Type paremters constraints in generic implementations description is inaccurate

https://doc.rust-lang.org/reference/items/implementations.html#generic-implementations

Generic parameters constrain an implementation if the parameter appears at least once in one of:

• ...
• As an associated type in the bounds of a type that contains another parameter that constrains the implementation

However, RFC 447 says that all parameters should be constrained:

Type parameters are legal if they are "constrained" according to the following inference rules:

• ...
• If <T0 as Trait<T1...Tn>>::U == V appears in the impl predicates, and T0...Tn are constrained and T0 as Trait<T1...Tn> is not the impl trait reference then V is constrained.

This is also what implemented by the compiler.

See also Why does Fn() -> T constrain T but Fn(T) -> T does not.

Start with the generic implementations section in the Rust Reference and compare its constraint description with RFC 447 and compiler/rustc_typeck/src/constrained_generic_params.rs around the linked implementation. Verify the associated-type inference rule, including the Fn() -> T versus Fn(T) -> T distinction. Done when the Reference accurately describes the compiler's constraint behavior.