Defaulted type parameters generate invalid code

[dtolnay]

#[derive(Template)]
#[template(path = "test")]
struct Test<T: Display = u8> {
    t: T,
}

error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions.
   --> tests/simple.rs:119:10
    |
119 | #[derive(Template)]
    |          ^^^^^^^^
    |
    = note: #[deny(invalid_type_param_default)] on by default
    = 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>

[djc]

Nice. 😄 Anything else I've missed?

And, thanks again for helping out so much.

[dtolnay]

LGTM. Nicely done.