[flang-commits] [flang] eda72fa - Fix OOM in FormatDiagnostic (2nd attempt) (#108866)

via flang-commits flang-commits at lists.llvm.org
Wed Sep 18 08:46:28 PDT 2024


Author: Vakhurin Sergei
Date: 2024-09-18T11:46:25-04:00
New Revision: eda72fac548f317cec997967494763e9a7bafa27

URL: https://github.com/llvm/llvm-project/commit/eda72fac548f317cec997967494763e9a7bafa27
DIFF: https://github.com/llvm/llvm-project/commit/eda72fac548f317cec997967494763e9a7bafa27.diff

LOG: Fix OOM in FormatDiagnostic (2nd attempt) (#108866)

Resolves: #70930 (and probably latest comments from clangd/clangd#251)
by fixing racing for the shared DiagStorage value which caused messing with args inside the storage and then formatting the following message with getArgSInt(1) == 2:

def err_module_odr_violation_function : Error<
  "%q0 has different definitions in different modules; "
  "%select{definition in module '%2'|defined here}1 "
  "first difference is "

which causes HandleSelectModifier to go beyond the ArgumentLen so the recursive call to FormatDiagnostic was made with DiagStr > DiagEnd that leads to infinite while (DiagStr != DiagEnd).

The Main Idea:
Reuse the existing DiagStorageAllocator logic to make all DiagnosticBuilders having independent states.
Also, encapsulating the rest of state (e.g. ID and Loc) into DiagnosticBuilder.

The last attempt failed -
https://github.com/llvm/llvm-project/pull/108187#issuecomment-2353122096
so was reverted - #108838

Added: 
    clang/test/PCH/race-condition.cpp

Modified: 
    clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
    clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
    clang/include/clang/Basic/Diagnostic.h
    clang/include/clang/Basic/DiagnosticIDs.h
    clang/include/clang/Basic/PartialDiagnostic.h
    clang/include/clang/Sema/Sema.h
    clang/lib/Basic/Diagnostic.cpp
    clang/lib/Basic/DiagnosticIDs.cpp
    clang/lib/Basic/SourceManager.cpp
    clang/lib/Frontend/Rewrite/FixItRewriter.cpp
    clang/lib/Frontend/TextDiagnosticPrinter.cpp
    clang/lib/Sema/Sema.cpp
    clang/lib/Sema/SemaBase.cpp
    clang/lib/Serialization/ASTReader.cpp
    clang/unittests/Basic/DiagnosticTest.cpp
    clang/unittests/Driver/DXCModeTest.cpp
    flang/lib/Frontend/TextDiagnosticPrinter.cpp

Removed: 
    


################################################################################
diff  --git a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
index 200bb87a5ac3cb..4c75b422701148 100644
--- a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -380,7 +380,6 @@ void ClangTidyDiagnosticConsumer::HandleDiagnostic(
     ++Context.Stats.ErrorsIgnoredNOLINT;
     // Ignored a warning, should ignore related notes as well
     LastErrorWasIgnored = true;
-    Context.DiagEngine->Clear();
     for (const auto &Error : SuppressionErrors)
       Context.diag(Error);
     return;
@@ -457,7 +456,6 @@ void ClangTidyDiagnosticConsumer::HandleDiagnostic(
   if (Info.hasSourceManager())
     checkFilters(Info.getLocation(), Info.getSourceManager());
 
-  Context.DiagEngine->Clear();
   for (const auto &Error : SuppressionErrors)
     Context.diag(Error);
 }

diff  --git a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
index 021d731f8f1768..cf9b42828568da 100644
--- a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
@@ -305,33 +305,33 @@ TEST_F(ConfigCompileTests, DiagnosticSuppression) {
   {
     auto D = DiagEngine.Report(diag::warn_unreachable);
     EXPECT_TRUE(isDiagnosticSuppressed(
-        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+        Diag{&DiagEngine, D}, Conf.Diagnostics.Suppress, LangOptions()));
   }
   // Subcategory not respected/suppressed.
   {
     auto D = DiagEngine.Report(diag::warn_unreachable_break);
     EXPECT_FALSE(isDiagnosticSuppressed(
-        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+        Diag{&DiagEngine, D}, Conf.Diagnostics.Suppress, LangOptions()));
   }
   {
     auto D = DiagEngine.Report(diag::warn_unused_variable);
     EXPECT_TRUE(isDiagnosticSuppressed(
-        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+        Diag{&DiagEngine, D}, Conf.Diagnostics.Suppress, LangOptions()));
   }
   {
     auto D = DiagEngine.Report(diag::err_typecheck_bool_condition);
     EXPECT_TRUE(isDiagnosticSuppressed(
-        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+        Diag{&DiagEngine, D}, Conf.Diagnostics.Suppress, LangOptions()));
   }
   {
     auto D = DiagEngine.Report(diag::err_unexpected_friend);
     EXPECT_TRUE(isDiagnosticSuppressed(
-        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+        Diag{&DiagEngine, D}, Conf.Diagnostics.Suppress, LangOptions()));
   }
   {
     auto D = DiagEngine.Report(diag::warn_alloca);
     EXPECT_TRUE(isDiagnosticSuppressed(
-        Diag{&DiagEngine}, Conf.Diagnostics.Suppress, LangOptions()));
+        Diag{&DiagEngine, D}, Conf.Diagnostics.Suppress, LangOptions()));
   }
 
   Frag.Diagnostics.Suppress.emplace_back("*");

diff  --git a/clang/include/clang/Basic/Diagnostic.h b/clang/include/clang/Basic/Diagnostic.h
index 54b69e98540239..e17ed8f98afa9a 100644
--- a/clang/include/clang/Basic/Diagnostic.h
+++ b/clang/include/clang/Basic/Diagnostic.h
@@ -183,6 +183,41 @@ struct DiagnosticStorage {
   DiagnosticStorage() = default;
 };
 
+/// An allocator for DiagnosticStorage objects, which uses a small cache to
+/// objects, used to reduce malloc()/free() traffic for partial diagnostics.
+class DiagStorageAllocator {
+  static const unsigned NumCached = 16;
+  DiagnosticStorage Cached[NumCached];
+  DiagnosticStorage *FreeList[NumCached];
+  unsigned NumFreeListEntries;
+
+public:
+  DiagStorageAllocator();
+  ~DiagStorageAllocator();
+
+  /// Allocate new storage.
+  DiagnosticStorage *Allocate() {
+    if (NumFreeListEntries == 0)
+      return new DiagnosticStorage;
+
+    DiagnosticStorage *Result = FreeList[--NumFreeListEntries];
+    Result->NumDiagArgs = 0;
+    Result->DiagRanges.clear();
+    Result->FixItHints.clear();
+    return Result;
+  }
+
+  /// Free the given storage object.
+  void Deallocate(DiagnosticStorage *S) {
+    if (S >= Cached && S <= Cached + NumCached) {
+      FreeList[NumFreeListEntries++] = S;
+      return;
+    }
+
+    delete S;
+  }
+};
+
 /// Concrete class used by the front-end to report problems and issues.
 ///
 /// This massages the diagnostics (e.g. handling things like "report warnings
@@ -522,27 +557,6 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
   void *ArgToStringCookie = nullptr;
   ArgToStringFnTy ArgToStringFn;
 
