error[E0128]: generic parameters with a default cannot use forward declared identifiers
  --> $DIR/issue-26812.rs:1:10
   |
LL | fn avg<T=T::Item>(_: T) {}
   |          ^^^^^^^ defaulted generic parameters cannot be forward declared

error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
  --> $DIR/issue-26812.rs:1:8
   |
LL | fn avg<T=T::Item>(_: T) {}
   |        ^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
   = note: `#[deny(invalid_type_param_default)]` on by default

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0128`.
Future incompatibility report: Future breakage diagnostic:
error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
  --> $DIR/issue-26812.rs:1:8
   |
LL | fn avg<T=T::Item>(_: T) {}
   |        ^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
   = note: `#[deny(invalid_type_param_default)]` on by default