From e2a1f8ec27b88be670cd867c43588f24516d2bbf Mon Sep 17 00:00:00 2001
From: Yi Kong <yikong@google.com>
Date: Wed, 16 Mar 2022 14:06:23 +0800
Subject: [PATCH] Reland "[clang][driver] Emit a warning if -xc/-xc++ is after
 the last input file"

This reverts commit ba59476515cf4598dd25bcfacfbca11b4f4da3d4.
---
 clang/include/clang/Basic/DiagnosticDriverKinds.td | 3 +++
 clang/lib/Driver/Driver.cpp                        | 8 ++++++++
 clang/test/Driver/x-args.c                         | 7 +++++++
 3 files changed, 18 insertions(+)
 create mode 100644 clang/test/Driver/x-args.c

diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
index afedb37797e3..0a62d4b85c9d 100644
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/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>;
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 70822490e63b..8099c1d71f0e 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2308,6 +2308,14 @@ 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 (LastXArg && LastInputArg && 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();
diff --git a/clang/test/Driver/x-args.c b/clang/test/Driver/x-args.c
new file mode 100644
index 000000000000..b49f474babf0
--- /dev/null
+++ b/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