-  /// ID of the "delayed" diagnostic, which is a (typically
-  /// fatal) diagnostic that had to be delayed because it was found
-  /// while emitting another diagnostic.
-  unsigned DelayedDiagID;
-
-  /// First string argument for the delayed diagnostic.
-  std::string DelayedDiagArg1;
-
-  /// Second string argument for the delayed diagnostic.
-  std::string DelayedDiagArg2;
-
-  /// Third string argument for the delayed diagnostic.
-  std::string DelayedDiagArg3;
-
-  /// Optional flag value.
-  ///
-  /// Some flags accept values, for instance: -Wframe-larger-than=<value> and
-  /// -Rpass=<value>. The content of this string is emitted after the flag name
-  /// and '='.
-  std::string FlagValue;
-
 public:
   explicit DiagnosticsEngine(IntrusiveRefCntPtr<DiagnosticIDs> Diags,
                              IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts,
@@ -949,70 +963,18 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
 
   void Report(const StoredDiagnostic &storedDiag);
 
-  /// Determine whethere there is already a diagnostic in flight.
-  bool isDiagnosticInFlight() const {
-    return CurDiagID != std::numeric_limits<unsigned>::max();
-  }
-
-  /// Set the "delayed" diagnostic that will be emitted once
-  /// the current diagnostic completes.
-  ///
-  ///  If a diagnostic is already in-flight but the front end must
-  ///  report a problem (e.g., with an inconsistent file system
-  ///  state), this routine sets a "delayed" diagnostic that will be
-  ///  emitted after the current diagnostic completes. This should
-  ///  only be used for fatal errors detected at inconvenient
-  ///  times. If emitting a delayed diagnostic causes a second delayed
-  ///  diagnostic to be introduced, that second delayed diagnostic
-  ///  will be ignored.
-  ///
-  /// \param DiagID The ID of the diagnostic being delayed.
-  ///
-  /// \param Arg1 A string argument that will be provided to the
-  /// diagnostic. A copy of this string will be stored in the
-  /// DiagnosticsEngine object itself.
-  ///
-  /// \param Arg2 A string argument that will be provided to the
-  /// diagnostic. A copy of this string will be stored in the
-  /// DiagnosticsEngine object itself.
-  ///
-  /// \param Arg3 A string argument that will be provided to the
-  /// diagnostic. A copy of this string will be stored in the
-  /// DiagnosticsEngine object itself.
-  void SetDelayedDiagnostic(unsigned DiagID, StringRef Arg1 = "",
-                            StringRef Arg2 = "", StringRef Arg3 = "");
-
-  /// Clear out the current diagnostic.
-  void Clear() { CurDiagID = std::numeric_limits<unsigned>::max(); }
-
-  /// Return the value associated with this diagnostic flag.
-  StringRef getFlagValue() const { return FlagValue; }
-
 private:
   // This is private state used by DiagnosticBuilder.  We put it here instead of
   // in DiagnosticBuilder in order to keep DiagnosticBuilder a small lightweight
-  // object.  This implementation choice means that we can only have one
-  // diagnostic "in flight" at a time, but this seems to be a reasonable
-  // tradeoff to keep these objects small.  Assertions verify that only one
-  // diagnostic is in flight at a time.
+  // object.  This implementation choice means that we can only have a few
+  // diagnostics "in flight" at a time, but this seems to be a reasonable
+  // tradeoff to keep these objects small.
   friend class Diagnostic;
   friend class DiagnosticBuilder;
   friend class DiagnosticErrorTrap;
   friend class DiagnosticIDs;
   friend class PartialDiagnostic;
 
-  /// Report the delayed diagnostic.
-  void ReportDelayed();
-
-  /// The location of the current diagnostic that is in flight.
-  SourceLocation CurDiagLoc;
-
-  /// The ID of the current diagnostic that is in flight.
-  ///
-  /// This is set to std::numeric_limits<unsigned>::max() when there is no
-  /// diagnostic in flight.
-  unsigned CurDiagID;
-
   enum {
     /// The maximum number of arguments we can hold.
     ///
@@ -1022,7 +984,7 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
     MaxArguments = DiagnosticStorage::MaxArguments,
   };
 
-  DiagnosticStorage DiagStorage;
+  DiagStorageAllocator DiagAllocator;
 
   DiagnosticMapping makeUserMapping(diag::Severity Map, SourceLocation L) {
     bool isPragma = L.isValid();
@@ -1042,8 +1004,8 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
   /// Used to report a diagnostic that is finally fully formed.
   ///
   /// \returns true if the diagnostic was emitted, false if it was suppressed.
-  bool ProcessDiag() {
-    return Diags->ProcessDiag(*this);
+  bool ProcessDiag(const DiagnosticBuilder &DiagBuilder) {
+    return Diags->ProcessDiag(*this, DiagBuilder);
   }
 
   /// @name Diagnostic Emission
@@ -1058,14 +1020,10 @@ class DiagnosticsEngine : public RefCountedBase<DiagnosticsEngine> {
   // Sema::Diag() patterns.
   friend class Sema;
 
-  /// Emit the current diagnostic and clear the diagnostic state.
+  /// Emit the diagnostic
   ///
   /// \param Force Emit the diagnostic regardless of suppression settings.
-  bool EmitCurrentDiagnostic(bool Force = false);
-
-  unsigned getCurrentDiagID() const { return CurDiagID; }
-
-  SourceLocation getCurrentDiagLoc() const { return CurDiagLoc; }
+  bool EmitDiagnostic(const DiagnosticBuilder &DB, bool Force = false);
 
   /// @}
 };
@@ -1118,40 +1076,7 @@ class DiagnosticErrorTrap {
 ///
 class StreamingDiagnostic {
 public:
-  /// An allocator for DiagnosticStorage objects, which uses a small cache to
-  /// objects, used to reduce malloc()/free() traffic for partial diagnostics.
-  class DiagStorageAllocator {
-    static const unsigned NumCached = 16;
-    DiagnosticStorage Cached[NumCached];
-    DiagnosticStorage *FreeList[NumCached];
-    unsigned NumFreeListEntries;
-
-  public:
-    DiagStorageAllocator();
-    ~DiagStorageAllocator();
-
-    /// Allocate new storage.
-    DiagnosticStorage *Allocate() {
-      if (NumFreeListEntries == 0)
-        return new DiagnosticStorage;
-
-      DiagnosticStorage *Result = FreeList[--NumFreeListEntries];
-      Result->NumDiagArgs = 0;
-      Result->DiagRanges.clear();
-      Result->FixItHints.clear();
-      return Result;
-    }
-
-    /// Free the given storage object.
-    void Deallocate(DiagnosticStorage *S) {
-      if (S >= Cached && S <= Cached + NumCached) {
-        FreeList[NumFreeListEntries++] = S;
-        return;
-      }
-
-      delete S;
-    }
-  };
+  using DiagStorageAllocator = clang::DiagStorageAllocator;
 
 protected:
   mutable DiagnosticStorage *DiagStorage = nullptr;
@@ -1240,11 +1165,6 @@ class StreamingDiagnostic {
 protected:
   StreamingDiagnostic() = default;
 
