diff --git a/clang/docs/checkedc/3C/CONTRIBUTING.md b/clang/docs/checkedc/3C/CONTRIBUTING.md
index a4e330e7a33e..8443c201b5b9 100644
--- a/clang/docs/checkedc/3C/CONTRIBUTING.md
+++ b/clang/docs/checkedc/3C/CONTRIBUTING.md
@@ -94,6 +94,58 @@ in subdirectories, so `*.c` will not pick up all of them; instead you
 can use `llvm-lit -vv .` to specify all test files under the current
 directory.
 
+### Diagnostic verification
+
+3C supports the standard Clang diagnostic verifier
+([`VerifyDiagnosticConsumer`](https://clang.llvm.org/doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details))
+for testing errors and warnings reported by 3C via its main `DiagnosticsEngine`.
+(Some 3C errors and warnings are reported via other means and cannot be tested
+this way; the best solution we have for them right now is to `grep` the stderr
+of 3C.) Diagnostic verification can be enabled via the usual `-Xclang -verify`
+compiler option; other diagnostic verification options (`-Xclang
+-verify=PREFIX`, etc.) should also work as normal. These must be passed as
+_compiler_ options, not `3c` options; for example, if you are using `--` on the
+`3c` command line, these options must be passed _after_ the `--`.
+
+Some notes about diagnostic verification in the context of 3C:
+
+* Parsing of the source files uses some of the compiler logic and thus may
+  generate compiler warnings, just as if you ran `clang` on the code. These are
+  sent to the diagnostic verifier along with diagnostics generated by 3C's
+  analysis. If you find it distracting to have to include the compiler warnings
+  in the set of expected diagnostics for a test, you can turn them off via the
+  `-Wno-everything` compiler option (which does not affect diagnostics generated
+  by 3C's analysis).
+
+* The `3c` tool works in several passes, where each pass runs on all translation
+  units: first `3c` parses the source files, then it runs several passes of
+  analysis. If a pass encounters at least one error, `3c` exits at the end of
+  that pass. Diagnostic verification does not change the _point_ at which `3c`
+  exits, but it changes the exit _code_ to indicate the result of verification
+  rather than the presence of errors. The verification includes the diagnostics
+  from all passes up to the point at which `3c` exits (i.e., the same
+  diagnostics that would be displayed if verification were not used). However,
+  an error that doesn't go via the main `DiagnosticsEngine` will cause an
+  unsuccessful exit code regardless of diagnostic verification. (This is
+  typically the behavior you want for a test.)
+
+* Diagnostic verification is independent for each translation unit, so in tests
+  with multiple translation units, you'll have to be careful that preprocessing
+  of each translation unit sees the correct set of `expected-*` directives for
+  the diagnostics generated for that translation unit (or an
+  `expected-no-diagnostics` directive if that translation unit generates no
+  diagnostics, even if other translation units do generate diagnostics). Be
+  warned that since which translation unit generated a given diagnostic isn't
+  visible to a normal user, we don't put much work into coming up with sensible
+  rules for this, but it should at least be deterministic for testing.
+
+Note that some 3C tests use diagnostic verification on calls to `clang` rather
+than `3c`, so if you see `expected-*` directives in a test, you can look at the
+`RUN` commands to see which command has `-Xclang -verify` and is using the
+directives. If you want to verify diagnostics of more than one `RUN` command in
+the same test, you can use different directive prefixes (`-Xclang
+-verify=PREFIX`).
+
 ## Coding guidelines
 
 Please follow [LLVM coding
diff --git a/clang/include/clang/3C/3C.h b/clang/include/clang/3C/3C.h
index a3dfabcf158b..76f9d7679889 100644
--- a/clang/include/clang/3C/3C.h
+++ b/clang/include/clang/3C/3C.h
@@ -57,7 +57,7 @@ struct _3COptions {
 
   bool AddCheckedRegions;
 
-  bool DisableCCTypeChecker;
+  bool EnableCCTypeChecker;
 
   bool WarnRootCause;
 
@@ -68,11 +68,6 @@ struct _3COptions {
   bool ForceItypes;
 #endif
 
-  // Currently applies only to the rewriting phase (because it is the only phase
-  // that generates diagnostics, except for the declaration merging diagnostics
-  // that are currently fatal) and uses the default "expected" prefix.
-  bool VerifyDiagnosticOutput;
-
   bool DumpUnwritableChanges;
   bool AllowUnwritableChanges;
 
@@ -104,6 +99,8 @@ class _3CInterface {
          const std::vector<std::string> &SourceFileList,
          clang::tooling::CompilationDatabase *CompDB);
 
+  ~_3CInterface();
+
   // Call clang to provide the data
   bool parseASTs();
 
@@ -141,10 +138,25 @@ class _3CInterface {
   // Dump all stats related to performance.
   bool dumpStats();
 
+  // Determine the exit code that the `3c` tool should exit with after the last
+  // 3C stage, considering diagnostic verification. Must be called exactly once
+  // before the _3CInterface is destructed (unless construction failed).
+  int determineExitCode();
+
 private:
   _3CInterface(const struct _3COptions &CCopt,
                const std::vector<std::string> &SourceFileList,
-               clang::tooling::CompilationDatabase *CompDB, bool &Failed);
+               clang::tooling::CompilationDatabase *CompDB);
+
+  bool ConstructionFailed = false;
+  bool DeterminedExitCode = false;
+
+  bool HadNonDiagnosticError = false;
+
+  // Determine whether 3C can continue to the next stage of processing. Checks
+  // HadNonDiagnosticError and error diagnostics but ignores diagnostic
+  // verification.
+  bool isSuccessfulSoFar();
 
   // saved ASTs
   std::vector< std::unique_ptr< ASTUnit >> ASTs;
diff --git a/clang/lib/3C/3C.cpp b/clang/lib/3C/3C.cpp
index c361983389e4..35a0beb2b86d 100644
--- a/clang/lib/3C/3C.cpp
+++ b/clang/lib/3C/3C.cpp
@@ -15,6 +15,7 @@
 #include "clang/3C/ConstraintBuilder.h"
 #include "clang/3C/IntermediateToolHook.h"
 #include "clang/3C/RewriteUtils.h"
+#include "clang/Frontend/VerifyDiagnosticConsumer.h"
 #include "clang/Tooling/ArgumentsAdjusters.h"
 #include "llvm/Support/TargetSelect.h"
 
@@ -51,11 +52,10 @@ std::string PerWildPtrInfoJson;
 bool AllTypes;
 std::string BaseDir;
 bool AddCheckedRegions;
-bool DisableCCTypeChecker;
+bool EnableCCTypeChecker;
 bool WarnRootCause;
 bool WarnAllRootCause;
 std::set<std::string> FilePaths;
-bool VerifyDiagnosticOutput;
 bool DumpUnwritableChanges;
 bool AllowUnwritableChanges;
 bool AllowRewriteFailures;
@@ -68,59 +68,217 @@ bool ForceItypes;
 static CompilationDatabase *CurrCompDB = nullptr;
 static tooling::CommandLineArguments SourceFiles;
 
