error[E0308]: mismatched types
  --> $DIR/issue-15896.rs:11:11
   |
LL |           Tau{t: x},
   |           ^^^^^^^^^ expected enum `main::R`, found struct `main::Tau`
   |
   = note: expected type `main::R`
              found type `main::Tau`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.