Rollup merge of rust-lang#88851 - fee1-dead:dup-bound, r=oli-obk

Fix duplicate bounds for const_trait_impl

Fixes rust-lang#88383.

Compare the constness of the candidates before winnowing and removing a `~const` `BoundCandidate`.

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1487,10 +1487,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ) => false,
 
             (ParamCandidate(other), ParamCandidate(victim)) => {
-                let value_same_except_bound_vars = other.value.skip_binder()
+                let same_except_bound_vars = other.value.skip_binder()
                     == victim.value.skip_binder()
+                    && other.constness == victim.constness
                     && !other.value.skip_binder().has_escaping_bound_vars();
-                if value_same_except_bound_vars {
+                if same_except_bound_vars {
                     // See issue #84398. In short, we can generate multiple ParamCandidates which are
                     // the same except for unused bound vars. Just pick the one with the fewest bound vars
                     // or the current one if tied (they should both evaluate to the same answer). This is

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-dup-bound.rs

@@ -16,12 +16,17 @@ impl const PartialEq for S {
 
 // This duplicate bound should not result in ambiguities. It should be equivalent to a single ~const
 // bound.
-// const fn equals_self<T: PartialEq + ~const PartialEq>(t: &T) -> bool {
-// FIXME(fee1-dead)^ why should the order matter here?
-const fn equals_self<T: ~const PartialEq + PartialEq>(t: &T) -> bool {
+const fn equals_self<T: PartialEq + ~const PartialEq>(t: &T) -> bool {
     *t == *t
 }
 
-pub const EQ: bool = equals_self(&S);
+trait A: PartialEq {}
+impl<T: PartialEq> A for T {}
+
+const fn equals_self2<T: A + ~const PartialEq>(t: &T) -> bool {
+    *t == *t
+}
+
+pub const EQ: bool = equals_self(&S) && equals_self2(&S);
 
 fn main() {}