fix: replace implicitly added named generics with fresh type vars in check_trait_impl_where_clause_matches_trait_where_clause

compiler/noirc_frontend/src/elaborator/mod.rs

@@ -9,7 +9,7 @@ use crate::{
     ast::{IdentOrQuotedType, ItemVisibility, UnresolvedType},
     graph::CrateGraph,
     hir::def_collector::dc_crate::UnresolvedTrait,
-    hir_def::traits::ResolvedTraitBound,
+    hir_def::traits::{NamedType, ResolvedTraitBound},
     node_interner::{GlobalValue, QuotedTypeId},
     token::SecondaryAttributeKind,
     usage_tracker::UsageTracker,
@@ -1392,22 +1392,37 @@ impl<'context> Elaborator<'context> {
             else {
                 continue;
             };
+            let trait_constraint_trait_name = trait_constraint_trait.name.to_string();
 
             let trait_constraint_type = trait_constraint.typ.substitute(&bindings);
             let trait_bound = &trait_constraint.trait_bound;
 
+            let mut named_generics = trait_bound.trait_generics.named.clone();
+
+            // If the trait bound is over a trait that has associated types, the ones that
+            // aren't explicit will be in `named_generics` as implicitly added ones.
+            // If they are unbound, they won't be bound until monomorphization, in which cae
+            // the below trait implementation lookup will fail (an unbound named generic will
+            // never unify in this case). In this case we replace them with fresh type variables
+            // so they'll unify (the bindings aren't applied here so this is fine).
+            // If they are bound though, we won't replace them as we want to ensure the binding
+            // matches.
+            self.replace_implicitly_added_unbound_named_generics_with_fresh_type_variables(
+                &mut named_generics,
+            );
+
             if self
                 .interner
                 .try_lookup_trait_implementation(
                     &trait_constraint_type,
                     trait_bound.trait_id,
                     &trait_bound.trait_generics.ordered,
-                    &trait_bound.trait_generics.named,
+                    &named_generics,
                 )
                 .is_err()
             {
                 let missing_trait =
-                    format!("{}{}", trait_constraint_trait.name, trait_bound.trait_generics);
+                    format!("{}{}", trait_constraint_trait_name, trait_bound.trait_generics);
                 self.push_err(ResolverError::TraitNotImplemented {
                     impl_trait: impl_trait.clone(),
                     missing_trait,
@@ -1419,6 +1434,22 @@ impl<'context> Elaborator<'context> {
         }
     }
 
+    fn replace_implicitly_added_unbound_named_generics_with_fresh_type_variables(
+        &mut self,
+        named_generics: &mut [NamedType],
+    ) {
+        for named_type in named_generics.iter_mut() {
+            match &named_type.typ {
+                Type::NamedGeneric(NamedGeneric { type_var, implicit: true, .. })
+                    if type_var.borrow().is_unbound() =>
+                {
+                    named_type.typ = self.interner.next_type_variable();
+                }
+                _ => (),
+            };
+        }
+    }
+
     fn check_parent_traits_are_implemented(&mut self, trait_impl: &UnresolvedTraitImpl) {
         let Some(trait_id) = trait_impl.trait_id else {
             return;

compiler/noirc_frontend/src/tests/traits.rs

@@ -2022,6 +2022,30 @@ fn associated_constant_mul_of_other_constants() {
     assert_no_errors!(src);
 }
 
+#[named]
+#[test]
+fn trait_bound_with_associated_constant() {
+    let src = r#"
+    pub trait Other {
+        let N: u32;
+    }
+
+    pub trait Trait<T>
+    where
+        T: Other,
+    {}
+
+    impl Other for Field {
+        let N: u32 = 1;
+    }
+
+    impl Trait<Field> for i32 {}
+
+    fn main() {}
+    "#;
+    assert_no_errors!(src);
+}
+
 #[named]
 #[test]
 fn trait_method_call_when_it_has_bounds_on_generic() {

test_programs/compile_success_no_bug/noirc_frontend_tests_traits_trait_bound_with_associated_constant/Nargo.toml

@@ -0,0 +1,7 @@
+
+            [package]
+            name = "noirc_frontend_tests_traits_trait_bound_with_associated_constant"
+            type = "bin"
+            authors = [""]
+
+            [dependencies]

test_programs/compile_success_no_bug/noirc_frontend_tests_traits_trait_bound_with_associated_constant/src/main.nr

@@ -0,0 +1,18 @@
+
+    pub trait Other {
+        let N: u32;
+    }
+
+    pub trait Trait<T>
+    where
+        T: Other,
+    {}
+
+    impl Other for Field {
+        let N: u32 = 1;
+    }
+
+    impl Trait<Field> for i32 {}
+
+    fn main() {}
+

test_programs/compile_success_no_bug/noirc_frontend_tests_traits_trait_bound_with_associated_constant/src_hash.txt

@@ -0,0 +1 @@
+1955914972083164529