diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -203,8 +203,6 @@
 def warn_drv_missing_plugin_arg : Warning<
   "missing plugin argument for plugin %0 in %1">,
   InGroup<InvalidCommandLineArgument>;
-def err_drv_invalid_libcxx_deployment : Error<
-  "invalid deployment target for -stdlib=libc++ (requires %0 or later)">;
 def err_drv_invalid_argument_to_option : Error<
   "invalid argument '%0' to -%1">;
 def err_drv_missing_sanitizer_ignorelist : Error<
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -2869,7 +2869,6 @@
   // First get the generic Apple args, before moving onto Darwin-specific ones.
   DerivedArgList *DAL =
       MachO::TranslateArgs(Args, BoundArch, DeviceOffloadKind);
-  const OptTable &Opts = getDriver().getOpts();
 
   // If no architecture is bound, none of the translations here are relevant.
   if (BoundArch.empty())
@@ -2901,26 +2900,6 @@
     }
   }
 
-  if (!Args.getLastArg(options::OPT_stdlib_EQ) &&
-      GetCXXStdlibType(Args) == ToolChain::CST_Libcxx)
-    DAL->AddJoinedArg(nullptr, Opts.getOption(options::OPT_stdlib_EQ),
-                      "libc++");
-
-  // Validate the C++ standard library choice.
-  CXXStdlibType Type = GetCXXStdlibType(*DAL);
-  if (Type == ToolChain::CST_Libcxx) {
-    // Check whether the target provides libc++.
-    StringRef where;
-
-    // Complain about targeting iOS < 5.0 in any way.
-    if (isTargetIOSBased() && isIPhoneOSVersionLT(5, 0))
-      where = "iOS 5.0";
-
-    if (where != StringRef()) {
-      getDriver().Diag(clang::diag::err_drv_invalid_libcxx_deployment) << where;
-    }
-  }
-
   auto Arch = tools::darwin::getArchTypeForMachOArchName(BoundArch);
   if ((Arch == llvm::Triple::arm || Arch == llvm::Triple::thumb)) {
     if (Args.hasFlag(options::OPT_fomit_frame_pointer,
diff --git a/clang/test/Driver/darwin-header-search-libcxx.cpp b/clang/test/Driver/darwin-header-search-libcxx.cpp
--- a/clang/test/Driver/darwin-header-search-libcxx.cpp
+++ b/clang/test/Driver/darwin-header-search-libcxx.cpp
@@ -159,3 +159,16 @@
 // RUN:               --check-prefix=CHECK-LIBCXX-MISSING-BOTH %s
 // CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[SYSROOT]]/usr/include/c++/v1"
+
+// Make sure that on Darwin, we use libc++ header search paths by default even when
+// -stdlib= isn't passed.
+//
+// RUN: %clang -### %s -fsyntax-only 2>&1 \
+// RUN:     --target=x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
+// RUN:               --check-prefix=CHECK-LIBCXX-STDLIB-UNSPECIFIED %s
+// CHECK-LIBCXX-STDLIB-UNSPECIFIED: "-cc1"
+// CHECK-LIBCXX-STDLIB-UNSPECIFIED: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
diff --git a/clang/test/Driver/darwin-stdlib.cpp b/clang/test/Driver/darwin-stdlib-dont-pass-in-c.c
copy from clang/test/Driver/darwin-stdlib.cpp
copy to clang/test/Driver/darwin-stdlib-dont-pass-in-c.c
--- a/clang/test/Driver/darwin-stdlib.cpp
+++ b/clang/test/Driver/darwin-stdlib-dont-pass-in-c.c
@@ -1,10 +1,8 @@
-// This test will fail if CLANG_DEFAULT_CXX_STDLIB is set to libstdc++.
-// XFAIL: default-cxx-stdlib=libstdc++
+// This test ensures that the Driver doesn't pass -stdlib= down to CC1 when compiling C code.
 
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64 -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -mmacosx-version-min=10.9 %s -### 2>&1 | FileCheck %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s
 // RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k %s -### 2>&1 | FileCheck %s
 
-// CHECK: "-stdlib=libc++"
-// CHECK-NOT: "-stdlib=libstdc++"
+// CHECK-NOT: "-stdlib="
diff --git a/clang/test/Driver/darwin-stdlib.cpp b/clang/test/Driver/darwin-stdlib.cpp
--- a/clang/test/Driver/darwin-stdlib.cpp
+++ b/clang/test/Driver/darwin-stdlib.cpp
@@ -1,10 +1,29 @@
-// This test will fail if CLANG_DEFAULT_CXX_STDLIB is set to libstdc++.
-// XFAIL: default-cxx-stdlib=libstdc++
+// Make sure that the Driver passes down -stdlib=libc++ to CC1 when specified explicitly.
+//
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64  -miphoneos-version-min=7.0 -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/              -mmacosx-version-min=10.9  -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k                            -stdlib=libc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBCXX %s
+// CHECK-LIBCXX: "-stdlib=libc++"
+// CHECK-LIBCXX-NOT: "-stdlib=libstdc++"
 
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64 -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -mmacosx-version-min=10.9 %s -### 2>&1 | FileCheck %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s
-// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k %s -### 2>&1 | FileCheck %s
+// Make sure that the Driver passes down -stdlib=libstdc++ to CC1 when specified explicitly. Note that this
+// shouldn't really be used on Darwin because libstdc++ is not supported anymore, but we still pin down the
+// Driver behavior for now.
+//
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64  -miphoneos-version-min=7.0 -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/              -mmacosx-version-min=10.9  -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k                            -stdlib=libstdc++ %s -### 2>&1 | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
+// CHECK-LIBSTDCXX: "-stdlib=libstdc++"
+// CHECK-LIBSTDCXX-NOT: "-stdlib=libc++"
 
-// CHECK: "-stdlib=libc++"
-// CHECK-NOT: "-stdlib=libstdc++"
+// Make sure that the Driver does not spuriously pass down any -stdlib=<...> option to CC1 if none is
+// specified on the command-line. In that case, CC1 should use the default standard library, which is
+// going to be libc++.
+//
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch arm64  -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/              -mmacosx-version-min=10.9  %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7s -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
+// RUN: %clang -target x86_64-apple-darwin -ccc-install-dir %S/Inputs/darwin_toolchain_tree/bin/ -arch armv7k                            %s -### 2>&1 | FileCheck --check-prefix=CHECK-NONE %s
+// CHECK-NONE-NOT: "-stdlib="
diff --git a/compiler-rt/test/profile/lit.cfg.py b/compiler-rt/test/profile/lit.cfg.py
--- a/compiler-rt/test/profile/lit.cfg.py
+++ b/compiler-rt/test/profile/lit.cfg.py
@@ -42,6 +42,14 @@
 clang_cflags = target_cflags + extra_link_flags
 clang_cxxflags = config.cxx_mode_flags + clang_cflags
 
+# TODO: target_cflags can sometimes contain C++ only flags like -stdlib=<FOO>, which are
+#       ignored when compiling as C code. Passing this flag when compiling as C results in
+#       warnings that break tests that use -Werror.
+#       We remove -stdlib= from the cflags here to avoid problems, but the interaction between
+#       CMake and compiler-rt's tests should be reworked so that cflags don't contain C++ only
+#       flags.
+clang_cflags = [flag.replace('-stdlib=libc++', '').replace('-stdlib=libstdc++', '') for flag in clang_cflags]
+
 def build_invocation(compile_flags, with_lto = False):
   lto_flags = []
   if with_lto and config.lto_supported: