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
@@ -20,6 +20,7 @@
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     --sysroot="" \
 // RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain --check-prefix=CHECK-LIBCXX-TOOLCHAIN-1 %s
 // CHECK-LIBCXX-TOOLCHAIN-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
diff --git a/clang/test/Driver/darwin-header-search-system.cpp b/clang/test/Driver/darwin-header-search-system.cpp
--- a/clang/test/Driver/darwin-header-search-system.cpp
+++ b/clang/test/Driver/darwin-header-search-system.cpp
@@ -95,6 +95,7 @@
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --sysroot="" \
 // RUN:   | FileCheck -DRESOURCE=%S/Inputs/resource_dir \
 // RUN:               --check-prefix=CHECK-NOSYSROOT %s
 // CHECK-NOSYSROOT: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
diff --git a/clang/test/Driver/mingw-sysroot.cpp b/clang/test/Driver/mingw-sysroot.cpp
--- a/clang/test/Driver/mingw-sysroot.cpp
+++ b/clang/test/Driver/mingw-sysroot.cpp
@@ -17,7 +17,7 @@
 // If we find a gcc in the path with the right triplet prefix, pick that as
 // sysroot:
 
-// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s
+// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ --sysroot="" -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}5.3-posix{{/|\\\\}}include{{/|\\\\}}c++"
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}5.3-posix{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}x86_64-w64-mingw32"
 // CHECK_TESTROOT_GCC: "{{.*}}/testroot-gcc{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}5.3-posix{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}backward"
@@ -27,7 +27,7 @@
 // If there's a matching sysroot next to the clang binary itself, prefer that
 // over a gcc in the path:
 
-// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-clang/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=compiler-rt -stdlib=libstdc++ -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_CLANG %s
+// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-clang/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=compiler-rt -stdlib=libstdc++ --sysroot="" -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_CLANG %s
 // CHECK_TESTROOT_CLANG: "{{.*}}/testroot-clang{{/|\\\\}}x86_64-w64-mingw32{{/|\\\\}}include"
 
 
@@ -35,4 +35,4 @@
 // happens to be in the same directory as gcc, make sure we still can pick up
 // the libgcc directory:
 
-// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-gcc/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s
+// RUN: env "PATH=%T/testroot-gcc/bin:%PATH%" %T/testroot-gcc/bin/x86_64-w64-mingw32-clang -target x86_64-w64-mingw32 -rtlib=platform -stdlib=libstdc++ --sysroot="" -c -### %s 2>&1 | FileCheck -check-prefix=CHECK_TESTROOT_GCC %s