diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -1377,6 +1377,17 @@
   }
 }
 
+static bool isLaxVectorConversionsDefault(const llvm::Triple &Triple) {
+  switch (Triple.getArch()) {
+  default:
+    return true;
+
+  case llvm::Triple::thumb:
+  case llvm::Triple::thumbeb:
+    return Triple.getSubArch() != llvm::Triple::ARMSubArch_v8_1m_mainline;
+  }
+}
+
 static bool isNoCommonDefault(const llvm::Triple &Triple) {
   switch (Triple.getArch()) {
   default:
@@ -4678,7 +4689,11 @@
       (Args.hasArg(options::OPT_mkernel) && types::isCXX(InputType)))
     CmdArgs.push_back("-fapple-kext");
 
-  Args.AddLastArg(CmdArgs, options::OPT_flax_vector_conversions_EQ);
+  if (Args.getLastArg(options::OPT_flax_vector_conversions_EQ)) {
+    Args.AddLastArg(CmdArgs, options::OPT_flax_vector_conversions_EQ);
+  } else if (!isLaxVectorConversionsDefault(Triple)) {
+    CmdArgs.push_back("-flax-vector-conversions=none");
+  }
   Args.AddLastArg(CmdArgs, options::OPT_fobjc_sender_dependent_dispatch);
   Args.AddLastArg(CmdArgs, options::OPT_fdiagnostics_print_source_range_info);
   Args.AddLastArg(CmdArgs, options::OPT_fdiagnostics_parseable_fixits);
diff --git a/clang/test/Driver/lax-vector-conversions.c b/clang/test/Driver/lax-vector-conversions.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/lax-vector-conversions.c
@@ -0,0 +1,8 @@
+// RUN: %clang --target=arm-arm-none-eabi -march=armv8.1m.main -### -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-STRICT %s
+
+// RUN: %clang --target=arm-arm-none-eabi -march=armv8a -### -c %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-LAX %s
+
+// CHECK-STRICT: "-flax-vector-conversions=none"
+// CHECK-LAX-NOT: "-flax-vector-conversions=none"