Index: include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- include/clang/Basic/DiagnosticDriverKinds.td
+++ include/clang/Basic/DiagnosticDriverKinds.td
@@ -30,6 +30,7 @@
   "GPU arch %1 requires CUDA version at least %3, but installation at %0 is %2. "
   "Use --cuda-path to specify a different CUDA install, or pass "
   "--no-cuda-version-check.">;
+def err_drv_cuda_nvptx_host : Error<"unsupported use of NVPTX for host compilation.">;
 def err_drv_invalid_thread_model_for_target : Error<
   "invalid thread model '%0' in '%1' for this target">;
 def err_drv_invalid_linker_name : Error<
Index: lib/Driver/ToolChains.cpp
===================================================================
--- lib/Driver/ToolChains.cpp
+++ lib/Driver/ToolChains.cpp
@@ -4779,7 +4779,8 @@
   if (!DriverArgs.hasArg(options::OPT_nocudainc) &&
       !DriverArgs.hasArg(options::OPT_no_cuda_version_check)) {
     StringRef Arch = DriverArgs.getLastArgValue(options::OPT_march_EQ);
-    assert(!Arch.empty() && "Must have an explicit GPU arch.");
+    if (Arch.empty())
+      return; // Driver has already reported the error.
     CudaInstallation.CheckCudaVersionSupportsArch(StringToCudaArch(Arch));
   }
   Linux::AddCudaIncludeArgs(DriverArgs, CC1Args);
@@ -4828,6 +4829,8 @@
   if (BoundArch) {
     DAL->eraseArg(options::OPT_march_EQ);
     DAL->AddJoinedArg(nullptr, Opts.getOption(options::OPT_march_EQ), BoundArch);
+  } else {
+    getDriver().Diag(diag::err_drv_cuda_nvptx_host);
   }
   return DAL;
 }
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -11248,7 +11248,10 @@
   assert(TC.getTriple().isNVPTX() && "Wrong platform");
 
   // Obtain architecture from the action.
-  CudaArch gpu_arch = StringToCudaArch(JA.getOffloadingArch());
+  const char *ArchName = JA.getOffloadingArch();
+  if (ArchName == nullptr)
+    return;  // Bail out. Driver has already reported the error.
+  CudaArch gpu_arch = StringToCudaArch(ArchName);
   assert(gpu_arch != CudaArch::UNKNOWN &&
          "Device action expected to have an architecture.");
 
Index: test/Driver/cuda-bad-arch.cu
===================================================================
--- test/Driver/cuda-bad-arch.cu
+++ test/Driver/cuda-bad-arch.cu
@@ -19,4 +19,9 @@
 // RUN: %clang -### -target x86_64-linux-gnu -c %s 2>&1 \
 // RUN: | FileCheck -check-prefix OK %s
 
+// We don't allow using NVPTX for host compilation.
+// RUN: %clang -### --cuda-host-only -target nvptx-nvidia-cuda -c %s 2>&1 \
+// RUN: | FileCheck -check-prefix HOST_NVPTX %s
+
 // OK-NOT: error: Unsupported CUDA gpu architecture
+// HOST_NVPTX: error: unsupported use of NVPTX for host compilation.
Index: test/Preprocessor/cuda-types.cu
===================================================================
--- test/Preprocessor/cuda-types.cu
+++ test/Preprocessor/cuda-types.cu
@@ -19,9 +19,3 @@
 // RUN: grep 'define __[^ ]*\(TYPE\|MAX\|SIZEOF\|WIDTH\)' %T/powerpc64-host-defines   | grep -v '__LDBL\|_LONG_DOUBLE' > %T/powerpc64-host-defines-filtered
 // RUN: grep 'define __[^ ]*\(TYPE\|MAX\|SIZEOF\|WIDTH\)' %T/powerpc64-device-defines | grep -v '__LDBL\|_LONG_DOUBLE' > %T/powerpc64-device-defines-filtered
 // RUN: diff %T/powerpc64-host-defines-filtered %T/powerpc64-device-defines-filtered
-
-// RUN: %clang --cuda-host-only -nocudainc -target nvptx-nvidia-cuda -x cuda -E -dM -o - /dev/null > %T/nvptx-host-defines
-// RUN: %clang --cuda-device-only -nocudainc -target nvptx-nvidia-cuda -x cuda -E -dM -o - /dev/null > %T/nvptx-device-defines
-// RUN: grep 'define __[^ ]*\(TYPE\|MAX\|SIZEOF\|WIDTH\)' %T/nvptx-host-defines   | grep -v '__LDBL\|_LONG_DOUBLE' > %T/nvptx-host-defines-filtered
-// RUN: grep 'define __[^ ]*\(TYPE\|MAX\|SIZEOF\|WIDTH\)' %T/nvptx-device-defines | grep -v '__LDBL\|_LONG_DOUBLE' > %T/nvptx-device-defines-filtered
-// RUN: diff %T/nvptx-host-defines-filtered %T/nvptx-device-defines-filtered