[flang][driver] Add options for -Werror

[arnamoy10]

Summary

Add the following option for the flang-driver

-Werror

With the option given, warnings are treated as error.

A test case is added.

[arnamoy10]

Updating to use available option from Options.td instead of creating a new option for -Werror

[awarzynski]

Thank you for adding this @arnamoy10 !

I think that in order to match the semantics of -Werror from f18, this patch needs to be extended a bit. In the following two cases, f18 will exit immediately, whereas flang-new -fc1 will happily carry-on:

• https://github.com/llvm/llvm-project/blob/main/flang/tools/f18/f18.cpp#L212-L213
• https://github.com/llvm/llvm-project/blob/main/flang/tools/f18/f18.cpp#L241-L242

Could you try implementing similar semantics here?

I've left a few more comments inline.

flang/lib/Frontend/CompilerInvocation.cpp

352–355

1. This will capture almost anything that matches -W<warning-flag>. Could you verify that it's indeed -Werror?
2. This is a diagnostics rather than Sema option. We are likely to introduce much more with time. Could you move it to a separate method?

flang/test/Semantics/dosemantics03.f90

1–2 ↗

(On Diff #331009)

Rather than adding the 2nd line, could you update the first line to use %flang_fc1 instead of %f18?

[arnamoy10]

flang/test/Semantics/dosemantics03.f90

1–2 ↗

(On Diff #331009)

I am not modifying this test case, as it would create a dependency on -std=f2018 up-streamed (due to this particular test case having dependency on language standard). I create a new test case, we can come back to it later.

[arnamoy10]

Thanks @awarzynski for the comments.

1. Moving diagnostics options parsing to a separate function
2. Adding a check so that -Wblah does not trigger the treatment of warnings as werrors, only -Werror triggers it
3. Adding a new test case.

[arnamoy10]

rebasing on top of main

[awarzynski]

I see that -Werror changes the behavior of the driver in 5 different places. I would hope to see 5 new tests to verify each case.

flang/lib/Frontend/CompilerInvocation.cpp

353–354

Please update

flang/lib/Frontend/FrontendActions.cpp

67–70

Please add a test for this change.

102–105

Please add a test for this change.

116–119

Please add a test for this change.

273–276

Please add a test for this change.

flang/test/Driver/werror.f90

11–12 ↗

(On Diff #332997)

Does this work for f18?

[arnamoy10]

Modifying the test cases to:

1. Make it work for f18 (when flang-new is not installed)
2. Adding more options and one test case to check correct functionality with PrescanAction and PrescanAndSemaAction

[awarzynski]

I really like how you split your tests into two files:

• werror_scan.f captures warning generated by the prescanner
• werror.f captures warnings from the semantic analysis

In every case you added multiple RUN lines to make sure that the behavior is consistent across multiple actions. I think that that's very useful. Ideally, we'd have one central switch for turning warnings into errors and this would be unnecessary. But we're not there yet. In the meantime, could you add a comment explaining why multiple RUN lines are used?

I have 2 more suggestions:

• add yet another file for warnings from the parser
• rename werror.f as `werror_sema.f

These are non-blockers for me.

flang/lib/Frontend/CompilerInvocation.cpp

360

AFAIK, no other option is allowed in -W<option>. Perhaps throw a diagnostic here?

flang/test/Driver/werror.f90

2 ↗

(On Diff #333843)

[nit] This comment is not directly related to what's in this file (i.e. it doesn't really help understand it, does it)? I would remove it. Same in werror_scan.f.

4–6 ↗

(On Diff #333843)

DELETEME

flang/test/Driver/werror_scan.f

6–8

DELETEME

10–12

This is going to be either flang-new -fc1 or f18. I think that we can start skipping this part of the comment to make it more generic.

[arnamoy10]

1. Separated test cases to check prescan, parse and sema differently.
2. Diagnostics is thrown when anything other that error is given for -W. This behavior is reflected in f18 as well.
3. Minor changes to address other comments.

[arnamoy10]

Adding newline at the end of a test case.

[awarzynski]

LGTM, thank you for addressing my comments!

