cyx-6 opened a new pull request, #632:
URL: https://github.com/apache/tvm-ffi/pull/632
## Problem
The macOS clang CI job on `main` fails to compile with `-Werror`:
```
include/tvm/ffi/extra/structural_visit.h:575:29: error: the argument to
'__builtin_assume' has side effects that will be discarded [-Werror,-Wassume]
TVM_FFI_UNSAFE_ASSUME(result.type_index() == TypeIndex::kTVMFFIInt);
```
## Root cause
clang's `__builtin_assume` discards its argument's side effects, so
`-Wassume` rejects any argument that **contains a call expression** — this is a
purely syntactic check, independent of whether the call is actually pure.
This is a regression from #629, which inlined `result.type_index()` directly
into `TVM_FFI_UNSAFE_ASSUME` to silence `-Wunused-variable`. The two warnings
pull in opposite directions:
| | `-Wunused-variable` | `-Wassume` |
|---|---|---|
| pre-#629 (local var) | ❌ (when assume macro is a no-op) | ✅ |
| #629 (inlined call) | ✅ | ❌ ← CI fails |
| this PR | ✅ | ✅ |
Note `result` here is a C++ `Expected`, so `result.type_index()` is a
**method call**. The other 10 `TVM_FFI_UNSAFE_ASSUME` sites in the tree operate
on the raw `TVMFFIAny` C struct via the plain field `src->type_index`, which
contains no call expression and is therefore unaffected. This was the only
call-expression assume site in the codebase.
## Fix
Hoist the call back into a local and mark it `[[maybe_unused]]`:
```cpp
[[maybe_unused]] int32_t type_index = result.type_index();
TVM_FFI_UNSAFE_ASSUME(type_index == TypeIndex::kTVMFFIInt);
```
- The assume argument is now a plain variable read → satisfies `-Wassume`.
- `[[maybe_unused]]` keeps `-Wunused-variable` quiet on configs where the
assume macro compiles away to a no-op → satisfies what #629 was originally
fixing.
## Validation
- `clang++ -std=c++17 -Werror -Wassume -fsyntax-only
src/ffi/extra/structural_visit.cc` (Homebrew clang 22): **passes** after the
change, **fails** before it (reproduces the CI error).
- Confirmed `[[maybe_unused]]` suppresses `-Wunused-variable` when the
assume macro expands to a no-op.
- `clang-format` clean; lines within the 100-col limit.
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