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

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

Differential Revision: https://reviews.llvm.org/D115576

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

@@ -24,9 +24,11 @@ void ThrowKeywordMissingCheck::registerMatchers(MatchFinder *Finder) {
       cxxConstructExpr(
           hasType(cxxRecordDecl(
               isSameOrDerivedFrom(matchesName("[Ee]xception|EXCEPTION")))),
-          unless(anyOf(hasAncestor(stmt(
+          unless(anyOf(
-                           anyOf(cxxThrowExpr(), callExpr(), returnStmt()))),
+              hasAncestor(
+                  stmt(anyOf(cxxThrowExpr(), callExpr(), returnStmt()))),
               hasAncestor(decl(anyOf(varDecl(), fieldDecl()))),
+              hasAncestor(expr(cxxNewExpr(hasAnyPlacementArg(anything())))),
               allOf(hasAncestor(CtorInitializerList),
                     unless(hasAncestor(cxxCatchStmt()))))))
           .bind("temporary-exception-not-thrown"),

clang-tools-extra/docs/ReleaseNotes.rst

@@ -149,6 +149,10 @@ Changes in existing checks
 - 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
 ^^^^^^^^^^^^^^
 

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

@@ -175,3 +175,14 @@ struct 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{};
+}