[clang][dataflow] Correctly treat empty initializer lists for unions.

This fixes a crash introduced by #82348
but also adds additional handling to make sure that we treat empty initializer
lists for both unions and structs/classes correctly (see tests added in this
patch).

Author: martinboehme

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

@@ -983,7 +983,7 @@ StorageLocation &Environment::createObjectInternal(const ValueDecl *D,
   }
 
   Value *Val = nullptr;
-  if (InitExpr)
+  if (InitExpr) {
     // In the (few) cases where an expression is intentionally
     // "uninterpreted", `InitExpr` is not associated with a value.  There are
     // two ways to handle this situation: propagate the status, so that
@@ -998,6 +998,11 @@ StorageLocation &Environment::createObjectInternal(const ValueDecl *D,
     // default value (assuming we don't update the environment API to return
     // references).
     Val = getValue(*InitExpr);
+
+    if (!Val && isa<ImplicitValueInitExpr>(InitExpr) &&
+        InitExpr->getType()->isPointerType())
+      Val = &getOrCreateNullPointerValue(InitExpr->getType()->getPointeeType());
+  }
   if (!Val)
     Val = createValue(Ty);
 

clang/lib/Analysis/FlowSensitive/Transfer.cpp

@@ -685,9 +685,22 @@ class TransferVisitor : public ConstStmtVisitor<TransferVisitor> {
 
     // `S->inits()` contains all the initializer expressions, including the
     // ones for direct base classes.
-    auto Inits = S->inits();
+    ArrayRef<Expr *> Inits = S->inits();
     size_t InitIdx = 0;
 
+    // Unions initialized with an empty initializer list need special treatment.
+    // For structs/classes initialized with an empty initializer list, Clang
+    // puts `ImplicitValueInitExpr`s in `InitListExpr::inits()`, but for unions,
+    // it doesn't do this -- so we create an `ImplicitValueInitExpr` ourselves.
+    std::optional<ImplicitValueInitExpr> ImplicitValueInitForUnion;
+    SmallVector<Expr *> InitsForUnion;
+    if (S->getType()->isUnionType() && Inits.empty()) {
+      assert(FieldsForInit.size() == 1);
+      ImplicitValueInitForUnion.emplace(FieldsForInit.front()->getType());
+      InitsForUnion.push_back(&*ImplicitValueInitForUnion);
+      Inits = InitsForUnion;
+    }
+
     // Initialize base classes.
     if (auto* R = S->getType()->getAsCXXRecordDecl()) {
       assert(FieldsForInit.size() + R->getNumBases() == Inits.size());

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

@@ -2408,8 +2408,72 @@ TEST(TransferTest, InitListExprAsUnion) {
         auto &FLoc = getFieldLoc<RecordStorageLocation>(
             *Env.getThisPointeeStorageLocation(), "F", ASTCtx);
         auto *AVal = cast<PointerValue>(getFieldValue(&FLoc, "a", ASTCtx, Env));
-        ASSERT_EQ(AVal, &getValueForDecl<PointerValue>(ASTCtx, Env, "null"));
-        ASSERT_EQ(getFieldValue(&FLoc, "b", ASTCtx, Env), nullptr);
+        EXPECT_EQ(AVal, &getValueForDecl<PointerValue>(ASTCtx, Env, "null"));
+        EXPECT_EQ(getFieldValue(&FLoc, "b", ASTCtx, Env), nullptr);
+      });
+}
+
+TEST(TransferTest, EmptyInitListExprForUnion) {
+  // This is a crash repro.
+  std::string Code = R"cc(
+    class target {
+      union {
+        int *a;
+        bool *b;
+      } F;
+
+     public:
+      constexpr target() : F{} {
+        int *null = nullptr;
+        F.b;  // Make sure we reference 'b' so it is modeled.
+        // [[p]]
+      }
+    };
+  )cc";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        auto &FLoc = getFieldLoc<RecordStorageLocation>(
+            *Env.getThisPointeeStorageLocation(), "F", ASTCtx);
+        auto *AVal = cast<PointerValue>(getFieldValue(&FLoc, "a", ASTCtx, Env));
+        EXPECT_EQ(AVal, &getValueForDecl<PointerValue>(ASTCtx, Env, "null"));
+        EXPECT_EQ(getFieldValue(&FLoc, "b", ASTCtx, Env), nullptr);
+      });
+}
+
+TEST(TransferTest, EmptyInitListExprForStruct) {
+  std::string Code = R"cc(
+    class target {
+      struct {
+        int *a;
+        bool *b;
+      } F;
+
+     public:
+      constexpr target() : F{} {
+        int *NullIntPtr = nullptr;
+        bool *NullBoolPtr = nullptr;
+        // [[p]]
+      }
+    };
+  )cc";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        auto &FLoc = getFieldLoc<RecordStorageLocation>(
+            *Env.getThisPointeeStorageLocation(), "F", ASTCtx);
+        auto *AVal = cast<PointerValue>(getFieldValue(&FLoc, "a", ASTCtx, Env));
+        ASSERT_EQ(AVal,
+                  &getValueForDecl<PointerValue>(ASTCtx, Env, "NullIntPtr"));
+        auto *BVal = cast<PointerValue>(getFieldValue(&FLoc, "b", ASTCtx, Env));
+        ASSERT_EQ(BVal,
+                  &getValueForDecl<PointerValue>(ASTCtx, Env, "NullBoolPtr"));
       });
 }