error[E0428]: the name `a` is defined multiple times
  --> $DIR/issue-3099.rs:5:1
   |
LL | fn a(x: String) -> String {
   | ------------------------- previous definition of the value `a` here
...
LL | fn a(x: String, y: String) -> String {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `a` redefined here
   |
   = note: `a` must be defined only once in the value namespace of this module

error: aborting due to previous error

For more information about this error, try `rustc --explain E0428`.