[libcxx-commits] [libcxx] [libc++] basic_ios<wchar_t> cannot store fill character WCHAR_MAX (PR #89305)

[David Tenty via libcxx-commits]

================
@@ -590,7 +616,24 @@ protected:
 
 private:
   basic_ostream<char_type, traits_type>* __tie_;
-  mutable int_type __fill_;
+
+#if defined(_AIX) || (defined(__MVS__) && defined(__64BIT__))
+// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
+  using _FillType = _OptionalFill<_Traits>;
+#else
+#if defined(_WIN32)
+  static const bool _OptOutForABICompat = true;
+#else
+  static const bool _OptOutForABICompat = false;
+#endif
+
+  using _FillType = _If<
+      sizeof(char_type) >= sizeof(int_type) && !_OptOutForABICompat,
+      _OptionalFill<_Traits>,
+      _SentinelValueFill<_Traits>
+  >;
+#endif
----------------
daltenty wrote:

I feel like this platform ABI logic belongs in `include/__configuration/abi.h` (see Louis' recent refactor in https://github.com/llvm/llvm-project/commit/23e1ed65c2c3eb1f80f7eeb4897ec843492c5451), with flags like `_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL` and `_LIBCXX_IOS_FORCE_OPTIONAL_FILL`

In ABI version 2, `_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL` could simply be on, even on AIX and 64-bit MVS. In ABI version 1, we enable `_LIBCXX_IOS_FORCE_OPTIONAL_FILL` for AIX  and 64-bit MVS and leave every thing else alone (that way we don't need to special case Windows like we do here)

Then this becomes:
```suggestion
#if defined(_LIBCXX_IOS_FORCE_OPTIONAL_FILL)
// AIX and 64-bit MVS must use _OptionalFill for ABI backward compatibility.
  using _FillType = _OptionalFill<_Traits>;
#elif defined(_LIBCXX_IOS_MAY_USE_OPTIONAL_FILL)
  using _FillType = _If<
      sizeof(char_type) >= sizeof(int_type),
      _OptionalFill<_Traits>,
      _SentinelValueFill<_Traits>
  >;
#else
  using _FillType = _SentinelValueFill<_Traits>;
#endif
```

https://github.com/llvm/llvm-project/pull/89305