[clang] [clang][dataflow] Correctly treat empty initializer lists for unions. (PR #82986)

[Yitzhak Mandelbaum via cfe-commits]

================
@@ -2392,14 +2392,88 @@ TEST(TransferTest, InitListExprAsUnion) {
       } F;
 
      public:
-      constexpr target() : F{nullptr} {}
+      constexpr target() : F{nullptr} {
+        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) {
-        // Just verify that it doesn't crash.
+        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, 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));
----------------
ymand wrote:

Why is `a` set when the initializer is empty? I'm guessing it's because line 699 in Transfer.cpp uses `front()` and so initializes the first field of the union. If so, perhaps say this explicitly in the comment preceding those lines?

https://github.com/llvm/llvm-project/pull/82986