createcentury commented on issue #51834:
URL: https://github.com/apache/arrow/issues/51834#issuecomment-6088935425
Looking at how Chromium addresses this in `base/types/expected_macros.h`
(around `ASSIGN_OR_RETURN`), they faced the exact same language constraints:
1. Chromium also expands `ASSIGN_OR_RETURN` into multiple flat statements in
the caller's scope (using `BASE_UNIQUIFY(_expected_value)`).
2. Consequently, their temporaries live for the enclosing scope just as
Arrow's current implementation does. They document this trade-off explicitly:
```cpp
// WARNING: Expands into multiple statements; cannot be used in a single
// statement (e.g. as the body of an if statement without {})!
```
Because portable standard C++ cannot simultaneously:
* Allow `lhs` to be a variable declaration in the enclosing scope
* Early-return from the enclosing function on error
* Destruct the intermediate `Result`/`expected` at the statement/expression
boundary
Any attempt to solve this portably either breaks `auto var` declarations or
requires non-standard extensions.
Therefore, guarding the `__extension__({ ... })` statement expression
specifically for GCC / Clang (where deep recursion in server workloads is most
critical) while leaving the existing multi-statement expansion for MSVC seems
like the most practical approach. MSVC users would retain today's baseline
behavior without introducing breakage or unportable constructs.
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