[implied_bounds_in_impls]: avoid linting on overlapping associated types

Author: y21

clippy_lints/src/implied_bounds_in_impls.rs

@@ -317,6 +317,15 @@ fn check(cx: &LateContext<'_>, decl: &FnDecl<'_>) {
                 && let implied_bindings = path.args.map_or([].as_slice(), |a| a.bindings)
                 && let Some(def_id) = poly_trait.trait_ref.path.res.opt_def_id()
                 && let Some(bound) = find_bound_in_supertraits(cx, def_id, implied_args, &supertraits)
+                // If the implied bound has a type binding that also exists in the implied-by trait,
+                // then we shouldn't lint. See #11880 for an example.
+                && let assocs = cx.tcx.associated_items(bound.trait_def_id)
+                && !implied_bindings.iter().any(|binding| {
+                    assocs
+                        .filter_by_name_unhygienic(binding.ident.name)
+                        .next()
+                        .is_some_and(|assoc| assoc.kind == ty::AssocKind::Type)
+                    })
             {
                 emit_lint(cx, poly_trait, opaque_ty, index, implied_bindings, bound);
             }

tests/ui/implied_bounds_in_impls.fixed

@@ -121,4 +121,33 @@ mod issue11435 {
     fn f3() -> impl Trait4<i8, X = i32, Y = i128> {}
 }
 
+fn issue11880() {
+    trait X {
+        type T;
+        type U;
+    }
+    trait Y: X {
+        type T;
+        type V;
+    }
+    impl X for () {
+        type T = i32;
+        type U = String;
+    }
+    impl Y for () {
+        type T = u32;
+        type V = Vec<u8>;
+    }
+
+    // Can't constrain `X::T` through `Y`
+    fn f() -> impl X<T = i32> + Y {}
+    fn f2() -> impl X<T = i32> + Y<T = u32> {}
+
+    // X::T is never constrained in the first place, so it can be omitted
+    // and left unconstrained
+    fn f3() -> impl Y {}
+    fn f4() -> impl Y<T = u32> {}
+    fn f5() -> impl Y<T = u32, U = String> {}
+}
+
 fn main() {}

tests/ui/implied_bounds_in_impls.rs

@@ -121,4 +121,33 @@ mod issue11435 {
     fn f3() -> impl Trait3<i8, i16, i64, X = i32, Y = i128> + Trait4<i8, X = i32> {}
 }
 
+fn issue11880() {
+    trait X {
+        type T;
+        type U;
+    }
+    trait Y: X {
+        type T;
+        type V;
+    }
+    impl X for () {
+        type T = i32;
+        type U = String;
+    }
+    impl Y for () {
+        type T = u32;
+        type V = Vec<u8>;
+    }
+
+    // Can't constrain `X::T` through `Y`
+    fn f() -> impl X<T = i32> + Y {}
+    fn f2() -> impl X<T = i32> + Y<T = u32> {}
+
+    // X::T is never constrained in the first place, so it can be omitted
+    // and left unconstrained
+    fn f3() -> impl X + Y {}
+    fn f4() -> impl X + Y<T = u32> {}
+    fn f5() -> impl X<U = String> + Y<T = u32> {}
+}
+
 fn main() {}

tests/ui/implied_bounds_in_impls.stderr

@@ -192,5 +192,41 @@ LL -     fn f3() -> impl Trait3<i8, i16, i64, X = i32, Y = i128> + Trait4<i8, X
 LL +     fn f3() -> impl Trait4<i8, X = i32, Y = i128> {}
    |
 
-error: aborting due to 16 previous errors
+error: this bound is already specified as the supertrait of `Y`
+  --> $DIR/implied_bounds_in_impls.rs:148:21
+   |
+LL |     fn f3() -> impl X + Y {}
+   |                     ^
+   |
+help: try removing this bound
+   |
+LL -     fn f3() -> impl X + Y {}
+LL +     fn f3() -> impl Y {}
+   |
+
+error: this bound is already specified as the supertrait of `Y<T = u32>`
+  --> $DIR/implied_bounds_in_impls.rs:149:21
+   |
+LL |     fn f4() -> impl X + Y<T = u32> {}
+   |                     ^
+   |
+help: try removing this bound
+   |
+LL -     fn f4() -> impl X + Y<T = u32> {}
+LL +     fn f4() -> impl Y<T = u32> {}
+   |
+
+error: this bound is already specified as the supertrait of `Y<T = u32>`
+  --> $DIR/implied_bounds_in_impls.rs:150:21
+   |
+LL |     fn f5() -> impl X<U = String> + Y<T = u32> {}
+   |                     ^^^^^^^^^^^^^
+   |
+help: try removing this bound
+   |
+LL -     fn f5() -> impl X<U = String> + Y<T = u32> {}
+LL +     fn f5() -> impl Y<T = u32, U = String> {}
+   |
+
+error: aborting due to 19 previous errors