error: invalid `?` in type
  --> $DIR/issue-84148-1.rs:1:14
   |
LL | fn f(t:for<>t?)
   |              ^ `?` is only allowed on expressions, not types
   |
help: if you meant to express that the type might not contain a value, use the `Option` wrapper type
   |
LL | fn f(t:Option<for<>t>)
   |        +++++++      ~

error: expected one of `->`, `where`, or `{`, found `<eof>`
  --> $DIR/issue-84148-1.rs:1:15
   |
LL | fn f(t:for<>t?)
   |               ^ expected one of `->`, `where`, or `{`

error: aborting due to 2 previous errors