[clang-tidy][#51939] Exempt placement-new expressions from 'bugprone-throw-keyword-missing'

[zero9178]

Summary

The purpose of this checker is to flag a missing throw keyword, and does so by checking for the construction of an exception class that is then unused.
This works great except that placement new expressions are also flagged as those lead to the construction of an object as well, even though they are not temporary (as that is dependent on the storage).
This patch fixes the issue by exempting the match if it is within a placement-new.

Fixes https://github.com/llvm/llvm-project/issues/51939

[aaron.ballman]

LGTM! Can you also add a release note about the fix?

[zero9178]

In D115576#3194944, @aaron.ballman wrote:

LGTM! Can you also add a release note about the fix?

Addressed in the final commit

clang-tools-extra/clang-tidy/bugprone/ThrowKeywordMissingCheck.cpp

 void ThrowKeywordMissingCheck::registerMatchers(MatchFinder *Finder) {
   auto CtorInitializerList =
       cxxConstructorDecl(hasAnyConstructorInitializer(anything()));
 
   Finder->addMatcher(
       cxxConstructExpr(
           hasType(cxxRecordDecl(
               isSameOrDerivedFrom(matchesName("[Ee]xception|EXCEPTION")))),
-          unless(anyOf(hasAncestor(stmt(
-                           anyOf(cxxThrowExpr(), callExpr(), returnStmt()))),
-                       hasAncestor(decl(anyOf(varDecl(), fieldDecl()))),
-                       allOf(hasAncestor(CtorInitializerList),
-                             unless(hasAncestor(cxxCatchStmt()))))))
+          unless(anyOf(
+              hasAncestor(
+                  stmt(anyOf(cxxThrowExpr(), callExpr(), returnStmt()))),
+              hasAncestor(expr(cxxNewExpr(hasAnyPlacementArg(anything())))),
           .bind("temporary-exception-not-thrown"),
       this);
 }
 
 void ThrowKeywordMissingCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *TemporaryExpr =
       Result.Nodes.getNodeAs<Expr>("temporary-exception-not-thrown");
 
   diag(TemporaryExpr->getBeginLoc(), "suspicious exception object created but "
                                      "not thrown; did you mean 'throw %0'?")
       << TemporaryExpr->getType().getBaseTypeIdentifier()->getName();
 }
 
 } // namespace bugprone
 } // namespace tidy
 } // namespace clang

clang-tools-extra/docs/ReleaseNotes.rst

- Removed default setting ``cppcoreguidelines-explicit-virtual-functions.IgnoreDestructors = "true"``,
   to match the current state of the C++ Core Guidelines.
 
 - Updated :doc:`google-readability-casting
   <clang-tidy/checks/google-readability-casting>` to diagnose and fix functional
   casts, to achieve feature parity with the corresponding ``cpplint.py`` check.
 
 - Fixed a false positive in :doc:`fuchsia-trailing-return
   <clang-tidy/checks/fuchsia-trailing-return>` for C++17 deduction guides.
   
+- Fixed a false positive in :doc:`bugprone-throw-keyword-missing
+  <clang-tidy/checks/bugprone-throw-keyword-missing>` when creating an exception object
+  using placement new
+
 Removed checks
 ^^^^^^^^^^^^^^
 
 Improvements to include-fixer
 -----------------------------
 
 The improvements are...
 

clang-tools-extra/test/clang-tidy/checkers/bugprone-throw-keyword-missing.cpp

 struct ExceptionRAII {
   ExceptionRAII() {}
   ~ExceptionRAII() {}
 };
 
 void exceptionRAIITest() {
   ExceptionRAII E;
 }
+
+namespace std {
+typedef decltype(sizeof(void*)) size_t;
+}
+
+void* operator new(std::size_t, void*);
+
+void placeMentNewTest() {
+  alignas(RegularException) unsigned char expr[sizeof(RegularException)];
+  new (expr) RegularException{};
+}