Index: lib/Driver/ToolChain.cpp
===================================================================
--- lib/Driver/ToolChain.cpp
+++ lib/Driver/ToolChain.cpp
@@ -548,16 +548,23 @@
 ToolChain::CXXStdlibType ToolChain::GetCXXStdlibType(const ArgList &Args) const{
   ToolChain::CXXStdlibType Type;
   bool HasValidType = false;
+  bool ForcePlatformDefault = false;
 
   const Arg *A = Args.getLastArg(options::OPT_stdlib_EQ);
   if (A) {
-    HasValidType = ParseCXXStdlibType(A->getValue(), Type);
-    if (!HasValidType)
+    StringRef Value = A->getValue();
+    HasValidType = ParseCXXStdlibType(Value, Type);
+
+    // Only use in tests to override CLANG_DEFAULT_CXX_STDLIB!
+    if (Value == "platform")
+      ForcePlatformDefault = true;
+    else if (!HasValidType)
       getDriver().Diag(diag::err_drv_invalid_stdlib_name)
         << A->getAsString(Args);
   }
 
-  if (!HasValidType && !ParseCXXStdlibType(CLANG_DEFAULT_CXX_STDLIB, Type))
+  if (!HasValidType && (ForcePlatformDefault ||
+      !ParseCXXStdlibType(CLANG_DEFAULT_CXX_STDLIB, Type)))
     Type = GetDefaultCXXStdlibType();
 
   return Type;
Index: lib/Driver/ToolChains.h
===================================================================
--- lib/Driver/ToolChains.h
+++ lib/Driver/ToolChains.h
@@ -728,7 +728,7 @@
   bool IsMathErrnoDefault() const override { return false; }
   bool IsObjCNonFragileABIDefault() const override { return true; }
 
-  CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
+  CXXStdlibType GetDefaultCXXStdlibType() const override;
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
@@ -756,7 +756,7 @@
   bool IsMathErrnoDefault() const override { return false; }
   bool IsObjCNonFragileABIDefault() const override { return true; }
 
-  CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
+  CXXStdlibType GetDefaultCXXStdlibType() const override;
 
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
Index: lib/Driver/ToolChains.cpp
===================================================================
--- lib/Driver/ToolChains.cpp
+++ lib/Driver/ToolChains.cpp
@@ -3106,16 +3106,7 @@
     getFilePaths().push_back(getDriver().SysRoot + "/usr/lib");
 }
 
-ToolChain::CXXStdlibType FreeBSD::GetCXXStdlibType(const ArgList &Args) const {
-  if (Arg *A = Args.getLastArg(options::OPT_stdlib_EQ)) {
-    StringRef Value = A->getValue();
-    if (Value == "libstdc++")
-      return ToolChain::CST_Libstdcxx;
-    if (Value == "libc++")
-      return ToolChain::CST_Libcxx;
-
-    getDriver().Diag(diag::err_drv_invalid_stdlib_name) << A->getAsString(Args);
-  }
+ToolChain::CXXStdlibType FreeBSD::GetDefaultCXXStdlibType() const {
   if (getTriple().getOSMajorVersion() >= 10)
     return ToolChain::CST_Libcxx;
   return ToolChain::CST_Libstdcxx;
@@ -3258,17 +3249,7 @@
 
 Tool *NetBSD::buildLinker() const { return new tools::netbsd::Linker(*this); }
 
-ToolChain::CXXStdlibType NetBSD::GetCXXStdlibType(const ArgList &Args) const {
-  if (Arg *A = Args.getLastArg(options::OPT_stdlib_EQ)) {
-    StringRef Value = A->getValue();
-    if (Value == "libstdc++")
-      return ToolChain::CST_Libstdcxx;
-    if (Value == "libc++")
-      return ToolChain::CST_Libcxx;
-
-    getDriver().Diag(diag::err_drv_invalid_stdlib_name) << A->getAsString(Args);
-  }
-
+ToolChain::CXXStdlibType NetBSD::GetDefaultCXXStdlibType() const {
   unsigned Major, Minor, Micro;
   getTriple().getOSVersion(Major, Minor, Micro);
   if (Major >= 7 || Major == 0) {
Index: test/Driver/freebsd.cpp
===================================================================
--- test/Driver/freebsd.cpp
+++ test/Driver/freebsd.cpp
@@ -1,13 +1,13 @@
-// RUN: %clangxx %s -### -o %t.o -target amd64-unknown-freebsd10.0 2>&1 \
+// RUN: %clangxx %s -### -o %t.o -target amd64-unknown-freebsd10.0 -stdlib=platform 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-TEN %s
-// RUN: %clangxx %s -### -o %t.o -target amd64-unknown-freebsd9.2 2>&1 \
+// RUN: %clangxx %s -### -o %t.o -target amd64-unknown-freebsd9.2 -stdlib=platform 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NINE %s
 // CHECK-TEN: "-lc++" "-lm"
 // CHECK-NINE: "-lstdc++" "-lm"
 