-ArgumentsAdjuster getIgnoreCheckedPointerAdjuster() {
-  return [](const CommandLineArguments &Args, StringRef /*unused*/) {
-    CommandLineArguments AdjustedArgs;
-    bool HasAdjuster = false;
-    for (size_t I = 0, E = Args.size(); I < E; ++I) {
-      StringRef Arg = Args[I];
-      AdjustedArgs.push_back(Args[I]);
-      if (Arg == "-f3c-tool") {
-        HasAdjuster = true;
-        break;
+// _3CDiagnosticConsumer is a wrapper DiagnosticConsumer that delays the
+// EndSourceFile callback until 3C's analysis is complete, making it possible to
+// use VerifyDiagnosticConsumer to verify diagnostics generated by 3C.
+//
+// Normally, the AST parsing process calls BeginSourceFile, passes `expected-*`
+// directives and generated diagnostics to VerifyDiagnosticConsumer as they
+// occur, and then calls EndSourceFile, which triggers the verification. But 3C
+// does not perform its analysis (and generate diagnostics) until after AST
+// parsing, so we need to delay the EndSourceFile callback until 3C is done
+// generating diagnostics. The alternative of putting another
+// BeginSourceFile/EndSourceFile pair around 3C's diagnostics would be difficult
+// because VerifyDiagnosticConsumer would expect us to pass the `expected-*`
+// directives again.
+//
+// _3CDiagnosticConsumer serves one additional purpose: its NumErrors field
+// counts the errors originally generated by the compiler or 3C so that 3C can
+// stop after a stage that generates an error regardless of whether the error
+// was "expected", since it might not be safe to continue the analysis after an
+// error. (In contrast, NumErrors of a VerifyDiagnosticConsumer gives the status
+// of diagnostic verification and will be 0 until verification is completed.) It
+// may work to use DiagnosticsEngine::hasErrorOccurred for this instead.
+//
+// Currently, diagnostic verification is per translation unit. In our few
+// multi-translation-unit tests, ensuring that the `expected-*` directives are
+// in the right translation units can sometimes be a hassle. But it's unclear
+// whether it would be feasible to share all the necessary objects across
+// translation units to perform a single verification.
+//
+// _3CDiagnosticConsumer can also wrap an ordinary DiagnosticConsumer when we
+// aren't verifying diagnostics. In that case, diagnostics with source locations
+// still need to be generated in a BeginSourceFile/EndSourceFile interval, so
+// delaying the EndSourceFile (as we do) is OK, and starting a new
+// BeginSourceFile/EndSourceFile interval after AST parsing would also be OK.
+// _3CDiagnosticConsumer doesn't actually need to know whether diagnostic
+// verification is enabled, because the underlying DiagnosticConsumer's
+// getNumErrors() gives us the correct post-verification status either way.
+//
+// See clang/docs/checkedc/3C/clang-tidy.md#_3c-name-prefix
+// NOLINTNEXTLINE(readability-identifier-naming)
+class _3CDiagnosticConsumer : public DiagnosticConsumer {
+  std::unique_ptr<DiagnosticConsumer> UnderlyingConsumer;
+
+  // In general, a DiagnosticConsumer can be used for more than one source file,
+  // but we use _3CDiagnosticConsumer for only one file, so we can use this
+  // simple state machine.
+  enum State { S_Startup, S_ASTParsing, S_3CAnalysis, S_Done };
+  State CurrentState = S_Startup;
+
+public:
+  _3CDiagnosticConsumer(DiagnosticsEngine &Engine)
+      : UnderlyingConsumer(Engine.takeClient()) {
+    // This code currently only supports the default LibTooling setup in which
+    // the ClangTool has no global DiagnosticConsumer, so
+    // CompilerInstance::createDiagnostics creates one owned by the
+    // DiagnosticsEngine. If we needed to support the case in which the
+    // DiagnosticConsumer isn't owned, we could use an "UnderlyingConsumerOwner"
+    // pattern like VerifyDiagnosticConsumer does.
+    assert(UnderlyingConsumer);
+  }
+
+  void BeginSourceFile(const LangOptions &LangOpts,
+                       const Preprocessor *PP) override {
+    assert(CurrentState == S_Startup);
+    CurrentState = S_ASTParsing;
+    UnderlyingConsumer->BeginSourceFile(LangOpts, PP);
+  }
+
+  void EndSourceFile() override {
+    assert(CurrentState == S_ASTParsing);
+    CurrentState = S_3CAnalysis;
+    // Delay forwarding the EndSourceFile callback.
+  }
+
+  void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                        const Diagnostic &Info) override {
+    // Count the originally generated diagnostics.
+    DiagnosticConsumer::HandleDiagnostic(DiagLevel, Info);
+    UnderlyingConsumer->HandleDiagnostic(DiagLevel, Info);
+  }
+
+  // Returns true if this translation unit was successful after diagnostic
+  // verification, i.e., verification is enabled and it succeeded, or
+  // verification is disabled and there were no errors. The caller must check
+  // _3CInterface::HadNonDiagnosticError separately.
+  bool finish3CAnalysis() {
+    assert(CurrentState == S_3CAnalysis);
+    CurrentState = S_Done;
+    UnderlyingConsumer->EndSourceFile();
+    return UnderlyingConsumer->getNumErrors() == 0;
+  }
+
+  ~_3CDiagnosticConsumer() {
+    // We considered asserting that the state is S_Done here, but if
+    // ASTUnit::LoadFromCompilerInvocation fails and returns null, the
+    // _3CDiagnosticConsumer may be destructed without reaching S_Done. However,
+    // even without such an assertion, we're still well protected against
+    // forgetting to finish verification and missing an error because if the
+    // ASTUnit is null, _3CInterface::parseASTs will record a "non-diagnostic
+    // error", while if the ASTUnit is non-null, it will get added to
+    // _3CInterface::ASTs and determineExitCode will call finish3CAnalysis. (And
+    // the _3CInterface destructor enforces that determineExitCode is called.)
+  }
+};
+
+// Based on LibTooling's ASTBuilderAction but does several custom things that we
+// need.
+//
+// See clang/docs/checkedc/3C/clang-tidy.md#_3c-name-prefix
+// NOLINTNEXTLINE(readability-identifier-naming)
+class _3CASTBuilderAction : public ToolAction {
+  std::vector<std::unique_ptr<ASTUnit>> &ASTs;
+
+public:
+  _3CASTBuilderAction(std::vector<std::unique_ptr<ASTUnit>> &ASTs)
+      : ASTs(ASTs) {}
+
+  bool runInvocation(std::shared_ptr<CompilerInvocation> Invocation,
