[clang-tidy][readability-braces-around-statements] ignore false-positive for constexpr if statement in lambda expression

[HerrCai0907]

Summary

Fixed: #64545

When TreeTransform, Stmt in constexpr IfStmt will be transform to NullStmt.
This NullStmt has the different beginning token.
This patch add addtional check in checkStmt to handle this case.

[Eugene.Zelenko]

clang-tools-extra/docs/ReleaseNotes.rst

233

if constexpr? Please also enclose it in double back-ticks.

[PiotrZSL]

LGTM

[HerrCai0907]

update release note

clang-tools-extra/clang-tidy/readability/BracesAroundStatementsCheck.cpp

 /// Returns true if braces where added.
 bool BracesAroundStatementsCheck::checkStmt(
     const MatchFinder::MatchResult &Result, const Stmt *S,
     SourceLocation InitialLoc, SourceLocation EndLocHint) {
 
   while (const auto *AS = dyn_cast<AttributedStmt>(S))
     S = AS->getSubStmt();
 
+  const SourceManager &SM = *Result.SourceManager;
+  const ASTContext *Context = Result.Context;
+
   // 1) If there's a corresponding "else" or "while", the check inserts "} "
   // right before that token.
   // 2) If there's a multi-line block comment starting on the same line after
   // the location we're inserting the closing brace at, or there's a non-comment
   // token, the check inserts "\n}" right before that token.
   // 3) Otherwise the check finds the end of line (possibly after some block or
   // line comments) and inserts "\n}" right before that EOL.
   if (!S || isa<CompoundStmt>(S)) {
     // Already inside braces.
     return false;
   }
 
+  // When TreeTransform, Stmt in constexpr IfStmt will be transform to NullStmt.
+  // This NullStmt can be detected according to beginning token.
+  const SourceLocation StmtBeginLoc = S->getBeginLoc();
+  if (isa<NullStmt>(S) && StmtBeginLoc.isValid() &&
+      getTokenKind(StmtBeginLoc, SM, Context) == tok::l_brace)
+    return false;
+
   if (!InitialLoc.isValid())
     return false;
-  const SourceManager &SM = *Result.SourceManager;
-  const ASTContext *Context = Result.Context;
 
   // Convert InitialLoc to file location, if it's on the same macro expansion
   // level as the start of the statement. We also need file locations for
   // Lexer::getLocForEndOfToken working properly.
   InitialLoc = Lexer::makeFileCharRange(
                    CharSourceRange::getCharRange(InitialLoc, S->getBeginLoc()),
                    SM, Context->getLangOpts())
                    .getBegin();

clang-tools-extra/docs/ReleaseNotes.rst

 - Improved :doc:`performance-faster-string-find
   <clang-tidy/checks/performance/faster-string-find>` check to properly escape
   single quotes.
 
 - Improved :doc:`performance-noexcept-swap
   <clang-tidy/checks/performance/noexcept-swap>` check to enforce a stricter
   match with the swap function signature, eliminating false-positives.
 
+- Improved :doc:`readability-braces-around-statements
+  <clang-tidy/checks/readability/braces-around-statements>` check to
+  ignore false-positive for ``if constexpr`` in lambda expression.
+
 - Improved :doc:`readability-container-size-empty
   <clang-tidy/checks/readability/container-size-empty>` check to
   detect comparison between string and empty string literals.
 
 - Improved :doc:`readability-identifier-naming
   <clang-tidy/checks/readability/identifier-naming>` check to emit proper
   warnings when a type forward declaration precedes its definition.
 

clang-tools-extra/test/clang-tidy/checkers/readability/braces-around-statements.cpp

int test_macros(bool b) {
   // CHECK-MESSAGES: :[[@LINE-3]]:7: warning: statement should be inside braces
   // CHECK-FIXES: if (b) {
   // CHECK-FIXES-NEXT: return 1;
   // CHECK-FIXES-NEXT: } else {
   // CHECK-FIXES-NEXT: M(return 2);
   // CHECK-FIXES-NEXT: }
 
 }
+
+template <bool A>
+auto constexpr_lambda_1 = [] {
+  if constexpr (A) {
+    1;
+  }
+};
+
+template <bool A>
+auto constexpr_lambda_2 = [] {
+  // CHECK-MESSAGES: :[[@LINE+1]]:19: warning: statement should be inside braces
+  if constexpr (A)
+    1;
+  // CHECK-FIXES:if constexpr (A) {
+  // CHECK-FIXES-NEXT:1;
+  // CHECK-FIXES-NEXT:}
+};
+
+void test_constexpr() {
+  constexpr_lambda_1<false>();
+  constexpr_lambda_2<false>();
+}