error[E0049]: associated function `foo` has 1 type parameter but its trait declaration has 0 type parameters
  --> $DIR/gat-trait-path-missing-lifetime.rs:8:10
   |
LL |   fn foo<'a>(t : Self::Y<'a>) -> Self::Y<'a> { t }
   |          -- expected 0 type parameters
...
LL |   fn foo<'a, T1: X<Y = T1>>(t : T1) -> T1::Y<'a> {
   |          ^^  ^^
   |          |
   |          found 1 type parameter

error[E0046]: not all trait items implemented, missing: `Y`
  --> $DIR/gat-trait-path-missing-lifetime.rs:7:1
   |
LL |   type Y<'a>;
   |   ---------- `Y` from trait
...
LL | impl<T> X for T {
   | ^^^^^^^^^^^^^^^ missing `Y` in implementation

error[E0107]: missing generics for associated type `X::Y`
  --> $DIR/gat-trait-path-missing-lifetime.rs:8:20
   |
LL |   fn foo<'a, T1: X<Y = T1>>(t : T1) -> T1::Y<'a> {
   |                    ^ expected 1 lifetime argument
   |
note: associated type defined here, with 1 lifetime parameter: `'a`
  --> $DIR/gat-trait-path-missing-lifetime.rs:2:8
   |
LL |   type Y<'a>;
   |        ^ --
help: add missing lifetime argument
   |
LL |   fn foo<'a, T1: X<Y<'a> = T1>>(t : T1) -> T1::Y<'a> {
   |                     ++++

error[E0107]: missing generics for associated type `X::Y`
  --> $DIR/gat-trait-path-missing-lifetime.rs:8:20
   |
LL |   fn foo<'a, T1: X<Y = T1>>(t : T1) -> T1::Y<'a> {
   |                    ^ expected 1 lifetime argument
   |
note: associated type defined here, with 1 lifetime parameter: `'a`
  --> $DIR/gat-trait-path-missing-lifetime.rs:2:8
   |
LL |   type Y<'a>;
   |        ^ --
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
help: add missing lifetime argument
   |
LL |   fn foo<'a, T1: X<Y<'a> = T1>>(t : T1) -> T1::Y<'a> {
   |                     ++++

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0046, E0049, E0107.
For more information about an error, try `rustc --explain E0046`.