[libcxx-commits] [libcxx] [libc++] Fix iostream size ABI break (PR #185839)

[Louis Dionne via libcxx-commits]

================
@@ -16,21 +16,29 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class T>
+union stream {
+  constexpr stream() {}
+  stream(const stream&)            = delete;
+  stream& operator=(const stream&) = delete;
+  constexpr ~stream() {}
+
+  T value;
+};
+
 template <class StreamT, class BufferT>
 union stream_data {
   constexpr stream_data() {}
   constexpr ~stream_data() {}
   struct {
-    // The stream has to be the first element, since that's referenced by the stream declarations in <iostream>
-    StreamT stream;
     BufferT buffer;
     mbstate_t mb;
   };
 
-  void init(FILE* stdstream) {
+  void init(FILE* stdstream, stream<StreamT>& stream) {
     mb = {};
     std::construct_at(&buffer, stdstream, &mb);
-    std::construct_at(&stream, &buffer);
+    std::construct_at(&stream.value, &buffer);
   }
----------------
ldionne wrote:

Let's make this a free function and pass the `FILE*` and both the stream data and the actual stream object explicitly:

```
template <class StreamT, class BufferT>
void init_streams(FILE*, stream<StreamT>&, stream_data<StreamT, BufferT>&);
```

https://github.com/llvm/llvm-project/pull/185839