Exit with error when no checks enabled.

[alexfh]

Summary

This seems like a more appropriate reaction to the user specifying a
single check with a wrong name, for example.

[klimek]

LG

clang-tidy/tool/ClangTidyMain.cpp

int main(int argc, const char **argv) {
   Options.AnalyzeTemporaryDtors = AnalyzeTemporaryDtors;
   if (llvm::error_code Err =
           clang::tidy::parseLineFilter(LineFilter, Options)) {
     llvm::errs() << "Invalid LineFilter: " << Err.message() << "\n\nUsage:\n";
     llvm::cl::PrintHelpMessage(/*Hidden=*/false, /*Categorized=*/true);
     return 1;
   }
 
+  std::vector<std::string> EnabledChecks = clang::tidy::getCheckNames(Options);
+
   // FIXME: Allow using --list-checks without positional arguments.
   if (ListChecks) {
     llvm::outs() << "Enabled checks:";
-    for (auto CheckName : clang::tidy::getCheckNames(Options))
+    for (auto CheckName : EnabledChecks)
       llvm::outs() << "\n    " << CheckName;
     llvm::outs() << "\n\n";
     return 0;
   }
 
+  if (EnabledChecks.empty()) {
+    llvm::errs() << "Error: no checks enabled.\n";
+    llvm::cl::PrintHelpMessage(/*Hidden=*/false, /*Categorized=*/true);
+    return 1;
+  }
+
   std::vector<clang::tidy::ClangTidyError> Errors;
   clang::tidy::ClangTidyStats Stats =
       clang::tidy::runClangTidy(Options, OptionsParser.getCompilations(),
                                 OptionsParser.getSourcePathList(), &Errors);
   clang::tidy::handleErrors(Errors, Fix);
 
   printStats(Stats);
   return 0;

test/clang-tidy/diagnostic.cpp

 // RUN: clang-tidy %s.nonexistent.cpp -- | FileCheck -check-prefix=CHECK1 %s
 // RUN: clang-tidy -checks='google-explicit-constructor' %s -- -fan-unknown-option | FileCheck -check-prefix=CHECK2 %s
 // RUN: clang-tidy -checks='-*,google-explicit-constructor,clang-diagnostic-literal-conversion' %s -- -fan-unknown-option | FileCheck -check-prefix=CHECK3 %s
-// RUN: clang-tidy -checks='-*,clang-diagnostic-macro-redefined' %s -- -DMACRO_FROM_COMMAND_LINE | FileCheck -check-prefix=CHECK4 %s
+// RUN: clang-tidy -checks='-*,misc-use-override,clang-diagnostic-macro-redefined' %s -- -DMACRO_FROM_COMMAND_LINE | FileCheck -check-prefix=CHECK4 %s
 
 // CHECK1-NOT: warning
 // CHECK2-NOT: warning
 // CHECK3-NOT: warning
 
 // CHECK1: warning: error reading '{{.*}}.nonexistent.cpp'
 // CHECK2: warning: unknown argument: '-fan-unknown-option'
 

test/clang-tidy/select-checks.cpp

 // RUN: grep -Ev "// *[A-Z-]+:" %s > %t.cpp
 // RUN: clang-tidy %t.cpp -fix -checks='-*,llvm-*' --
 // RUN: FileCheck -input-file=%t.cpp %s
+// RUN: clang-tidy %s -checks='-*,an-unknown-check' -- 2>&1 | FileCheck -check-prefix=CHECK2 %s
+
+// CHECK2: Error: no checks enabled.
 
 namespace i {
 }
 // CHECK: } // namespace i
 
 class A { A(int i); }; // Not fixing this, because the check is in google-.
 // CHECK: class A { A(int i); };