fix: Check for overlapping implementations

compiler/noirc_frontend/src/elaborator/enums.rs

@@ -21,7 +21,7 @@ use crate::{
     },
     hir::{
         comptime::Value,
-        def_collector::dc_crate::{CompilationError, UnresolvedEnum},
+        def_collector::dc_crate::UnresolvedEnum,
         resolution::{errors::ResolverError, import::PathResolutionError},
         type_check::TypeCheckError,
     },
@@ -335,19 +335,6 @@ impl Elaborator<'_> {
 
         self.interner.push_fn_meta(meta, method_id);
         if let Err(error) = self.interner.add_method(self_type, name_string, method_id, None) {
-            let error = if matches!(
-                error,
-                CompilationError::ResolverError(ResolverError::DuplicateDefinition { .. })
-            ) {
-                // Expecting a DefCollectorErrorKind::Duplicate error in this case
-                TypeCheckError::ExpectingOtherError {
-                    message: "define_enum_variant_function: duplicate definition".to_string(),
-                    location,
-                }
-                .into()
-            } else {
-                error
-            };
             self.push_err(error);
         }
 

compiler/noirc_frontend/src/elaborator/impls.rs

@@ -239,9 +239,7 @@ impl Elaborator<'_> {
         for method_id in function_ids {
             let method_name = self.interner.function_name(method_id).to_owned();
 
-            if let Err(error) =
-                self.interner.add_method(self_type, method_name.clone(), *method_id, None)
-            {
+            if let Err(error) = self.interner.add_method(self_type, method_name, *method_id, None) {
                 self.push_err(error);
             }
         }

compiler/noirc_frontend/src/node_interner/methods.rs

@@ -1,5 +1,8 @@
+use std::collections::HashSet;
+
 use crate::{
-    Type, TypeBindings,
+    Kind, Type, TypeBindings, TypeVariableId,
+    hir_def::types::NamedGeneric,
     node_interner::{FuncId, TraitId},
 };
 
@@ -33,6 +36,7 @@ pub struct Methods {
 }
 
 impl Methods {
+    /// Adds a method to this collection, without checking for overlaps.
     pub(super) fn add_method(&mut self, method: FuncId, typ: Type, trait_id: Option<TraitId>) {
         if let Some(trait_id) = trait_id {
             let trait_impl_method = TraitImplMethod { typ, method, trait_id };
@@ -43,6 +47,116 @@ impl Methods {
         }
     }
 
+    /// Finds an existing direct (inherent) method whose type overlaps with the given type.
+    /// Returns `Some((method_id, method_type))` if an overlap is found.
+    ///
+    /// Two types overlap if there exist concrete types that could match both.
+    /// For example:
+    /// - `Foo<T>` and `Foo<U>` overlap
+    /// - `Foo<T>` and `Foo<i32>` overlap (T can be i32)
+    /// - `Foo<i32>` and `Foo<u64>` don't overlap
+    pub(super) fn find_overlapping_method(
+        &self,
+        typ: &Type,
+        interner: &NodeInterner,
+    ) -> Option<(FuncId, Type)> {
+        for existing in &self.direct {
+            // Check if two types overlap, by instantiating both types (replacing NamedGenerics
+            // with fresh TypeVariables) and then checking if they can unify.
+            let instantiate_existing = Self::instantiate_named_generics(&existing.typ, interner);
+            let instantiate_typ = Self::instantiate_named_generics(typ, interner);
+
+            if Self::types_can_unify(&instantiate_existing, &instantiate_typ) {
+                return Some((existing.method, existing.typ.clone()));
+            }
+        }
+        None
+    }
+
+    /// Instantiate a type by finding all NamedGenerics and replacing them with
+    /// fresh type variables.
+    fn instantiate_named_generics(typ: &Type, interner: &NodeInterner) -> Type {
+        let mut named_generics = Vec::new();
+        Self::collect_named_generics(typ, &mut named_generics, &mut HashSet::new());
+
+        if named_generics.is_empty() {
+            return typ.clone();
+        }
+
+        // Create substitutions from each NamedGeneric to a fresh type variable
+        let substitutions: TypeBindings = named_generics
+            .into_iter()
+            .map(|(id, type_var, kind)| {
+                let fresh = interner.next_type_variable_with_kind(kind.clone());
+                (id, (type_var, kind, fresh))
+            })
+            .collect();
+
+        typ.substitute(&substitutions)
+    }
+
+    /// Recursively collect all NamedGenerics from a type.
+    fn collect_named_generics(
+        typ: &Type,
+        result: &mut Vec<(TypeVariableId, crate::TypeVariable, Kind)>,
+        seen: &mut HashSet<TypeVariableId>,
+    ) {
+        match typ {
+            Type::NamedGeneric(NamedGeneric { type_var, .. }) => {
+                let id = type_var.id();
+                if !seen.contains(&id) {
+                    seen.insert(id);
+                    result.push((id, type_var.clone(), type_var.kind()));
+                }
+            }
+            Type::Array(size, element) => {
+                Self::collect_named_generics(size, result, seen);
+                Self::collect_named_generics(element, result, seen);
+            }
+            Type::Vector(element) | Type::String(element) | Type::Reference(element, _) => {
+                Self::collect_named_generics(element, result, seen);
+            }
+            Type::FmtString(size, fields) | Type::CheckedCast { from: size, to: fields } => {
+                Self::collect_named_generics(size, result, seen);
+                Self::collect_named_generics(fields, result, seen);
+            }
+            Type::DataType(_, args) | Type::Alias(_, args) => {
+                for arg in args {
+                    Self::collect_named_generics(arg, result, seen);
+                }
+            }
+            Type::Tuple(fields) => {
+                for field in fields {
+                    Self::collect_named_generics(field, result, seen);
+                }
+            }
+            Type::Function(args, ret, env, _) => {
+                for arg in args {
+                    Self::collect_named_generics(arg, result, seen);
+                }
+                Self::collect_named_generics(ret, result, seen);
+                Self::collect_named_generics(env, result, seen);
+            }
+            Type::Forall(_, inner) => {
+                Self::collect_named_generics(inner, result, seen);
+            }
+            Type::InfixExpr(left, _, right, _) => {
+                Self::collect_named_generics(left, result, seen);
+                Self::collect_named_generics(right, result, seen);
+            }
+            // These types don't contain NamedGenerics
+            Type::FieldElement
+            | Type::Integer(..)
+            | Type::Bool
+            | Type::Unit
+            | Type::Quoted(..)
+            | Type::Error
+            | Type::TypeVariable(..)
+            | Type::Constant(..)
+            | Type::TraitAsType(..) => {}
+        }
+    }
+
     pub(super) fn find_direct_method(
         &self,
         typ: &Type,

