[clang][dataflow] Treat BuiltinBitCastExpr correctly in PropagateResultObject(). (#88875)

This patch includes a test that assert-fails without the fix.

Author: martinboehme

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

@@ -419,7 +419,11 @@ class ResultObjectVisitor : public RecursiveASTVisitor<ResultObjectVisitor> {
     // below them can initialize the same object (or part of it).
     if (isa<CXXConstructExpr>(E) || isa<CallExpr>(E) || isa<LambdaExpr>(E) ||
         isa<CXXDefaultArgExpr>(E) || isa<CXXDefaultInitExpr>(E) ||
-        isa<CXXStdInitializerListExpr>(E)) {
+        isa<CXXStdInitializerListExpr>(E) ||
+        // We treat `BuiltinBitCastExpr` as an "original initializer" too as
+        // it may not even be casting from a record type -- and even if it is,
+        // the two objects are in general of unrelated type.
+        isa<BuiltinBitCastExpr>(E)) {
       return;
     }
     if (auto *Op = dyn_cast<BinaryOperator>(E);

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

@@ -3208,6 +3208,32 @@ TEST(TransferTest, ResultObjectLocationForStmtExpr) {
       });
 }
 
+TEST(TransferTest, ResultObjectLocationForBuiltinBitCastExpr) {
+  std::string Code = R"(
+    struct S { int i; };
+    void target(int i) {
+      S s = __builtin_bit_cast(S, i);
+      // [[p]]
+    }
+  )";
+  using ast_matchers::explicitCastExpr;
+  using ast_matchers::match;
+  using ast_matchers::selectFirst;
+  using ast_matchers::traverse;
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        auto *BuiltinBitCast = selectFirst<BuiltinBitCastExpr>(
+            "cast", match(explicitCastExpr().bind("cast"), ASTCtx));
+
+        EXPECT_EQ(&Env.getResultObjectLocation(*BuiltinBitCast),
+                  &getLocForDecl<RecordStorageLocation>(ASTCtx, Env, "s"));
+      });
+}
+
 TEST(TransferTest, ResultObjectLocationPropagatesThroughConditionalOperator) {
   std::string Code = R"(
     struct A {