error[E0121]: the placeholder `_` is not allowed within types on item signatures for structs
  --> $DIR/macro-span-issue-116502.rs:7:13
   |
LL |             _
   |             ^
   |             |
   |             not allowed in type signatures
   |             not allowed in type signatures
   |             not allowed in type signatures
...
LL |     struct S<T = m!()>(m!(), T)
   |                  ----  ---- in this macro invocation
   |                  |
   |                  in this macro invocation
LL |     where
LL |         T: Trait<m!()>;
   |                  ---- in this macro invocation
   |
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
help: use type parameters instead
   |
LL ~             U
LL |         };
LL |     }
LL ~     struct S<U>(m!(), T)
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0121`.