+                     FileManager *Files,
+                     std::shared_ptr<PCHContainerOperations> PCHContainerOps,
+                     DiagnosticConsumer *DiagConsumer) override {
+
+    // Adjust some compiler options. This is similar to what we could do with
+    // a LibTooling ArgumentsAdjuster, but we access the options in their parsed
+    // data structure rather than as strings, so it is much more robust.
+
+    if (!EnableCCTypeChecker)
+      // Corresponds to the -f3c-tool compiler option.
+      Invocation->LangOpts->_3C = true;
+
+    // Re-canonicalize the path of the main source file, in case it was
+    // overridden by the compilation database after it was originally
+    // canonicalized by the _3CInterface constructor. This completely fixes
+    // https://github.com/correctcomputation/checkedc-clang/issues/515 and
+    // https://github.com/correctcomputation/checkedc-clang/issues/604 for the
+    // main source file.
+    //
+    // If https://github.com/correctcomputation/checkedc-clang/issues/604 were
+    // fixed another way, then making the path absolute would be sufficient to
+    // fix https://github.com/correctcomputation/checkedc-clang/issues/515; the
+    // path wouldn't need to be canonical.
+    SmallVectorImpl<FrontendInputFile> &Inputs =
+        Invocation->getFrontendOpts().Inputs;
+    for (FrontendInputFile *Iter = Inputs.begin(); Iter < Inputs.end();
+         Iter++) {
+      FrontendInputFile &OldInput = *Iter;
+      // getFile will assert that the input is a file, which should be true for
+      // 3C.
+      std::string OldPath = std::string(OldInput.getFile()), NewPath;
+      std::error_code EC = tryGetCanonicalFilePath(OldPath, NewPath);
+      if (EC) {
+        // If the compilation database specifies a bogus, inaccessible file,
+        // that will normally be caught by Driver::DiagnoseInputExistence before
+        // we get here.
+        errs() << "3C error: Failed to re-canonicalize source file path "
+               << OldPath << " during compiler invocation: "
+               << EC.message() << "\n";
+        return false;
       }
+      *Iter =
+          FrontendInputFile(NewPath, OldInput.getKind(), OldInput.isSystem());
     }
-    if (!DisableCCTypeChecker && !HasAdjuster)
-      AdjustedArgs.push_back("-f3c-tool");
-    return AdjustedArgs;
-  };
-}
-ArgumentsAdjuster addVerifyAdjuster() {
-  return [](const CommandLineArguments &Args, StringRef /*unused*/) {
-    CommandLineArguments AdjustedArgs(Args);
-    if (std::find(AdjustedArgs.begin(),AdjustedArgs.end(),"-verify")
-        == AdjustedArgs.end()) {
-      AdjustedArgs.push_back("-Xclang");
-      AdjustedArgs.push_back("-verify");
-    }
-    return AdjustedArgs;
-  };
-}
-ArgumentsAdjuster canonicalizeSourceFilePathAdjuster() {
-  return [](const CommandLineArguments &Args, StringRef FileName) {
-    std::string AbsoluteFp;
-    std::error_code EC = tryGetCanonicalFilePath(std::string(FileName), AbsoluteFp);
-    assert(!EC && "We canonicalized the source file path during 3C "
-                  "initialization; doing it again in "
-                  "canonicalizeSourceFilePathAdjuster shouldn't fail.");
-    CommandLineArguments AdjustedArgs;
-    for (auto Arg : Args) {
-      if (Arg == FileName) {
-        // XXX We are assuming that any occurrence of the source file path as an
-        // argument should be replaced with the canonical file path. It might be
-        // safer to only match the argument that represents the input file.
-        // Unfortunately, it's hard to do here short of reimplementing compiler
-        // command line parsing. It would be better to adjust the parsed option
-        // data structures instead
-        // (https://github.com/correctcomputation/checkedc-clang/issues/536).
-        AdjustedArgs.push_back(AbsoluteFp);
+
+    // Canonicalize -I paths to address the same two issues for `#include`d
+    // files. The situation is analogous except that this is not a complete fix
+    // for https://github.com/correctcomputation/checkedc-clang/issues/604
+    // because the portion of the path in the `#include` directive may be
+    // non-canonical.
+    HeaderSearchOptions &HeaderOpts = *Invocation->HeaderSearchOpts;
+    std::vector<HeaderSearchOptions::Entry> NewUserEntries;
+    for (auto It = HeaderOpts.UserEntries.begin();
+         It != HeaderOpts.UserEntries.end();) {
+      HeaderSearchOptions::Entry &Entry = *It;
+      std::string OldPath = Entry.Path, NewPath;
+      std::error_code EC = tryGetCanonicalFilePath(OldPath, NewPath);
+      if (EC) {
+        // Normally, if an -I directory isn't accessible, Clang seems to ignore
+        // it with no diagnostic. Hopefully, removing it from the list here will
+        // have the same effect and not break anything. If we kept the
+        // non-canonical entry, we might benefit from any diagnostic that Clang
+        // might issue in the future, but it's harder to reason about whether
+        // that might re-expose the original bugs.
+        It = HeaderOpts.UserEntries.erase(It);
       } else {
-        AdjustedArgs.push_back(Arg);
+        Entry.Path = NewPath;
+        It++;
       }
     }
-    return AdjustedArgs;
-  };
-}
+
+    // Set up diagnostics.
+    IntrusiveRefCntPtr<DiagnosticsEngine> DiagEngine =
+        CompilerInstance::createDiagnostics(&Invocation->getDiagnosticOpts(),
+                                            DiagConsumer,
+                                            /*ShouldOwnClient=*/false);
+    // The _3CDiagnosticConsumer takes ownership of and wraps the engine's
+    // previous DiagnosticConsumer, i.e., the one created by
+    // CompilerInstance::createDiagnostics above (which will be a
+    // VerifyDiagnosticConsumer if requested via the options).
+    DiagEngine->setClient(new _3CDiagnosticConsumer(*DiagEngine));
+
+    // Finally, actually build the AST. This part is the same as in
+    // ASTBuilderAction::runInvocation.
+
+    std::unique_ptr<ASTUnit> AST = ASTUnit::LoadFromCompilerInvocation(
+        Invocation, std::move(PCHContainerOps), DiagEngine, Files);
+    if (!AST)
+      return false;
+
+    ASTs.push_back(std::move(AST));
+    return true;
+  }
+};
 
