E0195 on where Self: 'a bounds when they are unconditionally satisfied by Self

[ExpHP]

trait Test {
    fn boo<'ctx>() where Self: 'ctx;
}

impl<A, B> Test for (A, B) {
    fn boo<'ctx>() where A: 'ctx, B: 'ctx { }    // ok
}

impl Test for () {
    fn boo<'ctx>() { }   //  [E0195]: lifetime parameters or bounds on method `boo` do not match the trait declaration
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9d4125a483581bb47d66f002f1074fda

I expected this to succeed. And in fact, the first impl (the generic one) succeeds! (i.e. the compiler is able to successfully determine that Self: 'ctx given A: 'ctx and B: 'ctx)

However, an error is emitted on the impl for ():

error[E0195]: lifetime parameters or bounds on method `boo` do not match the trait declaration
  --> src/lib.rs:10:11
   |
2  |     fn boo<'ctx>() where Self: 'ctx;
   |           ------ lifetimes in impl do not match this method in trait
...
10 |     fn boo<'ctx>() { }
   |           ^^^^^^ lifetimes do not match method in trait

error: aborting due to previous error

In order to make this one compile, we need to add where (): 'ctx to the impl, which is, well, a bit silly!

impl Test for () {
    fn boo<'ctx>() where (): 'ctx { }   //  ok (workaround)
}

---

Addendum: In fact, any mention of the lifetime inside where bounds causes compilation to succeed.

impl Test for () {
    fn boo<'ctx>() where i32: 'ctx { }  // ok
}

impl Test for () {
    fn boo<'ctx>() where &'ctx i32: Clone { }  // also ok
}

[ExpHP]

Possibly related:

trait Test2 {
    fn boo<'ctx>();
}

impl Test2 for () {
    fn boo<'ctx>() where i32: 'ctx { }
}

   Compiling playground v0.0.1 (/playground)
error[E0195]: lifetime parameters or bounds on method `boo` do not match the trait declaration
 --> src/lib.rs:6:11
  |
2 |     fn boo<'ctx>();
  |           ------ lifetimes in impl do not match this method in trait
...
6 |     fn boo<'ctx>() where i32: 'ctx { }
  |           ^^^^^^ lifetimes do not match method in trait

error: aborting due to previous error

Here, deleting the where i32: 'ctx makes this succeed. Perhaps this error could be intentional. Nonetheless, seems related.

[estebank]

I thought that implied_bounds would make the code not fail, but it also fails. We could reliably detect the case and suggest the extra bound on the lifetime to match the impl, if that's how we want to solve this.