implied bounds from impl header are not used when comparing trait and impl methods

@rustbot

This doesn't compile:

trait Trait {
    fn get();
}
impl<'a, 'b> Trait for &'a &'b () { // There is an implied bound 'b: 'a
    fn get() where 'b: 'a, {}
    //~^ ERROR impl has stricter requirements than trait
}

Although it should for the same reason it compiles with the following change:

- impl<'a, 'b> Trait for &'a &'b () {
+ impl<'a, 'b> Trait for &'a &'b () where 'b: 'a, {

There should be no difference in behavior between implied and explicit bounds on impl header.

This is because we're not adding sufficient implied bounds in compare_predicate_entailment() here:

rust/compiler/rustc_hir_analysis/src/check/compare_method.rs

Line 252 in 14ca83a

let mut wf_tys = FxIndexSet::default();

@rustbot label T-types A-implied-bounds A-associated-items C-bug

This should be fixed for associated types as well:

trait Trait {
    type Ty;
}
impl<'a, 'b> Trait for &'a &'b () {
    type Ty = () where 'b: 'a;
}

another test case:

trait Trait {
    type Assoc;
    fn test(_: ());
}

impl<'a, T> Trait for &'a T {
    type Assoc = ();
    fn test(_: Self::Assoc) {}
}

CC related (?): #89236 (but: inherent impl, not trait impl; wf'ck'ing (?) array repeat exprs, not compare_impl_item).

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implied bounds from impl header are not used when comparing trait and impl methods #105495

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implied bounds from impl header are not used when comparing trait and impl methods #105495

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aliemjay commented on Dec 9, 2022

aliemjay commented on Mar 19, 2023

fmease commented on Dec 29, 2024

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