fix Conv.h

Summary: with clang 21, header units with values returned in lambdas have issues. Tweak Conv.h to avoid the returned value being dropped. Log output example here: https://www.internalfb.com/intern/everpaste/?handle=GH-kWCUQuyFjyH8HAJ2urzob_hEBbswMAAAz from D109588369

Reviewed By: skrueger

Differential Revision: D109888758

fbshipit-source-id: e1d554acababbbaf8575e1f92dd0fcca58eb74fb

Author: DanielC314

folly/Conv.h

@@ -1493,15 +1493,16 @@ inline
       detail::CheckTrailingSpace,
       detail::ReturnUnit<Error>>::type;
   auto tmp = detail::parseToWrap(src, result);
-  return tmp
+  // returning outside of lambda to avoid clang 21 header-unit bug
+  tmp.thenOrThrow(
+         Check(),
+         [&](Error e) { throw_exception(makeConversionError(e, src)); })
       .thenOrThrow(
-          Check(),
-          [&](Error e) { throw_exception(makeConversionError(e, src)); })
-      .thenOrThrow(
-          [&](Unit) { return std::move(result); },
+          [&](Unit) {},
           [&](Error e) {
             throw_exception(makeConversionError(e, tmp.value()));
           });
+  return result;
 }
 
 /**