[crownyanguan]

How about -Wl option, it will cause link error like " Only -Werror is supported currently"

[arnamoy10]

In D98657#2701673, @crownyanguan wrote:

How about -Wl option, it will cause link error like " Only -Werror is supported currently"

Thanks for pointing it out. The issue was due to restricting -W` options in f18 as well, this has been fixed with 4299ab6c5daf

clang/include/clang/Driver/Options.td

def Wp_COMMA : CommaJoined<["-"], "Wp,">,
   HelpText<"Pass the comma separated arguments in <arg> to the preprocessor">,
   MetaVarName<"<arg>">, Group<Preprocessor_Group>;
 def Wundef_prefix_EQ : CommaJoined<["-"], "Wundef-prefix=">, Group<W_value_Group>,
   Flags<[CC1Option, CoreOption, HelpHidden]>, MetaVarName<"<arg>">,
   HelpText<"Enable warnings for undefined macros with a prefix in the comma separated list <arg>">,
   MarshallingInfoStringVector<DiagnosticOpts<"UndefPrefixes">>;
 def Wwrite_strings : Flag<["-"], "Wwrite-strings">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
 def Wno_write_strings : Flag<["-"], "Wno-write-strings">, Group<W_Group>, Flags<[CC1Option, HelpHidden]>;
-def W_Joined : Joined<["-"], "W">, Group<W_Group>, Flags<[CC1Option, CoreOption]>,
+def W_Joined : Joined<["-"], "W">, Group<W_Group>, Flags<[CC1Option, CoreOption, FC1Option, FlangOption]>,
   MetaVarName<"<warning>">, HelpText<"Enable the specified warning">;
 def Xanalyzer : Separate<["-"], "Xanalyzer">,
   HelpText<"Pass <arg> to the static analyzer">, MetaVarName<"<arg>">,
   Group<StaticAnalyzer_Group>;
 def Xarch__ : JoinedAndSeparate<["-"], "Xarch_">, Flags<[NoXarchOption]>;
 def Xarch_host : Separate<["-"], "Xarch_host">, Flags<[NoXarchOption]>,
   HelpText<"Pass <arg> to the CUDA/HIP host compilation">, MetaVarName<"<arg>">;
 def Xarch_device : Separate<["-"], "Xarch_device">, Flags<[NoXarchOption]>,

clang/lib/Driver/ToolChains/Flang.cpp

void Flang::AddPreprocessingOptions(const ArgList &Args,
                                     ArgStringList &CmdArgs) const {
   Args.AddAllArgs(CmdArgs, {options::OPT_D, options::OPT_U, options::OPT_I});
 }
 
 void Flang::AddOtherOptions(const ArgList &Args, ArgStringList &CmdArgs) const {
   Args.AddAllArgs(CmdArgs,
                   {options::OPT_module_dir, options::OPT_fdebug_module_writer,
                    options::OPT_fintrinsic_modules_path, options::OPT_pedantic,
-                   options::OPT_std_EQ});
+                   options::OPT_std_EQ, options::OPT_W_Joined});
 }
 
 void Flang::ConstructJob(Compilation &C, const JobAction &JA,
                          const InputInfo &Output, const InputInfoList &Inputs,
                          const ArgList &Args, const char *LinkingOutput) const {
   const auto &TC = getToolChain();
   // TODO: Once code-generation is available, this will need to be commented
   // out.

flang/include/flang/Frontend/CompilerInvocation.h

 //===- CompilerInvocation.h - Compiler Invocation Helper Data ---*- C -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 #ifndef LLVM_FLANG_FRONTEND_COMPILERINVOCATION_H
 #define LLVM_FLANG_FRONTEND_COMPILERINVOCATION_H
 
 #include "flang/Frontend/FrontendOptions.h"
 #include "flang/Frontend/PreprocessorOptions.h"
 #include "flang/Parser/parsing.h"
 #include "flang/Semantics/semantics.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "llvm/Option/ArgList.h"
 #include <memory>
 
class CompilerInvocation : public CompilerInvocationBase {
 
   /// Semantic options
   // TODO: Merge with or translate to frontendOpts_. We shouldn't need two sets
   // of options.
   std::string moduleDir_ = ".";
 
   bool debugModuleDir_ = false;
 
+  bool warnAsErr_ = false;
+
   // Fortran Dialect options
   Fortran::common::IntrinsicTypeDefaultKinds defaultKinds_;
 
   bool EnableConformanceChecks_ = false;
 
 public:
   CompilerInvocation() = default;
 
public:
   }
 
   std::string &moduleDir() { return moduleDir_; }
   const std::string &moduleDir() const { return moduleDir_; }
 
   bool &debugModuleDir() { return debugModuleDir_; }
   const bool &debugModuleDir() const { return debugModuleDir_; }
 
