Index: clang/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -161,9 +161,8 @@
   "support for '/Yc' with more than one source file not implemented yet; flag ignored">,
   InGroup<ClangClPch>;
 
-def warn_drv_non_fallback_argument_clang_cl : Warning<
-  "option '%0' is ignored when /fallback happens">,
-  InGroup<OptionIgnored>;
+def err_drv_both_fallback_and_dllexport_inlines_cannot_be_used_clang_cl : Error<
+  "option '/Zc:dllexportInlines-' is ABI-changing and not compatible with '/fallback'">;
 
 def err_drv_invalid_value : Error<"invalid value '%1' in '%0'">;
 def err_drv_invalid_int_value : Error<"invalid integral value '%1' in '%0'">;
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -5525,8 +5525,14 @@
 
  if (Args.hasFlag(options::OPT__SLASH_Zc_dllexportInlines_,
                   options::OPT__SLASH_Zc_dllexportInlines,
-                  false))
+                  false)) {
+   Arg *dllexportInlines = Args.getLastArg(options::OPT__SLASH_Zc_dllexportInlines_);
+   if (Args.hasArg(options::OPT__SLASH_fallback) && dllexportInlines) {
+     D.Diag(clang::diag::err_drv_both_fallback_and_dllexport_inlines_cannot_be_used_clang_cl);
+   } else {
     CmdArgs.push_back("-fno-dllexport-inlines");
+   }
+ }
 
   Arg *MostGeneralArg = Args.getLastArg(options::OPT__SLASH_vmg);
   Arg *BestCaseArg = Args.getLastArg(options::OPT__SLASH_vmb);
Index: clang/lib/Driver/ToolChains/MSVC.cpp
===================================================================
--- clang/lib/Driver/ToolChains/MSVC.cpp
+++ clang/lib/Driver/ToolChains/MSVC.cpp
@@ -669,12 +669,6 @@
   // them too.
   Args.AddAllArgs(CmdArgs, options::OPT_UNKNOWN);
 
-  // Warning for ignored flag.
-  if (const Arg *dllexportInlines =
-      Args.getLastArg(options::OPT__SLASH_Zc_dllexportInlines_))
-    C.getDriver().Diag(clang::diag::warn_drv_non_fallback_argument_clang_cl)
-      << dllexportInlines->getAsString(Args);
-
   // Input filename.
   assert(Inputs.size() == 1);
   const InputInfo &II = Inputs[0];
Index: clang/test/Driver/cl-options.c
===================================================================
--- clang/test/Driver/cl-options.c
+++ clang/test/Driver/cl-options.c
@@ -495,7 +495,7 @@
 // RUN: %clang_cl /Zc:dllexportInlines /c -### -- %s 2>&1 | FileCheck -check-prefix=DllExportInlines %s
 // DllExportInlines-NOT: "-fno-dllexport-inlines"
 // RUN: %clang_cl /fallback /Zc:dllexportInlines- /c -### -- %s 2>&1 | FileCheck -check-prefix=DllExportInlinesFallback %s
-// DllExportInlinesFallback: warning: option '/Zc:dllexportInlines-' is ignored when /fallback happens [-Woption-ignored]
+// DllExportInlinesFallback: error: option '/Zc:dllexportInlines-' is ABI-changing and not compatible with '/fallback'
 
 // RUN: %clang_cl /Zi /c -### -- %s 2>&1 | FileCheck -check-prefix=Zi %s
 // Zi: "-gcodeview"