-  /// Construct with an external storage not owned by itself. The allocator
-  /// is a null pointer in this case.
-  explicit StreamingDiagnostic(DiagnosticStorage *Storage)
-      : DiagStorage(Storage) {}
-
   /// Construct with a storage allocator which will manage the storage. The
   /// allocator is not a null pointer in this case.
   explicit StreamingDiagnostic(DiagStorageAllocator &Alloc)
@@ -1275,9 +1195,20 @@ class StreamingDiagnostic {
 class DiagnosticBuilder : public StreamingDiagnostic {
   friend class DiagnosticsEngine;
   friend class PartialDiagnostic;
+  friend class Diagnostic;
 
   mutable DiagnosticsEngine *DiagObj = nullptr;
 
+  SourceLocation DiagLoc;
+  unsigned DiagID;
+
+  /// Optional flag value.
+  ///
+  /// Some flags accept values, for instance: -Wframe-larger-than=<value> and
+  /// -Rpass=<value>. The content of this string is emitted after the flag name
+  /// and '='.
+  mutable std::string FlagValue;
+
   /// Status variable indicating if this diagnostic is still active.
   ///
   // NOTE: This field is redundant with DiagObj (IsActive iff (DiagObj == 0)),
@@ -1291,16 +1222,8 @@ class DiagnosticBuilder : public StreamingDiagnostic {
 
   DiagnosticBuilder() = default;
 
-  explicit DiagnosticBuilder(DiagnosticsEngine *diagObj)
-      : StreamingDiagnostic(&diagObj->DiagStorage), DiagObj(diagObj),
-        IsActive(true) {
-    assert(diagObj && "DiagnosticBuilder requires a valid DiagnosticsEngine!");
-    assert(DiagStorage &&
-           "DiagnosticBuilder requires a valid DiagnosticStorage!");
-    DiagStorage->NumDiagArgs = 0;
-    DiagStorage->DiagRanges.clear();
-    DiagStorage->FixItHints.clear();
-  }
+  DiagnosticBuilder(DiagnosticsEngine *DiagObj, SourceLocation DiagLoc,
+                    unsigned DiagID);
 
 protected:
   /// Clear out the current diagnostic.
@@ -1326,7 +1249,7 @@ class DiagnosticBuilder : public StreamingDiagnostic {
     if (!isActive()) return false;
 
     // Process the diagnostic.
-    bool Result = DiagObj->EmitCurrentDiagnostic(IsForceEmit);
+    bool Result = DiagObj->EmitDiagnostic(*this, IsForceEmit);
 
     // This diagnostic is dead.
     Clear();
@@ -1337,13 +1260,7 @@ class DiagnosticBuilder : public StreamingDiagnostic {
 public:
   /// Copy constructor.  When copied, this "takes" the diagnostic info from the
   /// input and neuters it.
-  DiagnosticBuilder(const DiagnosticBuilder &D) : StreamingDiagnostic() {
-    DiagObj = D.DiagObj;
-    DiagStorage = D.DiagStorage;
-    IsActive = D.IsActive;
-    IsForceEmit = D.IsForceEmit;
-    D.Clear();
-  }
+  DiagnosticBuilder(const DiagnosticBuilder &D);
 
   template <typename T> const DiagnosticBuilder &operator<<(const T &V) const {
     assert(isActive() && "Clients must not add to cleared diagnostic!");
@@ -1375,7 +1292,7 @@ class DiagnosticBuilder : public StreamingDiagnostic {
     return *this;
   }
 
-  void addFlagValue(StringRef V) const { DiagObj->FlagValue = std::string(V); }
+  void addFlagValue(StringRef V) const { FlagValue = std::string(V); }
 };
 
 struct AddFlagValue {
@@ -1550,12 +1467,7 @@ const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
 
 inline DiagnosticBuilder DiagnosticsEngine::Report(SourceLocation Loc,
                                                    unsigned DiagID) {
-  assert(CurDiagID == std::numeric_limits<unsigned>::max() &&
-         "Multiple diagnostics in flight at once!");
-  CurDiagLoc = Loc;
-  CurDiagID = DiagID;
-  FlagValue.clear();
-  return DiagnosticBuilder(this);
+  return DiagnosticBuilder(this, Loc, DiagID);
 }
 
 const StreamingDiagnostic &operator<<(const StreamingDiagnostic &DB,
@@ -1570,24 +1482,29 @@ inline DiagnosticBuilder DiagnosticsEngine::Report(unsigned DiagID) {
 //===----------------------------------------------------------------------===//
 
 /// A little helper class (which is basically a smart pointer that forwards
-/// info from DiagnosticsEngine) that allows clients to enquire about the
-/// currently in-flight diagnostic.
+/// info from DiagnosticsEngine and DiagnosticStorage) that allows clients to
+/// enquire about the diagnostic.
 class Diagnostic {
   const DiagnosticsEngine *DiagObj;
+  SourceLocation DiagLoc;
+  unsigned DiagID;
+  std::string FlagValue;
+  const DiagnosticStorage &DiagStorage;
   std::optional<StringRef> StoredDiagMessage;
 
 public:
-  explicit Diagnostic(const DiagnosticsEngine *DO) : DiagObj(DO) {}
-  Diagnostic(const DiagnosticsEngine *DO, StringRef storedDiagMessage)
-      : DiagObj(DO), StoredDiagMessage(storedDiagMessage) {}
+  Diagnostic(const DiagnosticsEngine *DO, const DiagnosticBuilder &DiagBuilder);
+  Diagnostic(const DiagnosticsEngine *DO, SourceLocation DiagLoc,
+             unsigned DiagID, const DiagnosticStorage &DiagStorage,
+             StringRef StoredDiagMessage);
 
   const DiagnosticsEngine *getDiags() const { return DiagObj; }
-  unsigned getID() const { return DiagObj->CurDiagID; }
-  const SourceLocation &getLocation() const { return DiagObj->CurDiagLoc; }
+  unsigned getID() const { return DiagID; }
+  const SourceLocation &getLocation() const { return DiagLoc; }
   bool hasSourceManager() const { return DiagObj->hasSourceManager(); }
   SourceManager &getSourceManager() const { return DiagObj->getSourceManager();}
 
-  unsigned getNumArgs() const { return DiagObj->DiagStorage.NumDiagArgs; }
+  unsigned getNumArgs() const { return DiagStorage.NumDiagArgs; }
 
   /// Return the kind of the specified index.
   ///
@@ -1597,8 +1514,7 @@ class Diagnostic {
   /// \pre Idx < getNumArgs()
   DiagnosticsEngine::ArgumentKind getArgKind(unsigned Idx) const {
     assert(Idx < getNumArgs() && "Argument index out of range!");
-    return (DiagnosticsEngine::ArgumentKind)
-        DiagObj->DiagStorage.DiagArgumentsKind[Idx];
+    return (DiagnosticsEngine::ArgumentKind)DiagStorage.DiagArgumentsKind[Idx];
   }
 
   /// Return the provided argument string specified by \p Idx.
@@ -1606,7 +1522,7 @@ class Diagnostic {
   const std::string &getArgStdStr(unsigned Idx) const {
     assert(getArgKind(Idx) == DiagnosticsEngine::ak_std_string &&
            "invalid argument accessor!");
-    return DiagObj->DiagStorage.DiagArgumentsStr[Idx];
+    return DiagStorage.DiagArgumentsStr[Idx];
   }
 