+  bool &warnAsErr() { return warnAsErr_; }
+  const bool &warnAsErr() const { return warnAsErr_; }
+
   bool &enableConformanceChecks() { return EnableConformanceChecks_; }
   const bool &enableConformanceChecks() const {
     return EnableConformanceChecks_;
   }
 
   Fortran::common::IntrinsicTypeDefaultKinds &defaultKinds() {
     return defaultKinds_;
   }
public:
   // Enables the std=f2018 conformance check
   void set_EnableConformanceChecks() { EnableConformanceChecks_ = true; }
 
   /// Useful setters
   void SetModuleDir(std::string &moduleDir) { moduleDir_ = moduleDir; }
 
   void SetDebugModuleDir(bool flag) { debugModuleDir_ = flag; }
 
+  void SetWarnAsErr(bool flag) { warnAsErr_ = flag; }
+
   /// Set the Fortran options to predifined defaults. These defaults are
   /// consistend with f18/f18.cpp.
   // TODO: We should map frontendOpts_ to parserOpts_ instead. For that, we
   // need to extend frontendOpts_ first. Next, we need to add the corresponding
   // compiler driver options in libclangDriver.
   void SetDefaultFortranOpts();
 
   /// Set the default predefinitions.

flang/lib/Frontend/CompilerInvocation.cpp

 //===- CompilerInvocation.cpp ---------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "flang/Frontend/CompilerInvocation.h"
 #include "flang/Common/Fortran-features.h"
 #include "flang/Frontend/PreprocessorOptions.h"
 #include "flang/Semantics/semantics.h"
 #include "flang/Version.inc"
 #include "clang/Basic/AllDiagnostics.h"
 #include "clang/Basic/DiagnosticDriver.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Driver/DriverDiagnostic.h"
static void parseSemaArgs(CompilerInvocation &res, llvm::opt::ArgList &args,
   if (moduleDirList.size() == 1)
     res.SetModuleDir(moduleDirList[0]);
 
   // -fdebug-module-writer option
   if (args.hasArg(clang::driver::options::OPT_fdebug_module_writer)) {
     res.SetDebugModuleDir(true);
   }
 }
 
+/// Parses all diagnostics related arguments and populates the variables
+/// options accordingly.
+static void parseDiagArgs(CompilerInvocation &res, llvm::opt::ArgList &args,
+    clang::DiagnosticsEngine &diags) {
+  // -Werror option
+  // TODO: Currently throws a Diagnostic for anything other than -W<error>,
+  // this has to change when other -W<opt>'s are supported.
+  if (args.hasArg(clang::driver::options::OPT_W_Joined)) {
+    if (args.getLastArgValue(clang::driver::options::OPT_W_Joined)
+            .equals("error")) {
+      res.SetWarnAsErr(true);
+    } else {
+      const unsigned diagID =
+          diags.getCustomDiagID(clang::DiagnosticsEngine::Error,
+              "Only `-Werror` is supported currently.");
+      diags.Report(diagID);
+    }
+  }
+}
+
 /// Parses all Dialect related arguments and populates the variables
 /// options accordingly.
 static void parseDialectArgs(CompilerInvocation &res, llvm::opt::ArgList &args,
     clang::DiagnosticsEngine &diags) {
 
   // -fdefault* family
   if (args.hasArg(clang::driver::options::OPT_fdefault_real_8)) {
     res.defaultKinds().set_defaultRealKind(8);
bool CompilerInvocation::CreateFromArgs(CompilerInvocation &res,
   // Parse the frontend args
   ParseFrontendArgs(res.frontendOpts(), args, diags);
   // Parse the preprocessor args
   parsePreprocessorArgs(res.preprocessorOpts(), args);
   // Parse semantic args
   parseSemaArgs(res, args, diags);
   // Parse dialect arguments
   parseDialectArgs(res, args, diags);
+  // Parse diagnostic arguments
+  parseDiagArgs(res, args, diags);
 
   return success;
 }
 
 /// Collect the macro definitions provided by the given preprocessor
 /// options into the parser options.
 ///
 /// \param [in] ppOpts The preprocessor options
void CompilerInvocation::setSemanticsOpts(
     Fortran::parser::AllCookedSources &allCookedSources) {
   const auto &fortranOptions = fortranOpts();
 
   semanticsContext_ = std::make_unique<semantics::SemanticsContext>(
       defaultKinds(), fortranOptions.features, allCookedSources);
 
