[clang][dataflow] Assign aggregate storage locations to union stmts

[sgatev]

Summary

This patch ensures that the dataflow analysis framework does not crash
when it encounters access to members of union types.

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

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

LatticeJoinEffect Environment::join(const Environment &Other) {
   if (LocToValSizeBefore != LocToVal.size())
     Effect = LatticeJoinEffect::Changed;
 
   return Effect;
 }
 
 StorageLocation &Environment::createStorageLocation(QualType Type) {
   assert(!Type.isNull());
-  if (Type->isStructureOrClassType()) {
+  if (Type->isStructureOrClassType() || Type->isUnionType()) {
     // FIXME: Explore options to avoid eager initialization of fields as some of
     // them might not be needed for a particular analysis.
     llvm::DenseMap<const ValueDecl *, StorageLocation *> FieldLocs;
     for (const FieldDecl *Field : Type->getAsRecordDecl()->fields()) {
       FieldLocs.insert({Field, &createStorageLocation(Field->getType())});
     }
     return takeOwnership(
         std::make_unique<AggregateStorageLocation>(Type, std::move(FieldLocs)));

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

runDataflow(
 
           EXPECT_EQ(&QuuxVal->getChild(*BarDecl), BarArgVal);
           EXPECT_EQ(&BazVal->getChild(*FooDecl), FooArgVal);
           EXPECT_EQ(&QuuxVal->getChild(*QuxDecl), QuxArgVal);
         });
   }
 }
 
+TEST_F(TransferTest, AssignToUnionMember) {
+  std::string Code = R"(
+    union A {
+      int Foo;
+    };
+
+    void target(int Bar) {
+      A Baz;
+      Baz.Foo = Bar;
+      // [[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 *BazDecl = findValueDecl(ASTCtx, "Baz");
+                ASSERT_THAT(BazDecl, NotNull());
+                ASSERT_TRUE(BazDecl->getType()->isUnionType());
+
+                const auto *BazLoc = dyn_cast_or_null<AggregateStorageLocation>(
+                    Env.getStorageLocation(*BazDecl, SkipPast::None));
+                ASSERT_THAT(BazLoc, NotNull());
+
+                // FIXME: Add support for union types.
+                EXPECT_THAT(Env.getValue(*BazLoc), IsNull());
+              });
+}
+
 } // namespace