[clang][driver] Emit an error for /clang:-x

`/clang:-x` emits an error instead of a warning. And if the error is suppressed,
`/clang:-x` takes no effect.
Considering that `/clang:` is a recent addition in 2018-11 and there are MSVC
style alternatives, therefore `/clang:-x` doesn't seem useful and we just reject
it since properly supporting it would add lots of complexity.

Fixes llvm#59307

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D142757

Author: Fznamznon

clang/docs/ReleaseNotes.rst

@@ -60,6 +60,10 @@ Bug Fixes
 - Fix crash when diagnosing incorrect usage of ``_Nullable`` involving alias
   templates. This fixes
   `Issue 60344 <https://github.com/llvm/llvm-project/issues/60344>`_.
+- Fix confusing warning message when ``/clang:-x`` is passed in ``clang-cl``
+  driver mode and emit an error which suggests using ``/TC`` or ``/TP``
+  ``clang-cl`` options instead. This fixes
+  `Issue 59307 <https://github.com/llvm/llvm-project/issues/59307>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Driver/Driver.cpp

@@ -2566,17 +2566,21 @@ void Driver::BuildInputs(const ToolChain &TC, DerivedArgList &Args,
     }
     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
-  {
+  if (!IsCLMode()) {
     Arg *LastXArg = Args.getLastArgNoClaim(options::OPT_x);
     Arg *LastInputArg = Args.getLastArgNoClaim(options::OPT_INPUT);
-    if (LastXArg && LastInputArg && LastInputArg->getIndex() < LastXArg->getIndex())
+    if (LastXArg && LastInputArg &&
+        LastInputArg->getIndex() < LastXArg->getIndex())
       Diag(clang::diag::warn_drv_unused_x) << LastXArg->getValue();
+  } else {
+    // In CL mode suggest /TC or /TP since -x doesn't make sense if passed via
+    // /clang:.
+    if (auto *A = Args.getLastArg(options::OPT_x))
+      Diag(diag::err_drv_unsupported_opt_with_suggestion)
+          << A->getAsString(Args) << "/TC' or '/TP";
   }
 
   for (Arg *A : Args) {

clang/test/Driver/x-args.c

@@ -5,3 +5,8 @@
 // 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
+
+// RUN: not %clang_cl /WX /clang:-xc /clang:-E /clang:-dM %s 2>&1 | FileCheck --implicit-check-not="error:" -check-prefix=CL %s
+// RUN: not %clang_cl /WX /clang:-E /clang:-dM %s /clang:-xc 2>&1 | FileCheck --implicit-check-not="error:" -check-prefix=CL %s
+// RUN: not %clang_cl /TC /WX /clang:-xc /clang:-E /clang:-dM %s 2>&1 | FileCheck --implicit-check-not="error:" -check-prefix=CL %s
+// CL: error: unsupported option '-x c'; did you mean '/TC' or '/TP'?