "variant"
 #endif
diff --git a/stdlib/public/Cxx/libstdcxx.modulemap b/stdlib/public/Cxx/libstdcxx.modulemap
index 5a2dbff03adac..d8200ec6cf2e0 100644
--- a/stdlib/public/Cxx/libstdcxx.modulemap
+++ b/stdlib/public/Cxx/libstdcxx.modulemap
@@ -20,4 +20,13 @@ module std {
   header "libstdcxx.h"
   requires cplusplus
   export *
+
+  /// C compatibility headers.
+  module compat {
+    module cassert {
+      header "cassert"
+      requires cplusplus
+      export *
+    }
+  }
 }