[clang][driver] Emit a warning if -xc/-xc++ is after the last input file

This follows the same warning GCC produces.

Differential Revision: https://reviews.llvm.org/D121683

Author: kongy

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -365,6 +365,9 @@ def warn_drv_preprocessed_input_file_unused : Warning<
 def warn_drv_unused_argument : Warning<
   "argument unused during compilation: '%0'">,
   InGroup<UnusedCommandLineArgument>;
+def warn_drv_unused_x : Warning<
+  "‘-x %0’ after last input file has no effect">,
+  InGroup<UnusedCommandLineArgument>;
 def warn_drv_empty_joined_argument : Warning<
   "joined argument expects additional value: '%0'">,
   InGroup<UnusedCommandLineArgument>;

clang/lib/Driver/Driver.cpp

@@ -2308,6 +2308,15 @@ void Driver::BuildInputs(const ToolChain &TC, DerivedArgList &Args,
     assert(!Args.hasArg(options::OPT_x) && "-x and /TC or /TP is not allowed");
   }
 
+  // Warn -x after last input file has no effect
+  {
+    Arg *LastXArg = Args.getLastArgNoClaim(options::OPT_x);
+    Arg *LastInputArg = Args.getLastArgNoClaim(options::OPT_INPUT);
+    if (LastInputArg->getIndex() < LastXArg->getIndex()) {
+      Diag(clang::diag::warn_drv_unused_x) << LastXArg->getValue();
+    }
+  }
+
   for (Arg *A : Args) {
     if (A->getOption().getKind() == Option::InputClass) {
       const char *Value = A->getValue();

clang/test/Driver/x-args.c

@@ -0,0 +1,7 @@
+// RUN: %clang -fsyntax-only -Werror -xc %s
+// RUN: %clang -fsyntax-only -Werror %s -xc %s
+
+// RUN: %clang -fsyntax-only %s -xc++ -fsyntax-only 2>&1 | FileCheck %s
+// RUN: %clang -fsyntax-only -xc %s -xc++ -fsyntax-only 2>&1 | FileCheck %s
+// RUN: %clang -fsyntax-only %s -xc %s -xc++ -fsyntax-only 2>&1 | FileCheck %s
+// CHECK: ‘-x c++’ after last input file has no effect