[clang][dataflow] Fix bug in handling of `return` statements.

[ymandel]

Summary

The handling of return statements, added in support of context-sensitive
analysis, has a bug relating to functions that return reference
types. Specifically, interpretation of such functions can result in a crash from
a bad cast. This patch fixes the bug and guards all of that code with the
context-sensitive option, since there's no reason to execute at all when
context-sensitive analysis is off.

clang/lib/Analysis/FlowSensitive/Transfer.cpp

void VisitCXXThisExpr(const CXXThisExpr *S) {
 
     auto &Loc = Env.createStorageLocation(*S);
     Env.setStorageLocation(*S, Loc);
     Env.setValue(Loc, Env.takeOwnership(
                           std::make_unique<PointerValue>(*ThisPointeeLoc)));
   }
 
   void VisitReturnStmt(const ReturnStmt *S) {
+    if (!Options.ContextSensitiveOpts)
+      return;
+
     auto *Ret = S->getRetValue();
     if (Ret == nullptr)
       return;
 
     auto *Val = Env.getValue(*Ret, SkipPast::None);
     if (Val == nullptr)
       return;
 
     // FIXME: Support reference-type returns.
     if (Val->getKind() == Value::Kind::Reference)
       return;
 
     auto *Loc = Env.getReturnStorageLocation();
     assert(Loc != nullptr);
+    // FIXME: Support reference-type returns.
+    if (Loc->getType()->isReferenceType())
+      return;
+
     // FIXME: Model NRVO.
     Env.setValue(*Loc, *Val);
   }
 
   void VisitMemberExpr(const MemberExpr *S) {
     ValueDecl *Member = S->getMemberDecl();
     assert(Member != nullptr);
 

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

runDataflow(
 
         auto &FooVal = *cast<BoolValue>(Env.getValue(*FooDecl, SkipPast::None));
         EXPECT_FALSE(Env.flowConditionImplies(FooVal));
         EXPECT_FALSE(Env.flowConditionImplies(Env.makeNot(FooVal)));
       },
       {TransferOptions{/*.ContextSensitiveOpts=*/std::nullopt}});
 }
 
+// This test is a regression test, based on a real crash.
+TEST(TransferTest, ContextSensitiveReturnReferenceFromNonReferenceLvalue) {
+  // This code exercises an unusual code path. If we return an lvalue directly,
+  // the code will catch that it's an l-value based on the `Value`'s kind. If we
+  // pass through a dummy function, the framework won't populate a value at
+  // all. In contrast, this code results in a (fresh) value, but it is not
+  // `ReferenceValue`. This test verifies that we catch this case as well.
+  std::string Code = R"(
+    class S {};
+    S& target(bool b, S &s) {
+      return b ? s : s;
+      // [[p]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        ASSERT_THAT(Results.keys(), UnorderedElementsAre("p"));
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        auto *Loc = Env.getReturnStorageLocation();
+        ASSERT_THAT(Loc, NotNull());
+
+        EXPECT_THAT(Env.getValue(*Loc), IsNull());
+      },
+      {TransferOptions{ContextSensitiveOptions{}}});
+}
+
 TEST(TransferTest, ContextSensitiveDepthZero) {
   std::string Code = R"(
     bool GiveBool();
     void SetBool(bool &Var) { Var = true; }
 
     void target() {
       bool Foo = GiveBool();
       SetBool(Foo);