[clang/Diagnostic] Use `optional` to disambiguate between a `StoredDiagMessage` that is not set vs set as empty string

[akyrtzi]

Summary

"But when would you have a completely empty diagnostic message", you ask dear reader?
That is when there is an empty "#warning" in code.

rdar://106155415

[jansvoboda11]

LGTM

[akyrtzi]

Avoid passing a new IgnoringDiagConsumer for the test since it's unused.

[dblaikie]

"But when would you have a completely empty diagnostic message", you ask dear reader?
That is when there is an empty "#warning" in code.

Is this observable from a clang command line tool? Be nice to have a lit test.

[akyrtzi]

Is this observable from a clang command line tool? Be nice to have a lit test.

Added lit test here: https://reviews.llvm.org/D145473

clang/include/clang/Basic/Diagnostic.h

 // Diagnostic
 //===----------------------------------------------------------------------===//
 
 /// 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.
 class Diagnostic {
   const DiagnosticsEngine *DiagObj;
-  StringRef StoredDiagMessage;
+  std::optional<StringRef> StoredDiagMessage;
 
 public:
   explicit Diagnostic(const DiagnosticsEngine *DO) : DiagObj(DO) {}
   Diagnostic(const DiagnosticsEngine *DO, StringRef storedDiagMessage)
       : DiagObj(DO), StoredDiagMessage(storedDiagMessage) {}
 
   const DiagnosticsEngine *getDiags() const { return DiagObj; }
   unsigned getID() const { return DiagObj->CurDiagID; }

clang/lib/Basic/Diagnostic.cpp

static const char *getTokenDescForDiagnostic(tok::TokenKind Kind) {
   }
 }
 
 /// FormatDiagnostic - Format this diagnostic into a string, substituting the
 /// formal arguments into the %0 slots.  The result is appended onto the Str
 /// array.
 void Diagnostic::
 FormatDiagnostic(SmallVectorImpl<char> &OutStr) const {
-  if (!StoredDiagMessage.empty()) {
-    OutStr.append(StoredDiagMessage.begin(), StoredDiagMessage.end());
+  if (StoredDiagMessage.has_value()) {
+    OutStr.append(StoredDiagMessage->begin(), StoredDiagMessage->end());
     return;
   }
 
   StringRef Diag =
     getDiags()->getDiagnosticIDs()->getDescription(getID());
 
   FormatDiagnostic(Diag.begin(), Diag.end(), OutStr);
 }

clang/unittests/Basic/DiagnosticTest.cpp

 //===- unittests/Basic/DiagnosticTest.cpp -- Diagnostic engine tests ------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticError.h"
 #include "clang/Basic/DiagnosticIDs.h"
+#include "clang/Basic/DiagnosticLex.h"
 #include "gtest/gtest.h"
 #include <optional>
 
 using namespace llvm;
 using namespace clang;
 
 void clang::DiagnosticsTestHelper(DiagnosticsEngine &diag) {
   unsigned delayedDiagID = 0U;
TEST(DiagnosticTest, diagnosticError) {
   EXPECT_EQ(ErrDiag->first, SourceLocation());
   EXPECT_EQ(ErrDiag->second.getDiagID(), diag::err_cannot_open_file);
 
   Value = std::make_pair(20, 1);
   ASSERT_FALSE(!Value);
   EXPECT_EQ(*Value, std::make_pair(20, 1));
   EXPECT_EQ(Value->first, 20);
 }
+
+TEST(DiagnosticTest, storedDiagEmptyWarning) {
+  DiagnosticsEngine Diags(new DiagnosticIDs(), new DiagnosticOptions);
+
+  class CaptureDiagnosticConsumer : public DiagnosticConsumer {
+  public:
+    SmallVector<StoredDiagnostic> StoredDiags;
+
+    void HandleDiagnostic(DiagnosticsEngine::Level level,
+                          const Diagnostic &Info) override {
+      StoredDiags.push_back(StoredDiagnostic(level, Info));
+    }
+  };
+
+  CaptureDiagnosticConsumer CaptureConsumer;
+  Diags.setClient(&CaptureConsumer, /*ShouldOwnClient=*/false);
+  Diags.Report(diag::pp_hash_warning) << "";
+  ASSERT_TRUE(CaptureConsumer.StoredDiags.size() == 1);
+
+  // Make sure an empty warning can round-trip with \c StoredDiagnostic.
+  Diags.Report(CaptureConsumer.StoredDiags.front());
+}
 }