   semanticsContext_->set_moduleDirectory(moduleDir())
       .set_searchDirectories(fortranOptions.searchDirectories)
-      .set_warnOnNonstandardUsage(enableConformanceChecks());
+      .set_warnOnNonstandardUsage(enableConformanceChecks())
+      .set_warningsAreErrors(warnAsErr());
 }

flang/lib/Frontend/FrontendActions.cpp

 //===--- FrontendActions.cpp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "flang/Frontend/FrontendActions.h"
 #include "flang/Common/default-kinds.h"
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/FrontendOptions.h"
 #include "flang/Lower/PFTBuilder.h"
 #include "flang/Parser/dump-parse-tree.h"
 #include "flang/Parser/parsing.h"
 #include "flang/Parser/provenance.h"
 #include "flang/Parser/source.h"
if (ci.invocation().frontendOpts().fortranForm_ == FortranForm::Unknown) {
     // can't decide between fixed and free form based on the file extension
     // earlier than this.
     parserOptions.isFixedForm = currentInput().IsFixedForm();
   }
 
   // Prescan. In case of failure, report and return.
   ci.parsing().Prescan(currentInputPath, parserOptions);
 
-  if (ci.parsing().messages().AnyFatalError()) {
+  if (!ci.parsing().messages().empty() &&
+      (ci.invocation().warnAsErr() ||
+          ci.parsing().messages().AnyFatalError())) {
     const unsigned diagID = ci.diagnostics().getCustomDiagID(
         clang::DiagnosticsEngine::Error, "Could not scan %0");
     ci.diagnostics().Report(diagID) << GetCurrentFileOrBufferName();
     ci.parsing().messages().Emit(llvm::errs(), ci.allCookedSources());
 
     return false;
   }
 
   return true;
if (ci.invocation().frontendOpts().fortranForm_ == FortranForm::Unknown) {
     // can't decide between fixed and free form based on the file extension
     // earlier than this.
     parserOptions.isFixedForm = currentInput().IsFixedForm();
   }
 
   // Prescan. In case of failure, report and return.
   ci.parsing().Prescan(currentInputPath, parserOptions);
 
-  if (ci.parsing().messages().AnyFatalError()) {
+  if (!ci.parsing().messages().empty() &&
+      (ci.invocation().warnAsErr() ||
+          ci.parsing().messages().AnyFatalError())) {
     const unsigned diagID = ci.diagnostics().getCustomDiagID(
         clang::DiagnosticsEngine::Error, "Could not scan %0");
     ci.diagnostics().Report(diagID) << GetCurrentFileOrBufferName();
     ci.parsing().messages().Emit(llvm::errs(), ci.allCookedSources());
 
     return false;
   }
 
   // Parse. In case of failure, report and return.
   ci.parsing().Parse(llvm::outs());
 
-  if (ci.parsing().messages().AnyFatalError()) {
+  if (!ci.parsing().messages().empty() &&
+      (ci.invocation().warnAsErr() ||
+          ci.parsing().messages().AnyFatalError())) {
     unsigned diagID = ci.diagnostics().getCustomDiagID(
         clang::DiagnosticsEngine::Error, "Could not parse %0");
     ci.diagnostics().Report(diagID) << GetCurrentFileOrBufferName();
 
     ci.parsing().messages().Emit(
         llvm::errs(), this->instance().allCookedSources());
     return false;
   }
 

 }
 
 void DebugMeasureParseTreeAction::ExecuteAction() {
   CompilerInstance &ci = this->instance();
 
   // Parse. In case of failure, report and return.
   ci.parsing().Parse(llvm::outs());
 
-  if (ci.parsing().messages().AnyFatalError()) {
+  if (!ci.parsing().messages().empty() &&
+      (ci.invocation().warnAsErr() ||
+          ci.parsing().messages().AnyFatalError())) {
     unsigned diagID = ci.diagnostics().getCustomDiagID(
         clang::DiagnosticsEngine::Error, "Could not parse %0");
     ci.diagnostics().Report(diagID) << GetCurrentFileOrBufferName();
 
     ci.parsing().messages().Emit(
         llvm::errs(), this->instance().allCookedSources());
     return;
   }
 

flang/test/Driver/driver-help-hidden.f90

 ! CHECK-NEXT: -help     Display available options
 ! CHECK-NEXT: -I <dir>               Add directory to the end of the list of include search paths
 ! CHECK-NEXT: -module-dir <dir>      Put MODULE files in <dir>
 ! CHECK-NEXT: -o <file> Write output to <file>
 ! CHECK-NEXT: -pedantic              Warn on language extensions
 ! CHECK-NEXT: -std=<value>           Language standard to compile for
 ! CHECK-NEXT: -U <macro>             Undefine macro <macro>
 ! CHECK-NEXT: --version Print version information
+! CHECK-NEXT: -W<warning>            Enable the specified warning
 ! CHECK-NEXT: -Xflang <arg>          Pass <arg> to the flang compiler
 
