[libcxx-commits] [libcxx] Fix the LWG 3568 test for basic_istream_view (PR #215589)

[A. Jiang via libcxx-commits]

================
@@ -39,10 +39,8 @@ void test() {
 
   // LWG 3568. basic_istream_view needs to initialize value_
   {
-    auto iss = make_string_stream<CharT>("");
-    std::ranges::basic_istream_view<int, CharT> isv{iss};
-    auto iter = isv.begin();
-    assert(*iter == 0);
+    static auto fn_local_iss = make_string_stream<CharT>("");
+    [[maybe_unused]] constexpr auto isv = std::views::istream<int>(fn_local_iss);
----------------
frederick-vs-ja wrote:

Thanks! I used to think that we can't verify the zero-initialized value. But I guess we can verify it now with the following class.

```C++
struct NoopExtraction {
  int n_; // intentionally doesn't have default member initializer

  template <class CharT, class Traits>
  std::basic_istream<CharT, Traits>& operator>>(std::basic_istream<CharT, Traits>& is, const NoopExtraction&) {
    return is; // intentionally doesn't modify the NoopExtraction object
  }
};
```

We may assert that the member `n_` is `0` after extraction.

https://github.com/llvm/llvm-project/pull/215589