 void dumpConstraintOutputJson(const std::string &PostfixStr,
                               ProgramInfo &Info) {
@@ -161,12 +319,11 @@ std::unique_ptr<_3CInterface>
 _3CInterface::create(const struct _3COptions &CCopt,
                      const std::vector<std::string> &SourceFileList,
                      CompilationDatabase *CompDB) {
-  bool Failed = false;
   // See clang/docs/checkedc/3C/clang-tidy.md#_3c-name-prefix
   // NOLINTNEXTLINE(readability-identifier-naming)
   std::unique_ptr<_3CInterface> _3CInter(
-      new _3CInterface(CCopt, SourceFileList, CompDB, Failed));
-  if (Failed) {
+      new _3CInterface(CCopt, SourceFileList, CompDB));
+  if (_3CInter->ConstructionFailed) {
     return nullptr;
   }
   return _3CInter;
@@ -174,7 +331,7 @@ _3CInterface::create(const struct _3COptions &CCopt,
 
 _3CInterface::_3CInterface(const struct _3COptions &CCopt,
                            const std::vector<std::string> &SourceFileList,
-                           CompilationDatabase *CompDB, bool &Failed) {
+                           CompilationDatabase *CompDB) {
 
   DumpIntermediate = CCopt.DumpIntermediate;
   Verbose = CCopt.Verbose;
@@ -190,11 +347,10 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
   BaseDir = CCopt.BaseDir;
   AllTypes = CCopt.EnableAllTypes;
   AddCheckedRegions = CCopt.AddCheckedRegions;
-  DisableCCTypeChecker = CCopt.DisableCCTypeChecker;
+  EnableCCTypeChecker = CCopt.EnableCCTypeChecker;
   AllocatorFunctions = CCopt.AllocatorFunctions;
   WarnRootCause = CCopt.WarnRootCause || CCopt.WarnAllRootCause;
   WarnAllRootCause = CCopt.WarnAllRootCause;
-  VerifyDiagnosticOutput = CCopt.VerifyDiagnosticOutput;
   DumpUnwritableChanges = CCopt.DumpUnwritableChanges;
   AllowUnwritableChanges = CCopt.AllowUnwritableChanges;
   AllowRewriteFailures = CCopt.AllowRewriteFailures;
@@ -211,23 +367,16 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
 
   ConstraintsBuilt = false;
 
-  if (VerifyDiagnosticOutput) {
-    errs() << "3C initialization error: Diagnostic verification is currently "
-              "unsupported.\n";
-    Failed = true;
-    return;
-  }
-
   if (OutputPostfix != "-" && !OutputDir.empty()) {
     errs() << "3C initialization error: Cannot use both -output-postfix and "
               "-output-dir\n";
-    Failed = true;
+    ConstructionFailed = true;
     return;
   }
   if (OutputPostfix == "-" && OutputDir.empty() && SourceFileList.size() > 1) {
     errs() << "3C initialization error: Cannot specify more than one input "
               "file when output is to stdout\n";
-    Failed = true;
+    ConstructionFailed = true;
     return;
   }
 
@@ -244,7 +393,7 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
   if (EC) {
     errs() << "3C initialization error: Failed to canonicalize base directory "
            << "\"" << BaseDir << "\": " << EC.message() << "\n";
-    Failed = true;
+    ConstructionFailed = true;
     return;
   }
   BaseDir = TmpPath;
@@ -256,7 +405,7 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
     if (EC) {
       errs() << "3C initialization error: Failed to create output directory \""
              << OutputDir << "\": " << EC.message() << "\n";
-      Failed = true;
+      ConstructionFailed = true;
       return;
     }
     TmpPath = OutputDir;
@@ -264,7 +413,7 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
     if (EC) {
       errs() << "3C initialization error: Failed to canonicalize output "
              << "directory \"" << OutputDir << "\": " << EC.message() << "\n";
-      Failed = true;
+      ConstructionFailed = true;
       return;
     }
     OutputDir = TmpPath;
@@ -279,7 +428,7 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
     if (EC) {
       errs() << "3C initialization error: Failed to canonicalize source file "
              << "path \"" << S << "\": " << EC.message() << "\n";
-      Failed = true;
+      ConstructionFailed = true;
       continue;
     }
     FilePaths.insert(AbsPath);
@@ -297,12 +446,12 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
     errs() << "The base directory is currently \"" << BaseDir
            << "\" and can be changed with the -base-dir option.\n";
     if (OutputDir != "") {
-      Failed = true;
+      ConstructionFailed = true;
       errs() << "When using -output-dir, input files outside the base "
                 "directory cannot be handled because there is no way to "
                 "compute their output paths.\n";
     } else if (!CCopt.AllowSourcesOutsideBaseDir) {
-      Failed = true;
+      ConstructionFailed = true;
       errs() << "You can use the -allow-sources-outside-base-dir option to "
                 "temporarily downgrade this error to a warning.\n";
     }
@@ -313,39 +462,54 @@ _3CInterface::_3CInterface(const struct _3COptions &CCopt,
   GlobalProgramInfo.getPerfStats().startTotalTime();
 }
 
+_3CInterface::~_3CInterface() {
+  assert(ConstructionFailed || DeterminedExitCode);
+}
+
+bool _3CInterface::isSuccessfulSoFar() {
+  if (HadNonDiagnosticError)
+    return false;
+  for (auto &TU : ASTs)
+    if (TU->getDiagnostics().getClient()->getNumErrors() > 0)
+      return false;
+  return true;
+}
+
+int _3CInterface::determineExitCode() {
+  assert(!DeterminedExitCode);
+  DeterminedExitCode = true;
+
+  bool Success = isSuccessfulSoFar();
+
+  bool SuccessAfterDiagnosticVerification = !HadNonDiagnosticError;
+  for (auto &TU : ASTs)
+    SuccessAfterDiagnosticVerification &=
+        ((_3CDiagnosticConsumer *)TU->getDiagnostics().getClient())
+            ->finish3CAnalysis();
+
+  if (!Success && SuccessAfterDiagnosticVerification)
+    // In this case, the `3c` tool will typically just have printed a "Failure
+    // occurred while X... Exiting" message. If we let that be the last word, it
+    // could confuse the user.
+    llvm::errs() << "(Note: Exiting successfully because the failure was due "
+                    "solely to expected error diagnostics and diagnostic "
+                    "verification succeeded.)\n";
+
+  return (SuccessAfterDiagnosticVerification ? 0 : 1);
+}
+
 bool _3CInterface::parseASTs() {
 
   std::lock_guard<std::mutex> Lock(InterfaceMutex);
 
   auto *Tool = new ClangTool(*CurrCompDB, SourceFiles);
-  Tool->appendArgumentsAdjuster(getIgnoreCheckedPointerAdjuster());
-  // NOTE: This code is currently unreachable because VerifyDiagnosticOutput is
-  // rejected in the _3CInterface constructor.
-  //
-  // TODO: This currently only enables compiler diagnostic verification.
-  // see https://github.com/correctcomputation/checkedc-clang/issues/425
-  // for status.
-  if (VerifyDiagnosticOutput)
-    Tool->appendArgumentsAdjuster(addVerifyAdjuster());
-  // We canonicalize source file paths in the _3CInterface constructor, but the
-  // compilation database may override the path with a relative or otherwise
-  // non-canonical path, which may then affect the presumed filename, which is
-  // used in the PersistentSourceLoc. Since we need PersistentSourceLocs to be
-  // unambiguous, we canonicalize the source file path again here. We may still
-  // get relative presumed filenames from relative -I paths, though
-  // convert_project tries to change the -I options to avoid this.
-  Tool->appendArgumentsAdjuster(canonicalizeSourceFilePathAdjuster());
 
   // load the ASTs
-  if (Tool->buildASTs(ASTs))
-    return false;
+  _3CASTBuilderAction Action(ASTs);
+  int ToolExitStatus = Tool->run(&Action);
+  HadNonDiagnosticError |= (ToolExitStatus != 0);
 
-  // check for compile errors
-  unsigned int Errs = 0;
-  for (auto &TU : ASTs)
-    Errs += TU->getDiagnostics().getClient()->getNumErrors();
-
-  return Errs == 0;
+  return isSuccessfulSoFar();
 }
 
 bool _3CInterface::addVariables() {
@@ -354,15 +518,10 @@ bool _3CInterface::addVariables() {
 
