Emit a warning if -xc/-xc++ is after the last input file

[kongyi]

Summary

This follows the same warning GCC produces.

[srhines]

Looks great. Thanks for noticing and adding this.

[MaskRay]

clang/include/clang/Basic/DiagnosticDriverKinds.td

369

Use ASCII punctuation marks

clang/test/Driver/x-args.c

2

The first two RUN lines are unneeded

8

No newline at end of file

[MaskRay]

clang/lib/Driver/Driver.cpp

2316

https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

Don’t Use Braces on Simple Single-Statement Bodies of if/else/loop Statements

Is it guaranteed that both LastXArg and LastInputArg will be non-null?

[MaskRay]

clang/test/Driver/x-args.c

2

Sorry, they are needed because of -Werror.

clang/include/clang/Basic/DiagnosticDriverKinds.td

def warn_drv_input_file_unused_by_cpp : Warning<
   "%0: '%1' input unused in cpp mode">,
   InGroup<UnusedCommandLineArgument>;
 def warn_drv_preprocessed_input_file_unused : Warning<
   "%0: previously preprocessed input%select{ unused when '%2' is present|}1">,
   InGroup<UnusedCommandLineArgument>;
 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>;
 def warn_drv_diagnostics_hotness_requires_pgo : Warning<
   "argument '%0' requires profile-guided optimization information">,
   InGroup<UnusedCommandLineArgument>;
 def warn_drv_clang_unsupported : Warning<
   "the clang compiler does not support '%0'">;

clang/lib/Driver/Driver.cpp

if (Arg *TCTP = Args.getLastArgNoClaim(options::OPT__SLASH_TC,
     }
     if (ShowNote)
       Diag(clang::diag::note_drv_t_option_is_global);
 
     // No driver mode exposes -x and /TC or /TP; we don't support mixing them.
     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();
       types::ID Ty = types::TY_INVALID;
 
       // Infer the input type if necessary.
       if (InputType == types::TY_Nothing) {
         // If there was an explicit arg for this, claim it.

clang/test/Driver/x-args.c

+// 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
 No newline at end of file