   /// Return the specified C string argument.
@@ -1614,8 +1530,7 @@ class Diagnostic {
   const char *getArgCStr(unsigned Idx) const {
     assert(getArgKind(Idx) == DiagnosticsEngine::ak_c_string &&
            "invalid argument accessor!");
-    return reinterpret_cast<const char *>(
-        DiagObj->DiagStorage.DiagArgumentsVal[Idx]);
+    return reinterpret_cast<const char *>(DiagStorage.DiagArgumentsVal[Idx]);
   }
 
   /// Return the specified signed integer argument.
@@ -1623,7 +1538,7 @@ class Diagnostic {
   int64_t getArgSInt(unsigned Idx) const {
     assert(getArgKind(Idx) == DiagnosticsEngine::ak_sint &&
            "invalid argument accessor!");
-    return (int64_t)DiagObj->DiagStorage.DiagArgumentsVal[Idx];
+    return (int64_t)DiagStorage.DiagArgumentsVal[Idx];
   }
 
   /// Return the specified unsigned integer argument.
@@ -1631,7 +1546,7 @@ class Diagnostic {
   uint64_t getArgUInt(unsigned Idx) const {
     assert(getArgKind(Idx) == DiagnosticsEngine::ak_uint &&
            "invalid argument accessor!");
-    return DiagObj->DiagStorage.DiagArgumentsVal[Idx];
+    return DiagStorage.DiagArgumentsVal[Idx];
   }
 
   /// Return the specified IdentifierInfo argument.
@@ -1640,7 +1555,7 @@ class Diagnostic {
     assert(getArgKind(Idx) == DiagnosticsEngine::ak_identifierinfo &&
            "invalid argument accessor!");
     return reinterpret_cast<IdentifierInfo *>(
-        DiagObj->DiagStorage.DiagArgumentsVal[Idx]);
+        DiagStorage.DiagArgumentsVal[Idx]);
   }
 
   /// Return the specified non-string argument in an opaque form.
@@ -1648,37 +1563,32 @@ class Diagnostic {
   uint64_t getRawArg(unsigned Idx) const {
     assert(getArgKind(Idx) != DiagnosticsEngine::ak_std_string &&
            "invalid argument accessor!");
-    return DiagObj->DiagStorage.DiagArgumentsVal[Idx];
+    return DiagStorage.DiagArgumentsVal[Idx];
   }
 
   /// Return the number of source ranges associated with this diagnostic.
-  unsigned getNumRanges() const {
-    return DiagObj->DiagStorage.DiagRanges.size();
-  }
+  unsigned getNumRanges() const { return DiagStorage.DiagRanges.size(); }
 
   /// \pre Idx < getNumRanges()
   const CharSourceRange &getRange(unsigned Idx) const {
     assert(Idx < getNumRanges() && "Invalid diagnostic range index!");
-    return DiagObj->DiagStorage.DiagRanges[Idx];
+    return DiagStorage.DiagRanges[Idx];
   }
 
   /// Return an array reference for this diagnostic's ranges.
-  ArrayRef<CharSourceRange> getRanges() const {
-    return DiagObj->DiagStorage.DiagRanges;
-  }
+  ArrayRef<CharSourceRange> getRanges() const { return DiagStorage.DiagRanges; }
 
-  unsigned getNumFixItHints() const {
-    return DiagObj->DiagStorage.FixItHints.size();
-  }
+  unsigned getNumFixItHints() const { return DiagStorage.FixItHints.size(); }
 
   const FixItHint &getFixItHint(unsigned Idx) const {
     assert(Idx < getNumFixItHints() && "Invalid index!");
-    return DiagObj->DiagStorage.FixItHints[Idx];
+    return DiagStorage.FixItHints[Idx];
   }
 
-  ArrayRef<FixItHint> getFixItHints() const {
-    return DiagObj->DiagStorage.FixItHints;
-  }
+  ArrayRef<FixItHint> getFixItHints() const { return DiagStorage.FixItHints; }
+
+  /// Return the value associated with this diagnostic flag.
+  StringRef getFlagValue() const { return FlagValue; }
 
   /// Format this diagnostic into a string, substituting the
   /// formal arguments into the %0 slots.

diff  --git a/clang/include/clang/Basic/DiagnosticIDs.h b/clang/include/clang/Basic/DiagnosticIDs.h
index daad66f499538f..1fa38ed6066e26 100644
--- a/clang/include/clang/Basic/DiagnosticIDs.h
+++ b/clang/include/clang/Basic/DiagnosticIDs.h
@@ -24,6 +24,7 @@
 
 namespace clang {
   class DiagnosticsEngine;
+  class DiagnosticBuilder;
   class SourceLocation;
 
   // Import the diagnostic enums themselves.
@@ -486,11 +487,13 @@ class DiagnosticIDs : public RefCountedBase<DiagnosticIDs> {
   ///
   /// \returns \c true if the diagnostic was emitted, \c false if it was
   /// suppressed.
-  bool ProcessDiag(DiagnosticsEngine &Diag) const;
+  bool ProcessDiag(DiagnosticsEngine &Diag,
+                   const DiagnosticBuilder &DiagBuilder) const;
 
   /// Used to emit a diagnostic that is finally fully formed,
   /// ignoring suppression.
-  void EmitDiag(DiagnosticsEngine &Diag, Level DiagLevel) const;
+  void EmitDiag(DiagnosticsEngine &Diag, const DiagnosticBuilder &DiagBuilder,
+                Level DiagLevel) const;
 
   /// Whether the diagnostic may leave the AST in a state where some
   /// invariants can break.

diff  --git a/clang/include/clang/Basic/PartialDiagnostic.h b/clang/include/clang/Basic/PartialDiagnostic.h
index 507d789c54ff9b..4bf6049d08fdb4 100644
--- a/clang/include/clang/Basic/PartialDiagnostic.h
+++ b/clang/include/clang/Basic/PartialDiagnostic.h
@@ -166,13 +166,10 @@ class PartialDiagnostic : public StreamingDiagnostic {
 
   void EmitToString(DiagnosticsEngine &Diags,
                     SmallVectorImpl<char> &Buf) const {
-    // FIXME: It should be possible to render a diagnostic to a string without
-    //        messing with the state of the diagnostics engine.
     DiagnosticBuilder DB(Diags.Report(getDiagID()));
     Emit(DB);
-    Diagnostic(&Diags).FormatDiagnostic(Buf);
+    Diagnostic(&Diags, DB).FormatDiagnostic(Buf);
     DB.Clear();
-    Diags.Clear();
   }
 
   /// Clear out this partial diagnostic, giving it a new diagnostic ID

diff  --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 0809ac1b144ef6..e1c3a99cfa167e 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -626,10 +626,10 @@ class Sema final : public SemaBase {
   const llvm::MapVector<FieldDecl *, DeleteLocs> &
   getMismatchingDeleteExpressions() const;
 
-  /// Cause the active diagnostic on the DiagosticsEngine to be
-  /// emitted. This is closely coupled to the SemaDiagnosticBuilder class and
+  /// Cause the built diagnostic to be emitted on the DiagosticsEngine.
+  /// This is closely coupled to the SemaDiagnosticBuilder class and
   /// should not be used elsewhere.
-  void EmitCurrentDiagnostic(unsigned DiagID);
+  void EmitDiagnostic(unsigned DiagID, const DiagnosticBuilder &DB);
 
   void addImplicitTypedef(StringRef Name, QualType T);
 

diff  --git a/clang/lib/Basic/Diagnostic.cpp b/clang/lib/Basic/Diagnostic.cpp
index ecff80a5090630..0bd6845085b735 100644
--- a/clang/lib/Basic/Diagnostic.cpp
+++ b/clang/lib/Basic/Diagnostic.cpp
@@ -126,9 +126,7 @@ void DiagnosticsEngine::Reset(bool soft /*=false*/) {
   TrapNumErrorsOccurred = 0;
   TrapNumUnrecoverableErrorsOccurred = 0;
 