 !-------------------------------------------------------------
 ! EXPECTED OUTPUT FOR FLANG DRIVER (flang-new)
 !-------------------------------------------------------------
 ! ERROR-FLANG: error: unknown argument '-help-hidden'; did you mean '--help-hidden'?
 
 !-------------------------------------------------------------
 ! EXPECTED OUTPUT FOR FLANG FRONTEND DRIVER (flang-new -fc1)
 !-------------------------------------------------------------
 ! Frontend driver -help-hidden is not supported
 ! ERROR-FLANG-FC1: error: unknown argument: '{{.*}}'
 

flang/test/Driver/driver-help.f90

 ! HELP-NEXT: -help                  Display available options
 ! HELP-NEXT: -I <dir>               Add directory to the end of the list of include search paths
 ! HELP-NEXT: -module-dir <dir>      Put MODULE files in <dir>
 ! HELP-NEXT: -o <file>              Write output to <file>
 ! HELP-NEXT: -pedantic              Warn on language extensions
 ! HELP-NEXT: -std=<value>           Language standard to compile for
 ! HELP-NEXT: -U <macro>             Undefine macro <macro>
 ! HELP-NEXT: --version              Print version information
+! HELP-NEXT: -W<warning>            Enable the specified warning
 ! HELP-NEXT: -Xflang <arg>          Pass <arg> to the flang compiler
 
 !-------------------------------------------------------------
 ! EXPECTED OUTPUT FOR FLANG FRONTEND DRIVER (flang-new -fc1)
 !-------------------------------------------------------------
 ! HELP-FC1:USAGE: flang-new
 ! HELP-FC1-EMPTY:
 ! HELP-FC1-NEXT:OPTIONS:

 ! HELP-FC1-NEXT: -I <dir>               Add directory to the end of the list of include search paths
 ! HELP-FC1-NEXT: -module-dir <dir>      Put MODULE files in <dir>
 ! HELP-FC1-NEXT: -o <file>              Write output to <file>
 ! HELP-FC1-NEXT: -pedantic              Warn on language extensions
 ! HELP-FC1-NEXT: -std=<value>           Language standard to compile for
 ! HELP-FC1-NEXT: -test-io               Run the InputOuputTest action. Use for development and testing only.
 ! HELP-FC1-NEXT: -U <macro>             Undefine macro <macro>
 ! HELP-FC1-NEXT: --version              Print version information
+! HELP-FC1-NEXT: -W<warning>            Enable the specified warning
 
 !---------------
 ! EXPECTED ERROR
 !---------------
 ! ERROR: error: unknown argument '-helps'; did you mean '-help'

flang/test/Driver/werror_parse.f

+! Ensure argument -Werror work as expected, this file checks for the functional correctness for
+! actions that extend the PrescanAndSemaAction, particularly for Semantic warnings/errors.
+! Multiple RUN lines are added to make sure that the behavior is consistent across multiple actions.
+
+! RUN: not %flang_fc1 -fsyntax-only -std=f2018 -Werror %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -std=f2018 -Werror -fdebug-dump-parse-tree %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -std=f2018 -Werror -fdebug-unparse-with-symbols %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -std=f2018 -Werror -fdebug-unparse %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -std=f2018 -Werror -fdebug-dump-symbols %s  2>&1 | FileCheck %s --check-prefix=WITH
+
+
+! RUN: %flang_fc1 -fsyntax-only -std=f2018 %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -std=f2018 -fdebug-dump-parse-tree %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -std=f2018 -fdebug-unparse-with-symbols %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -std=f2018 -fdebug-unparse %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -std=f2018 -fdebug-dump-symbols %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITH -Werror
+!-----------------------------------------
+! WITH: Could not parse
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITHOUT -Werror
+!-----------------------------------------
+! WITHOUT-NOT: Could not parse
+
+#ifndef _OM_NO_IOSTREAM
+#ifdef WIN32
+#ifndef USE_IOSTREAM
+#define USE_IOSTREAM
+#endif USE_IOSTREAM
+#endif WIN32

