Associated type constraints and higher ranked trait bounds require impossible type annotations

[patrick-gu]

I tried this code (Playground):

trait TraitWithLifetime<'a> {
    type Assoc;
}

struct Implementor<T>(T);

impl<'a, T> TraitWithLifetime<'a> for Implementor<T> where
    T: for<'b> TraitWithLifetime<'b> + TraitWithLifetime<'a, Assoc = ()>
{
}

I expected to see this happen: The code compiles successfully because there are no additional type annotations are needed.

Instead, an invalid suggestion is produced:

error[E0283]: type annotations needed
 --> src\main.rs:8:62
  |
1 | trait TraitWithLifetime<'a> {
  | --------------------------- required by this bound in `TraitWithLifetime`
...
8 |     T: for<'b> TraitWithLifetime<'b> + TraitWithLifetime<'a, Assoc = ()>
  |                                                              ^^^^^^^^^^ cannot infer type for 
type parameter `T`
  |
  = note: cannot satisfy `T: TraitWithLifetime<'a>`
help: consider specifying the type arguments in the function call
  |
8 |     T: for<'b> TraitWithLifetime<'b> + TraitWithLifetime<'a, Assoc = ()::<Self, 'a>>
  |                                                                        ^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.

Meta

rustc --version --verbose:

rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-pc-windows-msvc
release: 1.54.0
LLVM version: 12.0.1

[SNCPlay42]

This is similar to #85735 - the actual problem is that there are two bounds on the trait TraitWithLifetime whose generic parameters differ only by lifetimes (see #21974).

A similar fix to the one for #85735 could be applied to avoid the bad suggestion, but I think the compiler should emit a completely different error message from "type annotations needed" when generics on bounds differ only by lifetimes. If you remove the associated type constraint, the error message is still not good:

error[E0283]: type annotations needed
 --> src/lib.rs:8:40
  |
1 | trait TraitWithLifetime<'a> {
  | --------------------------- required by this bound in `TraitWithLifetime`
...
8 |     T: for<'b> TraitWithLifetime<'b> + TraitWithLifetime<'a>
  |                                        ^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T`
  |
  = note: cannot satisfy `T: TraitWithLifetime<'a>`

I think the reason there's no special error currently when generics on bounds differ only by lifetimes is that this might not be supposed to be a limitation of the compiler, but #21974 is more than five years old.

[estebank]

Current output:

error[[E0283]](https://doc.rust-lang.org/stable/error_codes/E0283.html): type annotations needed: cannot satisfy `T: TraitWithLifetime<'a>`
 --> src/lib.rs:8:62
  |
8 |     T: for<'b> TraitWithLifetime<'b> + TraitWithLifetime<'a, Assoc = ()>
  |                                                              ^^^^^^^^^^
  |
note: multiple `impl`s or `where` clauses satisfying `T: TraitWithLifetime<'a>` found
 --> src/lib.rs:8:8
  |
8 |     T: for<'b> TraitWithLifetime<'b> + TraitWithLifetime<'a, Assoc = ()>
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^