-  CurDiagID = std::numeric_limits<unsigned>::max();
   LastDiagLevel = DiagnosticIDs::Ignored;
-  DelayedDiagID = 0;
 
   if (!soft) {
     // Clear state related to #pragma diagnostic.
@@ -143,23 +141,6 @@ void DiagnosticsEngine::Reset(bool soft /*=false*/) {
   }
 }
 
-void DiagnosticsEngine::SetDelayedDiagnostic(unsigned DiagID, StringRef Arg1,
-                                             StringRef Arg2, StringRef Arg3) {
-  if (DelayedDiagID)
-    return;
-
-  DelayedDiagID = DiagID;
-  DelayedDiagArg1 = Arg1.str();
-  DelayedDiagArg2 = Arg2.str();
-  DelayedDiagArg3 = Arg3.str();
-}
-
-void DiagnosticsEngine::ReportDelayed() {
-  unsigned ID = DelayedDiagID;
-  DelayedDiagID = 0;
-  Report(ID) << DelayedDiagArg1 << DelayedDiagArg2 << DelayedDiagArg3;
-}
-
 DiagnosticMapping &
 DiagnosticsEngine::DiagState::getOrAddMapping(diag::kind Diag) {
   std::pair<iterator, bool> Result =
@@ -503,39 +484,31 @@ void DiagnosticsEngine::setSeverityForAll(diag::Flavor Flavor,
 }
 
 void DiagnosticsEngine::Report(const StoredDiagnostic &storedDiag) {
-  assert(CurDiagID == std::numeric_limits<unsigned>::max() &&
-         "Multiple diagnostics in flight at once!");
-
-  CurDiagLoc = storedDiag.getLocation();
-  CurDiagID = storedDiag.getID();
-  DiagStorage.NumDiagArgs = 0;
-
-  DiagStorage.DiagRanges.clear();
+  DiagnosticStorage DiagStorage;
   DiagStorage.DiagRanges.append(storedDiag.range_begin(),
                                 storedDiag.range_end());
 
-  DiagStorage.FixItHints.clear();
   DiagStorage.FixItHints.append(storedDiag.fixit_begin(),
                                 storedDiag.fixit_end());
 
   assert(Client && "DiagnosticConsumer not set!");
   Level DiagLevel = storedDiag.getLevel();
-  Diagnostic Info(this, storedDiag.getMessage());
+  Diagnostic Info(this, storedDiag.getLocation(), storedDiag.getID(),
+                  DiagStorage, storedDiag.getMessage());
   Client->HandleDiagnostic(DiagLevel, Info);
   if (Client->IncludeInDiagnosticCounts()) {
     if (DiagLevel == DiagnosticsEngine::Warning)
       ++NumWarnings;
   }
-
-  CurDiagID = std::numeric_limits<unsigned>::max();
 }
 
-bool DiagnosticsEngine::EmitCurrentDiagnostic(bool Force) {
+bool DiagnosticsEngine::EmitDiagnostic(const DiagnosticBuilder &DB,
+                                       bool Force) {
   assert(getClient() && "DiagnosticClient not set!");
 
   bool Emitted;
   if (Force) {
-    Diagnostic Info(this);
+    Diagnostic Info(this, DB);
 
     // Figure out the diagnostic level of this message.
     DiagnosticIDs::Level DiagLevel
@@ -544,24 +517,50 @@ bool DiagnosticsEngine::EmitCurrentDiagnostic(bool Force) {
     Emitted = (DiagLevel != DiagnosticIDs::Ignored);
     if (Emitted) {
       // Emit the diagnostic regardless of suppression level.
-      Diags->EmitDiag(*this, DiagLevel);
+      Diags->EmitDiag(*this, DB, DiagLevel);
     }
   } else {
     // Process the diagnostic, sending the accumulated information to the
     // DiagnosticConsumer.
-    Emitted = ProcessDiag();
+    Emitted = ProcessDiag(DB);
   }
 
-  // Clear out the current diagnostic object.
-  Clear();
+  return Emitted;
+}
+
+DiagnosticBuilder::DiagnosticBuilder(DiagnosticsEngine *DiagObj,
+                                     SourceLocation DiagLoc, unsigned DiagID)
+    : StreamingDiagnostic(DiagObj->DiagAllocator), DiagObj(DiagObj),
+      DiagLoc(DiagLoc), DiagID(DiagID), IsActive(true) {
+  assert(DiagObj && "DiagnosticBuilder requires a valid DiagnosticsEngine!");
+}
 
-  // If there was a delayed diagnostic, emit it now.
-  if (!Force && DelayedDiagID)
-    ReportDelayed();
+DiagnosticBuilder::DiagnosticBuilder(const DiagnosticBuilder &D)
+    : StreamingDiagnostic() {
+  DiagLoc = D.DiagLoc;
+  DiagID = D.DiagID;
+  FlagValue = D.FlagValue;
+  DiagObj = D.DiagObj;
+  DiagStorage = D.DiagStorage;
+  D.DiagStorage = nullptr;
+  Allocator = D.Allocator;
+  IsActive = D.IsActive;
+  IsForceEmit = D.IsForceEmit;
+  D.Clear();
+}
 
-  return Emitted;
+Diagnostic::Diagnostic(const DiagnosticsEngine *DO,
+                       const DiagnosticBuilder &DiagBuilder)
+    : DiagObj(DO), DiagLoc(DiagBuilder.DiagLoc), DiagID(DiagBuilder.DiagID),
+      FlagValue(DiagBuilder.FlagValue), DiagStorage(*DiagBuilder.getStorage()) {
 }
 
+Diagnostic::Diagnostic(const DiagnosticsEngine *DO, SourceLocation DiagLoc,
+                       unsigned DiagID, const DiagnosticStorage &DiagStorage,
+                       StringRef StoredDiagMessage)
+    : DiagObj(DO), DiagLoc(DiagLoc), DiagID(DiagID), DiagStorage(DiagStorage),
+      StoredDiagMessage(StoredDiagMessage) {}
+
 DiagnosticConsumer::~DiagnosticConsumer() = default;
 
 void DiagnosticConsumer::HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
@@ -1216,13 +1215,13 @@ bool ForwardingDiagnosticConsumer::IncludeInDiagnosticCounts() const {
   return Target.IncludeInDiagnosticCounts();
 }
 
-PartialDiagnostic::DiagStorageAllocator::DiagStorageAllocator() {
+DiagStorageAllocator::DiagStorageAllocator() {
   for (unsigned I = 0; I != NumCached; ++I)
     FreeList[I] = Cached + I;
   NumFreeListEntries = NumCached;
 }
 
