[clang][dataflow] Add a test to justify skipping past references in UO_Deref

[sgatev]

Summary

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

[xazax.hun]

Is it planned to support templated code this way?
I'm not entirely sure if this is worth pursuing as opposed to checking instantiations.

[sgatev]

I also think that we should prefer checking instantiations. We have a single Clang-Tidy check that works on templates and I believe this is the only special support that it needs. We can revisit that later and remove this support if it proves to be unnecessary.

[thakis]

This breaks tests on windows: http://45.33.8.238/win/52841/step_7.txt

Please take a look and revert for now if it takes a while to fix.

[sgatev]

Thanks for sharing this. I reverted the patch and will look at why it fails on Windows.

[sgatev]

@gribozavr2 suggested that this is probably because we use delayed template parsing on Windows for MSVC compatibility. I reproduced the error using the --target=x86_64-windows-msvc arg and verified that adding -fno-delayed-template-parsing arg fixes it.

clang/lib/Analysis/FlowSensitive/Transfer.cpp

void VisitUnaryOperator(const UnaryOperator *S) {
     // The CFG does not contain `ParenExpr` as top-level statements in basic
     // blocks, however sub-expressions can still be of that type.
     assert(S->getSubExpr() != nullptr);
     const Expr *SubExpr = S->getSubExpr()->IgnoreParens();
     assert(SubExpr != nullptr);
 
     switch (S->getOpcode()) {
     case UO_Deref: {
+      // Skip past a reference to handle dereference of a dependent pointer.
       const auto *SubExprVal = cast_or_null<PointerValue>(
           Env.getValue(*SubExpr, SkipPast::Reference));
       if (SubExprVal == nullptr)
         break;
 
       auto &Loc = Env.createStorageLocation(*S);
       Env.setStorageLocation(*S, Loc);
       Env.setValue(Loc, Env.takeOwnership(std::make_unique<ReferenceValue>(

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

runDataflow(Code,
                 const auto *FooVal =
                     cast<PointerValue>(Env.getValue(*FooDecl, SkipPast::None));
                 const auto *BarVal =
                     cast<PointerValue>(Env.getValue(*BarDecl, SkipPast::None));
                 EXPECT_EQ(&BarVal->getPointeeLoc(), &FooVal->getPointeeLoc());
               });
 }
 
+TEST_F(TransferTest, DerefDependentPtr) {
+  std::string Code = R"(
+    template <typename T>
+    void target(T *Foo) {
+      T &Bar = *Foo;
+      /*[[p]]*/
+    }
+  )";
+  runDataflow(
+      Code, [](llvm::ArrayRef<
+                   std::pair<std::string, DataflowAnalysisState<NoopLattice>>>
+                   Results,
+               ASTContext &ASTCtx) {
+        ASSERT_THAT(Results, ElementsAre(Pair("p", _)));
+        const Environment &Env = Results[0].second.Env;
+
+        const ValueDecl *FooDecl = findValueDecl(ASTCtx, "Foo");
+        ASSERT_THAT(FooDecl, NotNull());
+
+        const ValueDecl *BarDecl = findValueDecl(ASTCtx, "Bar");
+        ASSERT_THAT(BarDecl, NotNull());
+
+        const auto *FooVal =
+            cast<PointerValue>(Env.getValue(*FooDecl, SkipPast::None));
+        const auto *BarVal =
+            cast<ReferenceValue>(Env.getValue(*BarDecl, SkipPast::None));
+        EXPECT_EQ(&BarVal->getPointeeLoc(), &FooVal->getPointeeLoc());
+      });
+}
+
 } // namespace