error: expected identifier, found reserved identifier `_`
  --> $DIR/issue-91461.rs:2:7
   |
LL |     a(_:b:,)
   |       ^ expected identifier, found reserved identifier

error: expected one of `)`, `,`, `.`, `?`, or an operator, found `:`
  --> $DIR/issue-91461.rs:2:8
   |
LL |     a(_:b:,)
   |        ^ expected one of `)`, `,`, `.`, `?`, or an operator

error: aborting due to 2 previous errors

