[clang-tidy] Sanity checks in ClangTidyTest header.

[njames93]

Summary

Motivated by a suspicously failing build, but also good to have anyway in general.

[aaron.ballman]

LGTM (the harbormaster failure looks unrelated to your changes from what I can tell)

clang-tools-extra/unittests/clang-tidy/ClangTidyTest.h

 #include "ClangTidyDiagnosticConsumer.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Tooling/Core/Diagnostic.h"
 #include "clang/Tooling/Core/Replacement.h"
 #include "clang/Tooling/Refactoring.h"
 #include "clang/Tooling/Tooling.h"
-#include "llvm/ADT/Optional.h"
 #include "llvm/Support/Path.h"
 #include <map>
 #include <memory>
 
 namespace clang {
 namespace tidy {
 namespace test {
 
std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &Compiler,
     Context.setASTContext(&Compiler.getASTContext());
 
     Preprocessor *PP = &Compiler.getPreprocessor();
 
     // Checks must be created here, _after_ `Context` has been initialized, so
     // that check constructors can access the context (for example, through
     // `getLangOpts()`).
     CheckFactory<CheckTypes...>::createChecks(&Context, Checks);
+    assert(!Checks.empty() && "No checks created");
     for (auto &Check : Checks) {
+      assert(Check.get() && "Checks can't be null");
       if (!Check->isLanguageVersionSupported(Context.getLangOpts()))
         continue;
       Check->registerMatchers(&Finder);
       Check->registerPPCallbacks(Compiler.getSourceManager(), PP, PP);
     }
     return Finder.newASTConsumer();
   }
 
   SmallVectorImpl<std::unique_ptr<ClangTidyCheck>> &Checks;
   ast_matchers::MatchFinder &Finder;
   ClangTidyContext &Context;
 };
 
 template <typename... CheckTypes>
 std::string
 runCheckOnCode(StringRef Code, std::vector<ClangTidyError> *Errors = nullptr,
                const Twine &Filename = "input.cc",
                ArrayRef<std::string> ExtraArgs = None,
                const ClangTidyOptions &ExtraOptions = ClangTidyOptions(),
                std::map<StringRef, StringRef> PathsToContent =
                    std::map<StringRef, StringRef>()) {
+  static_assert(sizeof...(CheckTypes) > 0, "No checks specified");
   ClangTidyOptions Options = ExtraOptions;
   Options.Checks = "*";
   ClangTidyContext Context(std::make_unique<DefaultOptionsProvider>(
       ClangTidyGlobalOptions(), Options));
   ClangTidyDiagnosticConsumer DiagConsumer(Context);
   DiagnosticsEngine DE(new DiagnosticIDs(), new DiagnosticOptions,
                        &DiagConsumer, false);
   Context.setDiagnosticsEngine(&DE);
runCheckOnCode(StringRef Code, std::vector<ClangTidyError> *Errors = nullptr,
   Args.push_back(Filename.str());
 
   ast_matchers::MatchFinder Finder;
   llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
       new llvm::vfs::InMemoryFileSystem);
   llvm::IntrusiveRefCntPtr<FileManager> Files(
       new FileManager(FileSystemOptions(), InMemoryFileSystem));
 
-  SmallVector<std::unique_ptr<ClangTidyCheck>, 1> Checks;
+  SmallVector<std::unique_ptr<ClangTidyCheck>, sizeof...(CheckTypes)> Checks;
   tooling::ToolInvocation Invocation(
       Args,
       std::make_unique<TestClangTidyAction<CheckTypes...>>(Checks, Finder,
                                                            Context),
       Files.get());
   InMemoryFileSystem->addFile(Filename, 0,
                               llvm::MemoryBuffer::getMemBuffer(Code));
   for (const auto &FileContent : PathsToContent) {