-PartialDiagnostic::DiagStorageAllocator::~DiagStorageAllocator() {
+DiagStorageAllocator::~DiagStorageAllocator() {
   // Don't assert if we are in a CrashRecovery context, as this invariant may
   // be invalidated during a crash.
   assert((NumFreeListEntries == NumCached ||

diff  --git a/clang/lib/Basic/DiagnosticIDs.cpp b/clang/lib/Basic/DiagnosticIDs.cpp
index cae6642bd4bd3e..031d9d7817d1f6 100644
--- a/clang/lib/Basic/DiagnosticIDs.cpp
+++ b/clang/lib/Basic/DiagnosticIDs.cpp
@@ -566,7 +566,7 @@ DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
 
   // If explicitly requested, map fatal errors to errors.
   if (Result == diag::Severity::Fatal &&
-      Diag.CurDiagID != diag::fatal_too_many_errors && Diag.FatalsAsError)
+      DiagID != diag::fatal_too_many_errors && Diag.FatalsAsError)
     Result = diag::Severity::Error;
 
   bool ShowInSystemHeader =
@@ -800,8 +800,9 @@ StringRef DiagnosticIDs::getNearestOption(diag::Flavor Flavor,
 
 /// ProcessDiag - This is the method used to report a diagnostic that is
 /// finally fully formed.
-bool DiagnosticIDs::ProcessDiag(DiagnosticsEngine &Diag) const {
-  Diagnostic Info(&Diag);
+bool DiagnosticIDs::ProcessDiag(DiagnosticsEngine &Diag,
+                                const DiagnosticBuilder &DiagBuilder) const {
+  Diagnostic Info(&Diag, DiagBuilder);
 
   assert(Diag.getClient() && "DiagnosticClient not set!");
 
@@ -867,22 +868,24 @@ bool DiagnosticIDs::ProcessDiag(DiagnosticsEngine &Diag) const {
     // stop a flood of bogus errors.
     if (Diag.ErrorLimit && Diag.NumErrors > Diag.ErrorLimit &&
         DiagLevel == DiagnosticIDs::Error) {
-      Diag.SetDelayedDiagnostic(diag::fatal_too_many_errors);
+      Diag.Report(diag::fatal_too_many_errors);
       return false;
     }
   }
 
   // Make sure we set FatalErrorOccurred to ensure that the notes from the
   // diagnostic that caused `fatal_too_many_errors` won't be emitted.
-  if (Diag.CurDiagID == diag::fatal_too_many_errors)
+  if (Info.getID() == diag::fatal_too_many_errors)
     Diag.FatalErrorOccurred = true;
   // Finally, report it.
-  EmitDiag(Diag, DiagLevel);
+  EmitDiag(Diag, DiagBuilder, DiagLevel);
   return true;
 }
 
-void DiagnosticIDs::EmitDiag(DiagnosticsEngine &Diag, Level DiagLevel) const {
-  Diagnostic Info(&Diag);
+void DiagnosticIDs::EmitDiag(DiagnosticsEngine &Diag,
+                             const DiagnosticBuilder &DiagBuilder,
+                             Level DiagLevel) const {
+  Diagnostic Info(&Diag, DiagBuilder);
   assert(DiagLevel != DiagnosticIDs::Ignored && "Cannot emit ignored diagnostics!");
 
   Diag.Client->HandleDiagnostic((DiagnosticsEngine::Level)DiagLevel, Info);
@@ -890,8 +893,6 @@ void DiagnosticIDs::EmitDiag(DiagnosticsEngine &Diag, Level DiagLevel) const {
     if (DiagLevel == DiagnosticIDs::Warning)
       ++Diag.NumWarnings;
   }
-
-  Diag.CurDiagID = ~0U;
 }
 
 bool DiagnosticIDs::isUnrecoverable(unsigned DiagID) const {

diff  --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
index d6ec26af80aadd..65a8a7253e054f 100644
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -130,13 +130,8 @@ ContentCache::getBufferOrNone(DiagnosticsEngine &Diag, FileManager &FM,
   // the file could also have been removed during processing. Since we can't
   // really deal with this situation, just create an empty buffer.
   if (!BufferOrError) {
-    if (Diag.isDiagnosticInFlight())
-      Diag.SetDelayedDiagnostic(diag::err_cannot_open_file,
-                                ContentsEntry->getName(),
-                                BufferOrError.getError().message());
-    else
-      Diag.Report(Loc, diag::err_cannot_open_file)
-          << ContentsEntry->getName() << BufferOrError.getError().message();
+    Diag.Report(Loc, diag::err_cannot_open_file)
+        << ContentsEntry->getName() << BufferOrError.getError().message();
 
     return std::nullopt;
   }
@@ -153,12 +148,7 @@ ContentCache::getBufferOrNone(DiagnosticsEngine &Diag, FileManager &FM,
   // ContentsEntry::getSize() could have the wrong size. Use
   // MemoryBuffer::getBufferSize() instead.
   if (Buffer->getBufferSize() >= std::numeric_limits<unsigned>::max()) {
-    if (Diag.isDiagnosticInFlight())
-      Diag.SetDelayedDiagnostic(diag::err_file_too_large,
-                                ContentsEntry->getName());
-    else
-      Diag.Report(Loc, diag::err_file_too_large)
-        << ContentsEntry->getName();
+    Diag.Report(Loc, diag::err_file_too_large) << ContentsEntry->getName();
 
     return std::nullopt;
   }
@@ -168,12 +158,7 @@ ContentCache::getBufferOrNone(DiagnosticsEngine &Diag, FileManager &FM,
   // have come from a stat cache).
   if (!ContentsEntry->isNamedPipe() &&
       Buffer->getBufferSize() != (size_t)ContentsEntry->getSize()) {
-    if (Diag.isDiagnosticInFlight())
-      Diag.SetDelayedDiagnostic(diag::err_file_modified,
-                                ContentsEntry->getName());
-    else
-      Diag.Report(Loc, diag::err_file_modified)
-        << ContentsEntry->getName();
+    Diag.Report(Loc, diag::err_file_modified) << ContentsEntry->getName();
 
     return std::nullopt;
   }

diff  --git a/clang/lib/Frontend/Rewrite/FixItRewriter.cpp b/clang/lib/Frontend/Rewrite/FixItRewriter.cpp
index 44dfaf20eae73f..7309553e3bc0b4 100644
--- a/clang/lib/Frontend/Rewrite/FixItRewriter.cpp
+++ b/clang/lib/Frontend/Rewrite/FixItRewriter.cpp
@@ -200,10 +200,8 @@ void FixItRewriter::HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
 /// Emit a diagnostic via the adapted diagnostic client.
 void FixItRewriter::Diag(SourceLocation Loc, unsigned DiagID) {
   // When producing this diagnostic, we temporarily bypass ourselves,
-  // clear out any current diagnostic, and let the downstream client
-  // format the diagnostic.
+  // and let the downstream client format the diagnostic.
   Diags.setClient(Client, false);
-  Diags.Clear();
   Diags.Report(Loc, DiagID);
   Diags.setClient(this, false);
 }

diff  --git a/clang/lib/Frontend/TextDiagnosticPrinter.cpp b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
index c2fea3d03f0c0f..28f7218dc23f54 100644
--- a/clang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -84,7 +84,7 @@ static void printDiagnosticOptions(raw_ostream &OS,
     if (!Opt.empty()) {
       OS << (Started ? "," : " [")
          << (Level == DiagnosticsEngine::Remark ? "-R" : "-W") << Opt;
-      StringRef OptValue = Info.getDiags()->getFlagValue();
+      StringRef OptValue = Info.getFlagValue();
       if (!OptValue.empty())
         OS << "=" << OptValue;
       Started = true;

diff  --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 85cbbe7750c2b3..69d72412471809 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1590,7 +1590,7 @@ LangAS Sema::getDefaultCXXMethodAddrSpace() const {
   return LangAS::Default;
 }
 