     // 1. Add Variables.
   VariableAdderConsumer VA = VariableAdderConsumer(GlobalProgramInfo, nullptr);
-  unsigned int Errs = 0;
-  for (auto &TU : ASTs) {
-    TU->enableSourceFileDiagnostics();
+  for (auto &TU : ASTs)
     VA.HandleTranslationUnit(TU->getASTContext());
-    TU->getDiagnostics().getClient()->EndSourceFile();
-    Errs += TU->getDiagnostics().getClient()->getNumErrors();
-  }
 
-  return Errs == 0;
+  return isSuccessfulSoFar();
 }
 
 bool _3CInterface::buildInitialConstraints() {
@@ -371,23 +530,20 @@ bool _3CInterface::buildInitialConstraints() {
 
   if (!GlobalProgramInfo.link()) {
     errs() << "Linking failed!\n";
-    return false;
+    HadNonDiagnosticError = true;
+    return isSuccessfulSoFar(); // False, of course, but follow the pattern.
   }
 
   // 2. Gather constraints.
   ConstraintBuilderConsumer CB = ConstraintBuilderConsumer(GlobalProgramInfo, nullptr);
-  unsigned int Errs = 0;
-  for (auto &TU : ASTs) {
-    TU->enableSourceFileDiagnostics();
+  for (auto &TU : ASTs)
     CB.HandleTranslationUnit(TU->getASTContext());
-    TU->getDiagnostics().getClient()->EndSourceFile();
-    Errs += TU->getDiagnostics().getClient()->getNumErrors();
-  }
-  if (Errs > 0) return false;
+  if (!isSuccessfulSoFar())
+    return false;
 
   ConstraintsBuilt = true;
 
-  return true;
+  return isSuccessfulSoFar();
 }
 
 bool _3CInterface::solveConstraints() {
@@ -427,14 +583,10 @@ bool _3CInterface::solveConstraints() {
 
     // 4. Infer the bounds based on calls to malloc and calloc
     AllocBasedBoundsInference ABBI = AllocBasedBoundsInference(GlobalProgramInfo, nullptr);
-    unsigned int Errs = 0;
-    for (auto &TU : ASTs) {
-      TU->enableSourceFileDiagnostics();
+    for (auto &TU : ASTs)
       ABBI.HandleTranslationUnit(TU->getASTContext());
-      TU->getDiagnostics().getClient()->EndSourceFile();
-      Errs += TU->getDiagnostics().getClient()->getNumErrors();
-    }
-    if (Errs > 0) return false;
+    if (!isSuccessfulSoFar())
+      return false;
 
     // Propagate the information from allocator bounds.
     GlobalProgramInfo.getABoundsInfo().performFlowAnalysis(&GlobalProgramInfo);
@@ -443,14 +595,10 @@ bool _3CInterface::solveConstraints() {
   // 5. Run intermediate tool hook to run visitors that need to be executed
   // after constraint solving but before rewriting.
   IntermediateToolHook ITH = IntermediateToolHook(GlobalProgramInfo, nullptr);
-  unsigned int Errs = 0;
-  for (auto &TU : ASTs) {
-    TU->enableSourceFileDiagnostics();
+  for (auto &TU : ASTs)
     ITH.HandleTranslationUnit(TU->getASTContext());
-    TU->getDiagnostics().getClient()->EndSourceFile();
-    Errs += TU->getDiagnostics().getClient()->getNumErrors();
-  }
-  if (Errs > 0) return false;
+  if (!isSuccessfulSoFar())
+    return false;
 
   if (AllTypes) {
     // Propagate data-flow information for Array pointers.
@@ -490,7 +638,7 @@ bool _3CInterface::solveConstraints() {
     }
   }*/
 
-  return true;
+  return isSuccessfulSoFar();
 }
 
 bool _3CInterface::writeAllConvertedFilesToDisk() {
@@ -498,18 +646,12 @@ bool _3CInterface::writeAllConvertedFilesToDisk() {
 
   // 6. Rewrite the input files.
   RewriteConsumer RC = RewriteConsumer(GlobalProgramInfo);
-  unsigned int Errs = 0;
-  for (auto &TU : ASTs) {
-    TU->enableSourceFileDiagnostics();
+  for (auto &TU : ASTs)
     RC.HandleTranslationUnit(TU->getASTContext());
-    TU->getDiagnostics().getClient()->EndSourceFile();
-    Errs += TU->getDiagnostics().getClient()->getNumErrors();
-  }
-  if (Errs > 0) return false;
 
   GlobalProgramInfo.getPerfStats().endTotalTime();
   GlobalProgramInfo.getPerfStats().startTotalTime();
-  return true;
+  return isSuccessfulSoFar();
 }
 
 bool _3CInterface::dumpStats() {
@@ -546,7 +688,7 @@ bool _3CInterface::dumpStats() {
       PerWildPtrInfo.close();
     }
   }
-  return true;
+  return isSuccessfulSoFar();
 }
 
 ConstraintsInfo &_3CInterface::getWildPtrsInfo() {
diff --git a/clang/test/3C/base_subdir/canwrite_constraints.c b/clang/test/3C/base_subdir/canwrite_constraints.c
index 4fcb5538a921..b02afe263c52 100644
--- a/clang/test/3C/base_subdir/canwrite_constraints.c
+++ b/clang/test/3C/base_subdir/canwrite_constraints.c
@@ -10,9 +10,10 @@
 // not allow canwrite_constraints.h to change, and the internal types of q and
 // the return should remain wild.
 //
-// The root cause warning verification part of this test is currently disabled
-// because of https://github.com/correctcomputation/checkedc-clang/issues/503;
-// the rest of the test still works. TODO: Re-enable warning verification.
+// TODO: Fix the discrepancies in warnings that were introduced while the
+// diagnostic verifier was disabled
+// (https://github.com/correctcomputation/checkedc-clang/issues/609)
+// and then re-enable diagnostic verification here.
 //
 // RUN: cd %S && 3c -alltypes -addcr -output-dir=%t.checked/base_subdir -warn-all-root-cause %s --
 // RUN: FileCheck -match-full-lines -check-prefixes=CHECK_LOWER --input-file %t.checked/base_subdir/canwrite_constraints.c %s
diff --git a/clang/test/3C/base_subdir/canwrite_constraints_symlink.c b/clang/test/3C/base_subdir/canwrite_constraints_symlink.c
index f8b1a69eadbd..86f9091b4606 100644
--- a/clang/test/3C/base_subdir/canwrite_constraints_symlink.c
+++ b/clang/test/3C/base_subdir/canwrite_constraints_symlink.c
@@ -1,7 +1,3 @@
-// TODO: refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
-// XFAIL: *
-
 // Like canwrite_constraints_unimplemented.c but tests the third former base dir
 // matching bug from
 // https://github.com/correctcomputation/checkedc-clang/issues/327: symlinks.
@@ -24,7 +20,7 @@
 // Now 3C should know that it can't write to base_subdir_partial_defn.h because
 // the symlink goes out of the base dir.
 //
-// RUN: cd %t.base && 3c -addcr -verify canwrite_constraints_symlink.c --
+// RUN: cd %t.base && 3c -addcr canwrite_constraints_symlink.c -- -Xclang -verify
 