compiler/noirc_frontend/src/node_interner/mod.rs

@@ -18,6 +18,7 @@ use crate::ast::{
 use crate::graph::CrateId;
 use crate::hir::comptime;
 use crate::hir::def_collector::dc_crate::{CompilationError, UnresolvedTrait, UnresolvedTypeAlias};
+use crate::hir::def_collector::errors::DefCollectorErrorKind;
 use crate::hir::def_map::{LocalModuleId, ModuleDefId, ModuleId};
 use crate::hir::resolution::errors::ResolverError;
 use crate::hir::type_check::generics::TraitGenerics;
@@ -990,10 +991,11 @@ impl NodeInterner {
         }
     }
 
-    /// Adds a non-trait method to a type.
+    /// Adds a method to a type.
     ///
-    /// Returns `Some(duplicate)` if a matching method was already defined.
-    /// Returns `None` otherwise.
+    /// For inherent (non-trait) methods, this checks for overlapping implementations.
+    /// Returns `Ok(())` if the method was added successfully.
+    /// Returns `Err(error)` if there was an error (e.g., overlapping impl or unsupported type).
     pub fn add_method(
         &mut self,
         self_type: &Type,
@@ -1016,25 +1018,29 @@ impl NodeInterner {
                     return Err(error.into());
                 };
 
-                if trait_id.is_none() && matches!(self_type, Type::DataType(..)) {
-                    let check_self_param = false;
-                    if let Some(existing) =
-                        self.lookup_direct_method(self_type, &method_name, check_self_param)
+                let typ = self_type.clone();
+
+                // For inherent (non-trait) methods, check for overlapping implementations.
+                if trait_id.is_none() {
+                    if let Some(existing_methods) =
+                        self.methods.get(&key).and_then(|m| m.get(&method_name))
                     {
-                        let first_location = self.function_ident(&existing).location();
-                        let second_location = self.function_ident(&method_id).location();
-                        let error = ResolverError::DuplicateDefinition {
-                            name: method_name,
-                            first_location,
-                            second_location,
-                        };
-                        return Err(error.into());
+                        if let Some((existing_method, existing_type)) =
+                            existing_methods.find_overlapping_method(&typ, self)
+                        {
+                            let prev_location = self.function_ident(&existing_method).location();
+                            let location = self.function_ident(&method_id).location();
+                            let error = DefCollectorErrorKind::OverlappingImpl {
+                                typ: existing_type,
+                                location,
+                                prev_location,
+                            };
+                            return Err(error.into());
+                        }
                     }
                 }
 
-                // Only remember the actual type if it's FieldOrInt,
-                // so later we can disambiguate on calls like `u32::call`.
-                let typ = self_type.clone();
+                // Add the method to the collection
                 self.methods
                     .entry(key)
                     .or_default()

compiler/noirc_frontend/src/tests/structs.rs

@@ -91,10 +91,10 @@ fn check_impl_duplicate_method_without_self() {
 
     impl Foo {
         fn foo() {}
-           ~~~ first definition found here
+           ~~~ Previous impl defined here
         fn foo() {}
-           ^^^ duplicate definitions of foo found
-           ~~~ second definition found here
+           ^^^ Impl for type `Foo` overlaps with existing impl
+           ~~~ Overlapping impl
     }
     ";
     check_errors(src);
@@ -345,3 +345,48 @@ fn abi_attribute_outside_contract() {
     "#;
     check_errors(src);
 }
+
+#[test]
+fn overlapping_inherent_impls() {
+    let src = r#"
+        struct Foo<T> { _x: T }
+
+        impl<T> Foo<T> {
+            fn method(_self: Self) {}
+               ^^^^^^ Impl for type `Foo<i32>` overlaps with existing impl
+               ~~~~~~ Overlapping impl
+        }
+
+        impl Foo<i32> {
+            fn method(_self: Self) {}
+               ~~~~~~ Previous impl defined here
+        }
+
+        fn main() {
+            let _ = Foo { _x: 1 };
+        }
+    "#;
+    check_errors(src);
+}
+
+#[test]
+fn non_overlapping_inherent_impls() {
+    // Different concrete types don't overlap
+    let src = r#"
+        struct Foo<T> { _x: T }
+
+        impl Foo<i32> {
+            fn method(_self: Self) {}
+        }
+
+        impl Foo<u64> {
+            fn method(_self: Self) {}
+        }
+
+        fn main() {
+            let _ = Foo { _x: 1_i32 };
+            let _ = Foo { _x: 1_u64 };
+        }
+    "#;
+    assert_no_errors(src);
+}