-void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
+void Sema::EmitDiagnostic(unsigned DiagID, const DiagnosticBuilder &DB) {
   // FIXME: It doesn't make sense to me that DiagID is an incoming argument here
   // and yet we also use the current diag ID on the DiagnosticsEngine. This has
   // been made more painfully obvious by the refactor that introduced this
@@ -1598,9 +1598,9 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
   // eliminated. If it truly cannot be (for example, there is some reentrancy
   // issue I am not seeing yet), then there should at least be a clarifying
   // comment somewhere.
+  Diagnostic DiagInfo(&Diags, DB);
   if (std::optional<TemplateDeductionInfo *> Info = isSFINAEContext()) {
-    switch (DiagnosticIDs::getDiagnosticSFINAEResponse(
-              Diags.getCurrentDiagID())) {
+    switch (DiagnosticIDs::getDiagnosticSFINAEResponse(DiagInfo.getID())) {
     case DiagnosticIDs::SFINAE_Report:
       // We'll report the diagnostic below.
       break;
@@ -1613,13 +1613,11 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
       // Make a copy of this suppressed diagnostic and store it with the
       // template-deduction information.
       if (*Info && !(*Info)->hasSFINAEDiagnostic()) {
-        Diagnostic DiagInfo(&Diags);
         (*Info)->addSFINAEDiagnostic(DiagInfo.getLocation(),
                        PartialDiagnostic(DiagInfo, Context.getDiagAllocator()));
       }
 
       Diags.setLastDiagnosticIgnored(true);
-      Diags.Clear();
       return;
 
     case DiagnosticIDs::SFINAE_AccessControl: {
@@ -1630,7 +1628,7 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
       if (!AccessCheckingSFINAE && !getLangOpts().CPlusPlus11)
         break;
 
-      SourceLocation Loc = Diags.getCurrentDiagLoc();
+      SourceLocation Loc = DiagInfo.getLocation();
 
       // Suppress this diagnostic.
       ++NumSFINAEErrors;
@@ -1638,16 +1636,13 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
       // Make a copy of this suppressed diagnostic and store it with the
       // template-deduction information.
       if (*Info && !(*Info)->hasSFINAEDiagnostic()) {
-        Diagnostic DiagInfo(&Diags);
         (*Info)->addSFINAEDiagnostic(DiagInfo.getLocation(),
                        PartialDiagnostic(DiagInfo, Context.getDiagAllocator()));
       }
 
       Diags.setLastDiagnosticIgnored(true);
-      Diags.Clear();
 
-      // Now the diagnostic state is clear, produce a C++98 compatibility
-      // warning.
+      // Now produce a C++98 compatibility warning.
       Diag(Loc, diag::warn_cxx98_compat_sfinae_access_control);
 
       // The last diagnostic which Sema produced was ignored. Suppress any
@@ -1660,14 +1655,12 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
       // Make a copy of this suppressed diagnostic and store it with the
       // template-deduction information;
       if (*Info) {
-        Diagnostic DiagInfo(&Diags);
         (*Info)->addSuppressedDiagnostic(DiagInfo.getLocation(),
                        PartialDiagnostic(DiagInfo, Context.getDiagAllocator()));
       }
 
       // Suppress this diagnostic.
       Diags.setLastDiagnosticIgnored(true);
-      Diags.Clear();
       return;
     }
   }
@@ -1677,7 +1670,7 @@ void Sema::EmitCurrentDiagnostic(unsigned DiagID) {
   Context.setPrintingPolicy(getPrintingPolicy());
 
   // Emit the diagnostic.
-  if (!Diags.EmitCurrentDiagnostic())
+  if (!Diags.EmitDiagnostic(DB))
     return;
 
   // If this is not a note, and we're in a template instantiation

diff  --git a/clang/lib/Sema/SemaBase.cpp b/clang/lib/Sema/SemaBase.cpp
index a2f12d622e8ccc..5c24f21b469b01 100644
--- a/clang/lib/Sema/SemaBase.cpp
+++ b/clang/lib/Sema/SemaBase.cpp
@@ -26,7 +26,7 @@ SemaBase::ImmediateDiagBuilder::~ImmediateDiagBuilder() {
   Clear();
 
   // Dispatch to Sema to emit the diagnostic.
-  SemaRef.EmitCurrentDiagnostic(DiagID);
+  SemaRef.EmitDiagnostic(DiagID, *this);
 }
 
 PartialDiagnostic SemaBase::PDiag(unsigned DiagID) {

diff  --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 4fae6ff02ea9a3..7efcc81e194d95 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1382,7 +1382,7 @@ bool ASTReader::ReadVisibleDeclContextStorage(ModuleFile &M,
 
 void ASTReader::Error(StringRef Msg) const {
   Error(diag::err_fe_pch_malformed, Msg);
-  if (PP.getLangOpts().Modules && !Diags.isDiagnosticInFlight() &&
+  if (PP.getLangOpts().Modules &&
       !PP.getHeaderSearchInfo().getModuleCachePath().empty()) {
     Diag(diag::note_module_cache_path)
       << PP.getHeaderSearchInfo().getModuleCachePath();
@@ -1391,10 +1391,7 @@ void ASTReader::Error(StringRef Msg) const {
 
 void ASTReader::Error(unsigned DiagID, StringRef Arg1, StringRef Arg2,
                       StringRef Arg3) const {
-  if (Diags.isDiagnosticInFlight())
-    Diags.SetDelayedDiagnostic(DiagID, Arg1, Arg2, Arg3);
-  else
-    Diag(DiagID) << Arg1 << Arg2 << Arg3;
+  Diag(DiagID) << Arg1 << Arg2 << Arg3;
 }
 
 void ASTReader::Error(llvm::Error &&Err) const {
@@ -2713,7 +2710,7 @@ InputFile ASTReader::getInputFile(ModuleFile &F, unsigned ID, bool Complain) {
 
   // For an overridden file, there is nothing to validate.
   if (!Overridden && FileChange.Kind != Change::None) {
-    if (Complain && !Diags.isDiagnosticInFlight()) {
+    if (Complain) {
       // Build a list of the PCH imports that got us here (in reverse).
       SmallVector<ModuleFile *, 4> ImportStack(1, &F);
       while (!ImportStack.back()->ImportedBy.empty())
@@ -3689,10 +3686,8 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
           SourceMgr.AllocateLoadedSLocEntries(F.LocalNumSLocEntries,
                                               SLocSpaceSize);
       if (!F.SLocEntryBaseID) {
-        if (!Diags.isDiagnosticInFlight()) {
-          Diags.Report(SourceLocation(), diag::remark_sloc_usage);
-          SourceMgr.noteSLocAddressSpaceUsage(Diags);
-        }
+        Diags.Report(SourceLocation(), diag::remark_sloc_usage);
+        SourceMgr.noteSLocAddressSpaceUsage(Diags);
         return llvm::createStringError(std::errc::invalid_argument,
                                        "ran out of source locations");
       }