 // expected-error@base_subdir_partial_defn.h:1 {{3C internal error: 3C generated changes to this file even though it is not allowed to write to the file}}
 // expected-note@*:* {{-dump-unwritable-changes}}
diff --git a/clang/test/3C/base_subdir/canwrite_constraints_unimplemented.c b/clang/test/3C/base_subdir/canwrite_constraints_unimplemented.c
index 069ae8931b10..694ecf48e57b 100644
--- a/clang/test/3C/base_subdir/canwrite_constraints_unimplemented.c
+++ b/clang/test/3C/base_subdir/canwrite_constraints_unimplemented.c
@@ -1,13 +1,9 @@
-// TODO: refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
-// XFAIL: *
-
 // An example of a case that is not handled by the canWrite constraints code and
 // causes 3C to generate a change to an unwritable file. Test that 3C generates
 // an error diagnostic.
 // (https://github.com/correctcomputation/checkedc-clang/issues/387)
 
-// RUN: cd %S && 3c -addcr -verify %s --
+// RUN: cd %S && 3c -addcr %s -- -Xclang -verify
 
 // expected-error@../base_subdir_partial_defn.h:1 {{3C internal error: 3C generated changes to this file even though it is not allowed to write to the file}}
 // expected-note@*:* {{-dump-unwritable-changes}}
diff --git a/clang/test/3C/canwrite_constraints.h b/clang/test/3C/canwrite_constraints.h
index 11a69b8dcb99..f086d0342ece 100644
--- a/clang/test/3C/canwrite_constraints.h
+++ b/clang/test/3C/canwrite_constraints.h
@@ -53,7 +53,7 @@ void unwritable_cast(void((*g)(int *q)) : itype(_Ptr<void(_Ptr<int>)>)) {
 //
 // TODO: Add the correct expected root cause warnings once diagnostic
 // verification is re-enabled
-// (https://github.com/correctcomputation/checkedc-clang/issues/503).
+// (https://github.com/correctcomputation/checkedc-clang/issues/609).
 void unwritable_func_with_itype(int *p : itype(_Array_ptr<int>)) {}
 
 void unwritable_func_with_itype_and_bounds(int *p
diff --git a/clang/test/3C/diag_verifier_fail.c b/clang/test/3C/diag_verifier_fail.c
index 3ecdc55d0468..9766cc65b2fc 100644
--- a/clang/test/3C/diag_verifier_fail.c
+++ b/clang/test/3C/diag_verifier_fail.c
@@ -1,7 +1,3 @@
-// TODO: refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
-// XFAIL: *
-
 // Test that the diagnostic verifier is functioning correctly in 3c, because if
 // it isn't, all the other regression tests that use the diagnostic verifier may
 // not be able to catch the problems they are supposed to catch.
@@ -11,7 +7,7 @@
 // fail.
 
 // RUN: rm -rf %t*
-// RUN: not 3c -base-dir=%S -extra-arg="-Wno-everything" -verify -warn-root-cause %s -- 2>%t.stderr
+// RUN: not 3c -base-dir=%S -warn-root-cause %s -- -Xclang -verify -Wno-everything 2>%t.stderr
 // RUN: grep -q "error: 'warning' diagnostics expected but not seen:" %t.stderr
 // RUN: grep -q "error: 'warning' diagnostics seen but not expected:" %t.stderr
 
diff --git a/clang/test/3C/diag_verifier_pass.c b/clang/test/3C/diag_verifier_pass.c
index 809240f7d33a..ea68c6379498 100644
--- a/clang/test/3C/diag_verifier_pass.c
+++ b/clang/test/3C/diag_verifier_pass.c
@@ -1,7 +1,3 @@
-// TODO: refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
-// XFAIL: *
-
 // Test that the diagnostic verifier is functioning correctly in 3c, because if
 // it isn't, all the other regression tests that use the diagnostic verifier may
 // not be able to catch the problems they are supposed to catch. This goes with
@@ -13,7 +9,7 @@
 // those errors and fail, so diag_verifier_{pass,fail}.c only add coverage for
 // unusual problems with the diagnostic verifier that we haven't seen yet.
 
-// RUN: 3c -base-dir=%S -extra-arg="-Wno-everything" -verify -warn-root-cause %s --
+// RUN: 3c -base-dir=%S -warn-root-cause %s -- -Xclang -verify -Wno-everything
 
 // Example warning borrowed from root_cause.c .
 void *x; // expected-warning {{Default void* type}}
diff --git a/clang/test/3C/difftypes1a.c b/clang/test/3C/difftypes1a.c
index 2a4f38bfef66..021c27f9360d 100644
--- a/clang/test/3C/difftypes1a.c
+++ b/clang/test/3C/difftypes1a.c
@@ -1,7 +1,13 @@
-//RUN: rm -rf %t*
-//RUN: not 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes1b.c -- 2>%t.stderr
-//RUN: grep -q "merging failed" %t.stderr
+// Since the RUN commands in difftypes1a.c and difftypes1b.c process the two
+// files in different orders and the location where the error is reported
+// depends on the order, we need to use a different diagnostic verification
+// prefix (and set of corresponding comments) for each RUN command. Verification
+// is per translation unit, so the translation unit with no diagnostic needs
+// `expected-no-diagnostics`.
 
-// The desired behavior in this case is to fail, so other checks are omitted
+//RUN: rm -rf %t*
+//RUN: 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes1b.c -- -Xclang -verify=ab-expected
 
+// ab-expected-no-diagnostics
+// ba-expected-error@+1 {{merging failed for 'foo'}}
 _Ptr<int> foo(int, char);
diff --git a/clang/test/3C/difftypes1b.c b/clang/test/3C/difftypes1b.c
index 3959be295978..be3a338812bb 100644
--- a/clang/test/3C/difftypes1b.c
+++ b/clang/test/3C/difftypes1b.c
@@ -1,7 +1,13 @@
-//RUN: rm -rf %t*
-//RUN: not 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes1a.c -- 2>%t.stderr
-//RUN: grep -q "merging failed" %t.stderr
+// Since the RUN commands in difftypes1a.c and difftypes1b.c process the two
+// files in different orders and the location where the error is reported
+// depends on the order, we need to use a different diagnostic verification
+// prefix (and set of corresponding comments) for each RUN command. Verification
+// is per translation unit, so the translation unit with no diagnostic needs
+// `expected-no-diagnostics`.
 
