Index: test/Driver/darwin-stdlib.cpp
===================================================================
--- test/Driver/darwin-stdlib.cpp
+++ test/Driver/darwin-stdlib.cpp
@@ -1,3 +1,7 @@
+// This test will fail if CLANG_DEFAULT_CXX_STDLIB is set to anything different
+// than the platform default. (see https://llvm.org/bugs/show_bug.cgi?id=30548)
+// XFAIL: default-cxx-stdlib-set
+
 // RUN: %clang -target x86_64-apple-darwin -arch arm64 -miphoneos-version-min=7.0 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-LIBCXX
 // RUN: %clang -target x86_64-apple-darwin -mmacosx-version-min=10.8 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-LIBSTDCXX
 // RUN: %clang -target x86_64-apple-darwin -mmacosx-version-min=10.9 %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-LIBCXX
Index: test/lit.cfg
===================================================================
--- test/lit.cfg
+++ test/lit.cfg
@@ -343,6 +343,9 @@
 
 # Set available features we allow tests to conditionalize on.
 #
+if config.clang_default_cxx_stdlib != '':
+    config.available_features.add('default-cxx-stdlib-set')
+
 # Enabled/disabled features
 if config.clang_staticanalyzer != 0:
     config.available_features.add("staticanalyzer")
Index: test/lit.site.cfg.in
===================================================================
--- test/lit.site.cfg.in
+++ test/lit.site.cfg.in
@@ -16,6 +16,7 @@
 config.llvm_use_sanitizer = "@LLVM_USE_SANITIZER@"
 config.have_zlib = "@HAVE_LIBZ@"
 config.clang_arcmt = @ENABLE_CLANG_ARCMT@
+config.clang_default_cxx_stdlib = "@CLANG_DEFAULT_CXX_STDLIB@"
 config.clang_staticanalyzer = @ENABLE_CLANG_STATIC_ANALYZER@
 config.clang_examples = @ENABLE_CLANG_EXAMPLES@
 config.enable_shared = @ENABLE_SHARED@