diff  --git a/clang/test/PCH/race-condition.cpp b/clang/test/PCH/race-condition.cpp
new file mode 100644
index 00000000000000..752b0cc3ff6286
--- /dev/null
+++ b/clang/test/PCH/race-condition.cpp
@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -fallow-pch-with-compiler-errors -std=c++20 -x c++-header -emit-pch %s -o %t -verify
+// RUN: %clang_cc1 -fallow-pch-with-compiler-errors -std=c++20 -include-pch %t %s -verify
+#ifndef HEADER_H
+#define HEADER_H
+
+#include "bad_include.h"
+// expected-error at 6{{'bad_include.h' file not found}}
+
+template <bool, class = void> struct enable_if {};
+template <class T> struct enable_if<true, T> { typedef T type; };
+template <bool B, class T = void> using enable_if_t = typename enable_if<B, T>::type;
+
+template <typename> struct meta { static constexpr int value = 0; };
+template <> struct meta<int> { static constexpr int value = 1; };
+template <> struct meta<float> { static constexpr int value = 2; };
+
+namespace N {
+inline namespace inner {
+
+template <class T>
+constexpr enable_if_t<meta<T>::value == 0, void> midpoint(T) {}
+
+template <class U>
+constexpr enable_if_t<meta<U>::value == 1, void> midpoint(U) {}
+
+template <class F>
+constexpr enable_if_t<meta<F>::value == 2, void> midpoint(F) {}
+
+} // namespace inner
+} // namespace N
+
+#else
+
+// expected-error at 27{{'N::midpoint' has 
diff erent definitions in 
diff erent modules; defined here first 
diff erence is 1st parameter with type 'F'}}
+// expected-error at 24{{'N::midpoint' has 
diff erent definitions in 
diff erent modules; defined here first 
diff erence is 1st parameter with type 'U'}}
+// expected-note at 21{{but in '' found 1st parameter with type 'T'}}
+int x = N::something;
+// expected-error at 37{{no member named 'something' in namespace 'N'}}
+// expected-note at 21{{but in '' found 1st parameter with type 'T'}}
+
+#endif

diff  --git a/clang/unittests/Basic/DiagnosticTest.cpp b/clang/unittests/Basic/DiagnosticTest.cpp
index 74690193917165..691d74f697f278 100644
--- a/clang/unittests/Basic/DiagnosticTest.cpp
+++ b/clang/unittests/Basic/DiagnosticTest.cpp
@@ -17,9 +17,6 @@ using namespace llvm;
 using namespace clang;
 
 void clang::DiagnosticsTestHelper(DiagnosticsEngine &diag) {
-  unsigned delayedDiagID = 0U;
-
-  EXPECT_EQ(diag.DelayedDiagID, delayedDiagID);
   EXPECT_FALSE(diag.DiagStates.empty());
   EXPECT_TRUE(diag.DiagStatesByLoc.empty());
   EXPECT_TRUE(diag.DiagStateOnPushStack.empty());
@@ -83,6 +80,21 @@ TEST(DiagnosticTest, fatalsAsError) {
   }
 }
 
+TEST(DiagnosticTest, tooManyErrorsIsAlwaysFatal) {
+  DiagnosticsEngine Diags(new DiagnosticIDs(), new DiagnosticOptions,
+                          new IgnoringDiagConsumer());
+  Diags.setFatalsAsError(true);
+
+  // Report a fatal_too_many_errors diagnostic to ensure that still
+  // acts as a fatal error despite downgrading fatal errors to errors.
+  Diags.Report(diag::fatal_too_many_errors);
+  EXPECT_TRUE(Diags.hasFatalErrorOccurred());
+
+  // Ensure that the severity of that diagnostic is really "fatal".
+  EXPECT_EQ(Diags.getDiagnosticLevel(diag::fatal_too_many_errors, {}),
+            DiagnosticsEngine::Level::Fatal);
+}
+
 // Check that soft RESET works as intended
 TEST(DiagnosticTest, softReset) {
   DiagnosticsEngine Diags(new DiagnosticIDs(), new DiagnosticOptions,
@@ -104,7 +116,6 @@ TEST(DiagnosticTest, softReset) {
   // Check for private variables of DiagnosticsEngine 
diff erentiating soft reset
   DiagnosticsTestHelper(Diags);
 
-  EXPECT_FALSE(Diags.isDiagnosticInFlight());
   EXPECT_TRUE(Diags.isLastDiagnosticIgnored());
 }
 

diff  --git a/clang/unittests/Driver/DXCModeTest.cpp b/clang/unittests/Driver/DXCModeTest.cpp
index 41ab30bc81d5f9..2a079a62f1bc13 100644
--- a/clang/unittests/Driver/DXCModeTest.cpp
+++ b/clang/unittests/Driver/DXCModeTest.cpp
@@ -51,7 +51,6 @@ static void validateTargetProfile(
   EXPECT_TRUE(C);
   EXPECT_EQ(Diags.getNumErrors(), NumOfErrors);
   EXPECT_STREQ(DiagConsumer->Errors.back().c_str(), ExpectError.data());
-  Diags.Clear();
   DiagConsumer->clear();
 }
 
@@ -160,7 +159,6 @@ TEST(DxcModeTest, ValidatorVersionValidation) {
       DiagConsumer->Errors.back().c_str(),
       "invalid validator version : 0.1; if validator major version is 0, "
       "minor version must also be 0");
-  Diags.Clear();
   DiagConsumer->clear();
 
   Args = TheDriver.ParseArgStrings({"-validator-version", "1"}, false,
@@ -176,7 +174,6 @@ TEST(DxcModeTest, ValidatorVersionValidation) {
   EXPECT_STREQ(DiagConsumer->Errors.back().c_str(),
                "invalid validator version : 1; format of validator version is "
                "\"<major>.<minor>\" (ex:\"1.4\")");
-  Diags.Clear();
   DiagConsumer->clear();
 
   Args = TheDriver.ParseArgStrings({"-validator-version", "-Tlib_6_7"}, false,
@@ -193,7 +190,6 @@ TEST(DxcModeTest, ValidatorVersionValidation) {
       DiagConsumer->Errors.back().c_str(),
       "invalid validator version : -Tlib_6_7; format of validator version is "
       "\"<major>.<minor>\" (ex:\"1.4\")");
-  Diags.Clear();
   DiagConsumer->clear();
 
   Args = TheDriver.ParseArgStrings({"-validator-version", "foo"}, false,
@@ -210,7 +206,6 @@ TEST(DxcModeTest, ValidatorVersionValidation) {
       DiagConsumer->Errors.back().c_str(),
       "invalid validator version : foo; format of validator version is "
       "\"<major>.<minor>\" (ex:\"1.4\")");
-  Diags.Clear();
   DiagConsumer->clear();
 }
 

diff  --git a/flang/lib/Frontend/TextDiagnosticPrinter.cpp b/flang/lib/Frontend/TextDiagnosticPrinter.cpp
index 8b00fb69b3cefb..dc182d68a1a979 100644
--- a/flang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/flang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -45,7 +45,7 @@ static void printRemarkOption(llvm::raw_ostream &os,
     // warning could be printed i.e. [-Wunknown-warning-option]
     os << " [" << (level == clang::DiagnosticsEngine::Remark ? "-R" : "-W")
        << opt;
-    llvm::StringRef optValue = info.getDiags()->getFlagValue();
+    llvm::StringRef optValue = info.getFlagValue();
     if (!optValue.empty())
       os << "=" << optValue;
     os << ']';


        


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