-// The desired behavior in this case is to fail, so other checks are omitted
+//RUN: rm -rf %t*
+//RUN: 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes1a.c -- -Xclang -verify=ba-expected
 
+// ba-expected-no-diagnostics
+// ab-expected-error@+1 {{merging failed for 'foo'}}
 int *foo(int, char *);
diff --git a/clang/test/3C/difftypes2a.c b/clang/test/3C/difftypes2a.c
index 3096543990f2..7969c47e7910 100644
--- a/clang/test/3C/difftypes2a.c
+++ b/clang/test/3C/difftypes2a.c
@@ -1,9 +1,17 @@
+// Since the RUN commands in difftypes2a.c and difftypes2b.c process the two
+// files in different orders and the location where the error is reported
+// depends on the order, we need to use a different diagnostic verification
+// prefix (and set of corresponding comments) for each RUN command. Verification
+// is per translation unit, so the translation unit with no diagnostic needs
+// `expected-no-diagnostics`.
+
 // RUN: rm -rf %t*
-// RUN: not 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes2b.c -- 2>%t.stderr
-// RUN: grep -q "merging failed" %t.stderr
+// RUN: 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes2b.c -- -Xclang -verify=ab-expected
 
 // The desired behavior in this case is to fail, so other checks are omitted
 
 // Test no body vs body
 
+// ab-expected-no-diagnostics
+// ba-expected-error@+1 {{merging failed for 'foo'}}
 void foo(char *x);
diff --git a/clang/test/3C/difftypes2b.c b/clang/test/3C/difftypes2b.c
index c50af4ce24a4..edc480d282a5 100644
--- a/clang/test/3C/difftypes2b.c
+++ b/clang/test/3C/difftypes2b.c
@@ -1,11 +1,17 @@
-// RUN: rm -rf %t*
-// RUN: not 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes2a.c -- 2>%t.stderr
-// RUN: grep -q "merging failed" %t.stderr
+// Since the RUN commands in difftypes2a.c and difftypes2b.c process the two
+// files in different orders and the location where the error is reported
+// depends on the order, we need to use a different diagnostic verification
+// prefix (and set of corresponding comments) for each RUN command. Verification
+// is per translation unit, so the translation unit with no diagnostic needs
+// `expected-no-diagnostics`.
 
-// The desired behavior in this case is to fail, so other checks are omitted
+// RUN: rm -rf %t*
+// RUN: 3c -base-dir=%S -output-dir=%t.checked %s %S/difftypes2a.c -- -Xclang -verify=ba-expected
 
 // Test body vs no body
 
+// ba-expected-no-diagnostics
+// ab-expected-error@+1 {{merging failed for 'foo'}}
 void foo(char **y) {
   // this is the body
 }
diff --git a/clang/test/3C/macro_function_call.c b/clang/test/3C/macro_function_call.c
index 1797c7ed9685..f568e80709d7 100644
--- a/clang/test/3C/macro_function_call.c
+++ b/clang/test/3C/macro_function_call.c
@@ -1,9 +1,9 @@
 // RUN: rm -rf %t*
-// RUN: 3c -base-dir=%S -alltypes -addcr %s -- | FileCheck -match-full-lines -check-prefixes="CHECK_ALL","CHECK" %s
-// RUN: 3c -base-dir=%S -addcr %s -- | FileCheck -match-full-lines -check-prefixes="CHECK_NOALL","CHECK" %s
-// RUN: 3c -base-dir=%S -alltypes -addcr %s -- | %clang -c -fcheckedc-extension -x c -o /dev/null -
-// RUN: 3c -base-dir=%S -alltypes -output-dir=%t.checked %s --
-// RUN: 3c -base-dir=%t.checked -alltypes %t.checked/macro_function_call.c -- | diff %t.checked/macro_function_call.c -
+// RUN: 3c -base-dir=%S -alltypes -addcr %s -- -Xclang -verify | FileCheck -match-full-lines -check-prefixes="CHECK_ALL","CHECK" %s
+// RUN: 3c -base-dir=%S -addcr %s -- -Xclang -verify | FileCheck -match-full-lines -check-prefixes="CHECK_NOALL","CHECK" %s
+// RUN: 3c -base-dir=%S -alltypes -addcr %s -- -Xclang -verify | %clang -c -fcheckedc-extension -x c -o /dev/null -
+// RUN: 3c -base-dir=%S -alltypes -output-dir=%t.checked %s -- -Xclang -verify
+// RUN: 3c -base-dir=%t.checked -alltypes %t.checked/macro_function_call.c -- -Xclang -verify | diff %t.checked/macro_function_call.c -
 
 // Test fix for https://github.com/correctcomputation/checkedc-clang/issues/439
 // We cannot insert casts on function calls inside macros, so constraints must
@@ -12,13 +12,6 @@
 
 // 3C emits a warning if it fails inserting a cast. Ensure the test fails if
 // this happens.
-//
-// NOTICE: This part of the test is disabled (the -verify option has been
-// removed from the 3c RUN commands) until we have a replacement for the
-// diagnostic verifier
-// (https://github.com/correctcomputation/checkedc-clang/issues/503).
-// TODO: Re-enable it when we do.
-//
 // expected-no-diagnostics
 
 // Unsafe call in macro. This would require an _Assume_bounds_cast, but we
diff --git a/clang/test/3C/macro_rewrite_error.c b/clang/test/3C/macro_rewrite_error.c
index 972b184e56a5..cf340f16ab15 100644
--- a/clang/test/3C/macro_rewrite_error.c
+++ b/clang/test/3C/macro_rewrite_error.c
@@ -1,8 +1,4 @@
-// TODO: refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
-// XFAIL: *
-
-// RUN: 3c -base-dir=%S -extra-arg="-Wno-everything" -verify -alltypes %s --
+// RUN: 3c -base-dir=%S -alltypes %s -- -Xclang -verify -Wno-everything
 
 // clang-format makes a mess of this file.
 // clang-format off
diff --git a/clang/test/3C/root_cause.c b/clang/test/3C/root_cause.c
index 2e88b4acbdca..bfbf1e369327 100644
--- a/clang/test/3C/root_cause.c
+++ b/clang/test/3C/root_cause.c
@@ -1,8 +1,9 @@
-// TODO: Refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
+// TODO: Fix the failures in this test that were introduced while the diagnostic
+// verifier was disabled
+// (https://github.com/correctcomputation/checkedc-clang/issues/609).
 // XFAIL: *
 
-// RUN: 3c -base-dir=%S -extra-arg="-Wno-everything" -verify -alltypes -warn-root-cause %s --
+// RUN: 3c -base-dir=%S -alltypes -warn-root-cause %s -- -Xclang -verify -Wno-everything
 
 // This test is unusual in that it checks for the errors in the code
 
diff --git a/clang/test/3C/stdout_mode_write_other.c b/clang/test/3C/stdout_mode_write_other.c
index aefcdf89c374..731ad45cdbd0 100644
--- a/clang/test/3C/stdout_mode_write_other.c
+++ b/clang/test/3C/stdout_mode_write_other.c
@@ -1,13 +1,14 @@
-// TODO: refactor this test
-// https://github.com/correctcomputation/checkedc-clang/issues/503
-// XFAIL: *
-
 // Like the base_subdir/canwrite_constraints_unimplemented test except that
 // writing base_subdir_partial_defn.h is not restricted by the base dir, so we
 // reach the stdout mode restriction.
+//
+// Since this is currently the simplest 3C regression test that tests an error
+// diagnostic, we also use it to test some things about error diagnostics in
+// general.
 