flang/test/Driver/werror_scan.f

+! Ensure argument -Werror work as expected, this file checks for the functional correctness for
+! actions that extend the PrescanAction
+! Multiple RUN lines are added to make sure that the behavior is consistent across multiple actions.
+
+! RUN: not %flang_fc1 -fsyntax-only -E -Werror %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -fdebug-dump-parsing-log -Werror %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -fdebug-dump-provenance -Werror %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -fdebug-measure-parse-tree -Werror %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: %flang_fc1 -fsyntax-only -E %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-dump-parsing-log %s  2>&1 | FileCheck %s --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-dump-provenance %s  2>&1 | FileCheck %s --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-measure-parse-tree %s  2>&1 | FileCheck %s --check-prefix=WITHOUT
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITH -Werror
+!-----------------------------------------
+! WITH: Could not scan
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITHOUT -Werror
+!-----------------------------------------
+! WITHOUT-NOT: Could not scan
+
+1 continue
+end

flang/test/Driver/werror_sema.f90

+! Ensure argument -Werror work as expected, this file checks for the functional correctness for
+! actions that extend the PrescanAndSemaAction, particularly for Semantic warnings/errors.
+! Multiple RUN lines are added to make sure that the behavior is consistent across multiple actions.
+
+! RUN: not %flang_fc1 -fsyntax-only -Werror %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -Werror -fdebug-dump-parse-tree %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -Werror -fdebug-unparse-with-symbols %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -Werror -fdebug-unparse %s  2>&1 | FileCheck %s --check-prefix=WITH
+! RUN: not %flang_fc1 -fsyntax-only -Werror -fdebug-dump-symbols %s  2>&1 | FileCheck %s --check-prefix=WITH
+
+
+! RUN: %flang_fc1 -fsyntax-only %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-dump-parse-tree %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-unparse-with-symbols %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-unparse %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+! RUN: %flang_fc1 -fsyntax-only -fdebug-dump-symbols %s  2>&1 | FileCheck %s --allow-empty --check-prefix=WITHOUT
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITH -Werror
+!-----------------------------------------
+! WITH: Semantic errors in
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITHOUT -Werror
+!-----------------------------------------
+! WITHOUT-NOT: Semantic errors in
+
+PROGRAM werror
+REAL, DIMENSION(20, 10) :: A
+FORALL (J=1:N)  A(I, I) = 1
+END PROGRAM werror

flang/test/Driver/werror_wrong.f90

+! Ensure that only argument -Werror is supported.
+
+! RUN: not %flang_fc1 -fsyntax-only -Wall %s  2>&1 | FileCheck %s --check-prefix=WRONG
+! RUN: not %flang_fc1 -fsyntax-only -WX %s  2>&1 | FileCheck %s --check-prefix=WRONG
+
+!-----------------------------------------
+! EXPECTED OUTPUT WITH -W<opt>
+!-----------------------------------------
+! WRONG: Only `-Werror` is supported currently.
 No newline at end of file

flang/tools/f18/f18.cpp

if (arg.empty()) {
         arg == "-std=f2018" || arg == "-pedantic") {
       driver.warnOnNonstandardUsage = true;
     } else if (arg == "-fopenacc") {
       options.features.Enable(Fortran::common::LanguageFeature::OpenACC);
       predefinitions.emplace_back("_OPENACC", "202011");
     } else if (arg == "-fopenmp") {
       options.features.Enable(Fortran::common::LanguageFeature::OpenMP);
       predefinitions.emplace_back("_OPENMP", "201511");
-    } else if (arg == "-Werror") {
+    } else if (arg.find("-W") != std::string::npos) {
+      if (arg == "-Werror")
         driver.warningsAreErrors = true;
+      else {
+        // Only -Werror is supported currently
+        llvm::errs() << "Only `-Werror` is supported currently.\n";
+        return EXIT_FAILURE;
+      }
     } else if (arg == "-ed") {
       options.features.Enable(Fortran::common::LanguageFeature::OldDebugLines);
     } else if (arg == "-E") {
       driver.dumpCookedChars = true;
     } else if (arg == "-fbackslash" || arg == "-fno-backslash") {
       options.features.Enable(
           Fortran::common::LanguageFeature::BackslashEscapes,
           arg == "-fbackslash");