-// RUN: %clangxx %s -### -pg -o %t.o -target amd64-unknown-freebsd10.0 2>&1 \
+// RUN: %clangxx %s -### -pg -o %t.o -target amd64-unknown-freebsd10.0 -stdlib=platform 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-PG-TEN %s
-// RUN: %clangxx %s -### -pg -o %t.o -target amd64-unknown-freebsd9.2 2>&1 \
+// RUN: %clangxx %s -### -pg -o %t.o -target amd64-unknown-freebsd9.2 -stdlib=platform 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-PG-NINE %s
 // CHECK-PG-TEN: "-lc++_p" "-lm_p"
 // CHECK-PG-NINE: "-lstdc++_p" "-lm_p"
Index: test/Driver/netbsd.cpp
===================================================================
--- test/Driver/netbsd.cpp
+++ test/Driver/netbsd.cpp
@@ -1,93 +1,93 @@
 // RUN: %clangxx -no-canonical-prefixes -target x86_64--netbsd \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=X86_64 %s
 // RUN: %clangxx -no-canonical-prefixes -target x86_64--netbsd7.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=X86_64-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target x86_64--netbsd6.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=X86_64-6 %s
 // RUN: %clangxx -no-canonical-prefixes -target arm--netbsd6.0.0-eabi \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=ARM %s
 // RUN: %clangxx -no-canonical-prefixes -target arm--netbsd7.0.0-eabi \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=ARM-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target aarch64--netbsd \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=AARCH64 %s
 // RUN: %clangxx -no-canonical-prefixes -target aarch64--netbsd7.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=AARCH64-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc--netbsd \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=SPARC %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc--netbsd6.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=SPARC-6 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc--netbsd7.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=SPARC-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc64--netbsd \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=SPARC64 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc64--netbsd6.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=SPARC64-6 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc64--netbsd7.0.0 \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=SPARC64-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target powerpc--netbsd \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=POWERPC %s
 // RUN: %clangxx -no-canonical-prefixes -target powerpc64--netbsd \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=POWERPC64 %s
 
 // RUN: %clangxx -no-canonical-prefixes -target x86_64--netbsd -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-X86_64 %s
 // RUN: %clangxx -no-canonical-prefixes -target x86_64--netbsd7.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-X86_64-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target x86_64--netbsd6.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-X86_64-6 %s
 // RUN: %clangxx -no-canonical-prefixes -target arm--netbsd6.0.0-eabi -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-ARM %s
 // RUN: %clangxx -no-canonical-prefixes -target arm--netbsd7.0.0-eabi -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-ARM-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target aarch64--netbsd -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-AARCH64 %s
 // RUN: %clangxx -no-canonical-prefixes -target aarch64--netbsd7.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-AARCH64-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc--netbsd -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-SPARC %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc--netbsd6.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-SPARC-6 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc--netbsd7.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-SPARC-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc64--netbsd -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-SPARC64 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc64--netbsd6.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-SPARC64-6 %s
 // RUN: %clangxx -no-canonical-prefixes -target sparc64--netbsd7.0.0 -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-SPARC64-7 %s
 // RUN: %clangxx -no-canonical-prefixes -target powerpc--netbsd -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-POWERPC %s
 // RUN: %clangxx -no-canonical-prefixes -target powerpc64--netbsd -static \
-// RUN: --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
+// RUN: -stdlib=platform --sysroot=%S/Inputs/basic_netbsd_tree %s -### 2>&1 \
 // RUN: | FileCheck -check-prefix=S-POWERPC64 %s
 
 // X86_64: clang{{.*}}" "-cc1" "-triple" "x86_64--netbsd"