 // RUN: rm -rf %t*
-// RUN: 3c -base-dir=%S -addcr -verify %s --
+// RUN: 3c -base-dir=%S -addcr %s -- -Xclang -verify 2>%t.stderr
+// RUN: grep -q 'Exiting successfully because the failure was due solely to expected error diagnostics and diagnostic verification succeeded' %t.stderr
 
 // Same as above, except instead of using the diagnostic verifier, manually test
 // that 3c exits nonzero and prints the error to stderr. This tests that 3c
diff --git a/clang/tools/3c/3CStandalone.cpp b/clang/tools/3c/3CStandalone.cpp
index d8aa1a6dadac..120bccdf3192 100644
--- a/clang/tools/3c/3CStandalone.cpp
+++ b/clang/tools/3c/3CStandalone.cpp
@@ -156,10 +156,13 @@ static cl::opt<bool> OptAddCheckedRegions("addcr",
                                           cl::init(false),
                                           cl::cat(_3CCategory));
 
-static cl::opt<bool>
-    OptDiableCCTypeChecker("disccty",
-                           cl::desc("Do not disable checked c type checker."),
-                           cl::init(false), cl::cat(_3CCategory));
+static cl::opt<bool> OptEnableCCTypeChecker(
+    "enccty",
+    cl::desc(
+        "Enable the Checked C type checker. 3c normally disables it (via the "
+        "equivalent of `clang -f3c-tool`) so that 3c can operate on partially "
+        "converted programs that may have Checked C type errors."),
+    cl::init(false), cl::cat(_3CCategory));
 
 static cl::opt<std::string> OptBaseDir(
     "base-dir",
@@ -191,20 +194,6 @@ static cl::opt<bool>
                                  "even those unlikely to be interesting."),
                         cl::init(false), cl::cat(_3CCategory));
 
-// https://clang.llvm.org/doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details
-//
-// Analogous to the -verify option of `clang -cc1`, but currently applies only
-// to the rewriting phase (because it is the only phase that generates
-// diagnostics, except for the declaration merging diagnostics that are
-// currently fatal). No checking of diagnostics from the other phases is
-// performed. We cannot simply have the caller pass `-extra-arg=-Xclang
-// -extra-arg=-verify` because that would expect each phase to produce the same
-// set of diagnostics.
-static cl::opt<bool> OptVerifyDiagnosticOutput(
-    "verify",
-    cl::desc("Verify diagnostic output (for automated testing of 3c)."),
-    cl::init(false), cl::cat(_3CCategory), cl::Hidden);
-
 // In the future, we may enhance this to write the output to individual files.
 // For now, the user has to copy and paste the correct portions of stderr.
 static cl::opt<bool> OptDumpUnwritableChanges(
@@ -297,10 +286,9 @@ int main(int argc, const char **argv) {
   CcOptions.PerPtrInfoJson = OptPerPtrWILDInfoJson.getValue();
   CcOptions.AddCheckedRegions = OptAddCheckedRegions;
   CcOptions.EnableAllTypes = OptAllTypes;
-  CcOptions.DisableCCTypeChecker = OptDiableCCTypeChecker;
+  CcOptions.EnableCCTypeChecker = OptEnableCCTypeChecker;
   CcOptions.WarnRootCause = OptWarnRootCause;
   CcOptions.WarnAllRootCause = OptWarnAllRootCause;
-  CcOptions.VerifyDiagnosticOutput = OptVerifyDiagnosticOutput;
   CcOptions.DumpUnwritableChanges = OptDumpUnwritableChanges;
   CcOptions.AllowUnwritableChanges = OptAllowUnwritableChanges;
   CcOptions.AllowRewriteFailures = OptAllowRewriteFailures;
@@ -347,7 +335,7 @@ int main(int argc, const char **argv) {
   // Build AST from source.
   if (!_3CInterface.parseASTs()) {
     errs() << "Failure occurred while parsing source files. Exiting.\n";
-    return 1;
+    return _3CInterface.determineExitCode();
   }
 
   if (OptVerbose) {
@@ -358,7 +346,7 @@ int main(int argc, const char **argv) {
   // Add variables.
   if (!_3CInterface.addVariables()) {
     errs() << "Failure occurred while trying to add variables. Exiting.\n";
-    return 1;
+    return _3CInterface.determineExitCode();
   }
 
   if (OptVerbose) {
@@ -369,7 +357,7 @@ int main(int argc, const char **argv) {
   // Build constraints.
   if (!_3CInterface.buildInitialConstraints()) {
     errs() << "Failure occurred while trying to build constraints. Exiting.\n";
-    return 1;
+    return _3CInterface.determineExitCode();
   }
 
   if (OptVerbose) {
@@ -380,7 +368,7 @@ int main(int argc, const char **argv) {
   // Solve the constraints.
   if (!_3CInterface.solveConstraints()) {
     errs() << "Failure occurred while trying to solve constraints. Exiting.\n";
-    return 1;
+    return _3CInterface.determineExitCode();
   }
 
   if (OptVerbose) {
@@ -392,14 +380,17 @@ int main(int argc, const char **argv) {
   if (!_3CInterface.writeAllConvertedFilesToDisk()) {
     errs() << "Failure occurred while trying to rewrite converted files back. "
               "Exiting.\n";
-    return 1;
+    return _3CInterface.determineExitCode();
   }
 
   // Write all the performance related stats.
   if (!_3CInterface.dumpStats()) {
     errs() << "Failure occurred while trying to write performance stats. "
               "Exiting.\n";
-    return 1;
+    return _3CInterface.determineExitCode();
   }
-  return 0;
+
+  // Even if all passes succeeded, we could still have a diagnostic verification
+  // failure.
+  return _3CInterface.determineExitCode();
 }
diff --git a/clang/tools/3c/utils/port_tools/generate_ccommands.py b/clang/tools/3c/utils/port_tools/generate_ccommands.py
index ad05eb77e4e2..4a74d91f93f1 100644
--- a/clang/tools/3c/utils/port_tools/generate_ccommands.py
+++ b/clang/tools/3c/utils/port_tools/generate_ccommands.py
@@ -133,7 +133,6 @@ def run3C(checkedc_bin, extra_3c_args,
 
     translation_units: List[TranslationUnitInfo] = []
     all_files = []
-    absolute_include_dirs = set()
     for i in cmds:
         file_to_add = i['file']
         compiler_path = None  # XXX Clean this up
@@ -165,9 +164,6 @@ def run3C(checkedc_bin, extra_3c_args,
                                      target_directory, file_to_add,
                                      output_filename)
             translation_units.append(tu)
-            for arg in compiler_x_args:
-                if arg.startswith('-I'):
-                    absolute_include_dirs.add(tu.realpath(arg[len('-I'):]))
 
     expandMacros(expand_macros_opts, compilation_base_dir, translation_units)
 
@@ -223,16 +219,6 @@ def run3C(checkedc_bin, extra_3c_args,
     args.append('-p')
     args.append(compile_commands_json)
     args.append('-extra-arg=-w')
-    # This is a workaround for a bug in Clang LibTooling that affects the use of
-    # relative paths with -I when different translation units have different
-    # working directories. For details, see
-    # https://github.com/correctcomputation/checkedc-clang/issues/515 .
-    #
-    # Even if the above issue were fixed, we may still need this in some cases
-    # to ensure that PersistentSourceLoc filenames are unambiguous: see the
-    # comment in _3CInterface::parseASTs in clang/lib/3C/3C.cpp .
-    for dir in absolute_include_dirs:
-        args.append('-extra-arg-before=-I' + dir)
     vcodewriter.addClangdArg("-log=verbose")
     vcodewriter.addClangdArg(args[1:])
     args.append('-base-dir="' + compilation_base_dir + '"')