Index: clang/lib/Driver/ToolChains/WebAssembly.cpp
===================================================================
--- clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -16,7 +16,7 @@
 #include "clang/Driver/Options.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
-#include "llvm/Option/ArgList.h"
+#include "llvm/Support/VirtualFileSystem.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;
@@ -334,7 +334,11 @@
 WebAssembly::GetCXXStdlibType(const ArgList &Args) const {
   if (Arg *A = Args.getLastArg(options::OPT_stdlib_EQ)) {
     StringRef Value = A->getValue();
-    if (Value != "libc++")
+    if (Value == "libc++")
+      return ToolChain::CST_Libcxx;
+    else if (Value == "libstdc++")
+      return ToolChain::CST_Libstdcxx;
+    else
       getDriver().Diag(diag::err_drv_invalid_stdlib_name)
           << A->getAsString(Args);
   }
@@ -380,17 +384,46 @@
 
 void WebAssembly::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                                ArgStringList &CC1Args) const {
-  if (!DriverArgs.hasArg(options::OPT_nostdlibinc) &&
-      !DriverArgs.hasArg(options::OPT_nostdincxx)) {
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
+      DriverArgs.hasArg(options::OPT_nostdincxx))
+    return;
+  std::string SysRoot = getDriver().SysRoot;
+  const std::string MultiarchTriple =
+      getMultiarchTriple(getDriver(), getTriple(), SysRoot);
+  switch (GetCXXStdlibType(DriverArgs)) {
+  case ToolChain::CST_Libcxx: {
     if (getTriple().getOS() != llvm::Triple::UnknownOS) {
-      const std::string MultiarchTriple =
-          getMultiarchTriple(getDriver(), getTriple(), getDriver().SysRoot);
       addSystemInclude(DriverArgs, CC1Args,
-                       getDriver().SysRoot + "/include/" + MultiarchTriple +
-                           "/c++/v1");
+                       SysRoot + "/include/" + MultiarchTriple + "/c++/v1");
+    }
+    addSystemInclude(DriverArgs, CC1Args, SysRoot + "/include/c++/v1");
+    break;
+  }
+  case ToolChain::CST_Libstdcxx: {
+    std::error_code EC;
+    Generic_GCC::GCCVersion Version = {"", -1, -1, -1, "", "", ""};
+    std::string CxxIncludePath = SysRoot + "/include/c++/";
+    // Walk the subdirs, and find the one with the newest gcc version:
+    for (llvm::vfs::directory_iterator
+             LI = getVFS().dir_begin(CxxIncludePath, EC),
+             LE;
+         !EC && LI != LE; LI = LI.increment(EC)) {
+      StringRef VersionText = llvm::sys::path::filename(LI->path());
+      auto CandidateVersion = Generic_GCC::GCCVersion::Parse(VersionText);
+      if (CandidateVersion.Major == -1)
+        continue;
+      if (CandidateVersion <= Version)
+        continue;
+      Version = CandidateVersion;
     }
+    if (Version.Major == -1)
+      break;
+    std::string LibstdcxxInclude = SysRoot + "/include/c++/" + Version.Text;
     addSystemInclude(DriverArgs, CC1Args,
-                     getDriver().SysRoot + "/include/c++/v1");
+                     LibstdcxxInclude + "/" + MultiarchTriple);
+    addSystemInclude(DriverArgs, CC1Args, LibstdcxxInclude);
+    break;
+  }
   }
 }
 
@@ -403,7 +436,8 @@
     CmdArgs.push_back("-lc++abi");
     break;
   case ToolChain::CST_Libstdcxx:
-    llvm_unreachable("invalid stdlib name");
+    CmdArgs.push_back("-lstdc++");
+    break;
   }
 }
 
Index: clang/test/Driver/wasm-toolchain.cpp
===================================================================
--- clang/test/Driver/wasm-toolchain.cpp
+++ clang/test/Driver/wasm-toolchain.cpp
@@ -14,35 +14,35 @@
 
 // A basic C++ link command-line with unknown OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo --stdlib=c++ %s 2>&1 \
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo --stdlib=libc++ %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK %s
 // LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ link command-line with optimization with unknown OS.
 
-// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s --stdlib=c++ 2>&1 \
+// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s --stdlib=libc++ 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK_OPT %s
 // LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ link command-line with known OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=c++ %s 2>&1 \
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=libc++ %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK_KNOWN %s
 // LINK_KNOWN: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_KNOWN: wasm-ld{{.*}}" "-L/foo/lib/wasm32-wasi" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ link command-line with optimization with known OS.
 
-// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo %s --stdlib=c++ 2>&1 \
+// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo %s --stdlib=libc++ 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK_OPT_KNOWN %s
 // LINK_OPT_KNOWN: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT_KNOWN: wasm-ld{{.*}}" "-L/foo/lib/wasm32-wasi" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ compile command-line with known OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=c++ %s 2>&1 \
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=libc++ %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=COMPILE %s
 // COMPILE: clang{{.*}}" "-cc1"
 // COMPILE: "-resource-dir" "[[RESOURCE_DIR:[^"]*]]"
@@ -52,3 +52,21 @@
 // COMPILE: "-internal-isystem" "[[RESOURCE_DIR]]{{(/|\\\\)}}include"
 // COMPILE: "-internal-isystem" "/foo/include/wasm32-wasi"
 // COMPILE: "-internal-isystem" "/foo/include"
+
+// A basic C++ link command-line with known OS for -stdlib=libstdc++.
+
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=libstdc++ %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=LINK_OPT_KNOWN_GCC %s
+// LINK_OPT_KNOWN_GCC: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_OPT_KNOWN_GCC: wasm-ld{{.*}}" "-L/foo/lib/wasm32-wasi" "crt1.o" "[[temp]]" "-lstdc++" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// A basic C++ compile command-line with known OS for -stdlib=libstdc++.
+
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=libstdc++ %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=COMPILE_GCC %s
+// COMPILE_GCC: clang{{.*}}" "-cc1"
+// COMPILE_GCC: "-resource-dir" "[[RESOURCE_DIR:[^"]*]]"
+// COMPILE_GCC: "-isysroot" "/foo"
+// COMPILE_GCC: "-internal-isystem" "[[RESOURCE_DIR]]{{(/|\\\\)}}include"
+// COMPILE_GCC: "-internal-isystem" "/foo/include/wasm32-wasi"
+// COMPILE_GCC: "-internal-isystem" "/foo/include"