Report errors for duplicate bound abilities

crates/compiler/can/src/annotation.rs

@@ -931,32 +931,38 @@ fn canonicalize_has_clause(
     let var_name = Lowercase::from(var_name);
 
     let mut can_abilities = Vec::with_capacity(abilities.len());
-    for ability in *abilities {
-        let ability = match ability.value {
+    for &Loc {
+        region,
+        value: ability,
+    } in *abilities
+    {
+        let ability = match ability {
             TypeAnnotation::Apply(module_name, ident, _type_arguments) => {
-                let symbol = make_apply_symbol(env, ability.region, scope, module_name, ident)?;
+                let symbol = make_apply_symbol(env, region, scope, module_name, ident)?;
 
                 // Ability defined locally, whose members we are constructing right now...
                 if !pending_abilities_in_scope.contains_key(&symbol)
                 // or an ability that was imported from elsewhere
                 && !scope.abilities_store.is_ability(symbol)
                 {
-                    let region = ability.region;
                     env.problem(roc_problem::can::Problem::HasClauseIsNotAbility { region });
                     return Err(Type::Erroneous(Problem::HasClauseIsNotAbility(region)));
                 }
                 symbol
             }
             _ => {
-                let region = ability.region;
                 env.problem(roc_problem::can::Problem::HasClauseIsNotAbility { region });
                 return Err(Type::Erroneous(Problem::HasClauseIsNotAbility(region)));
             }
         };
 
         references.insert(ability);
+        if can_abilities.contains(&ability) {
+            env.problem(roc_problem::can::Problem::DuplicateHasAbility { ability, region });
+        } else {
             can_abilities.push(ability);
         }
+    }
 
     if let Some(shadowing) = introduced_variables.named_var_by_name(&var_name) {
         let var_name_ident = var_name.to_string().into();

crates/compiler/problem/src/can.rs

@@ -117,6 +117,10 @@ pub enum Problem {
     IllegalHasClause {
         region: Region,
     },
+    DuplicateHasAbility {
+        ability: Symbol,
+        region: Region,
+    },
     AbilityMemberMissingHasClause {
         member: Symbol,
         ability: Symbol,

crates/reporting/src/error/canonicalize.rs

@@ -677,6 +677,24 @@ pub fn can_problem<'b>(
             severity = Severity::RuntimeError;
         }
 
+        Problem::DuplicateHasAbility { ability, region } => {
+            doc = alloc.stack([
+                alloc.concat([
+                    alloc.reflow("I already saw that this type variable is bound to the "),
+                    alloc.symbol_foreign_qualified(ability),
+                    alloc.reflow(" ability once before:"),
+                ]),
+                alloc.region(lines.convert_region(region)),
+                alloc.concat([
+                    alloc.reflow("Abilities only need to bound to a type variable once in a "),
+                    alloc.keyword("has"),
+                    alloc.reflow(" clause!"),
+                ]),
+            ]);
+            title = "DUPLICATE BOUND ABILITY".to_string();
+            severity = Severity::Warning;
+        }
+
         Problem::AbilityMemberMissingHasClause {
             member,
             ability,

crates/reporting/tests/test_reporting.rs

@@ -11531,4 +11531,26 @@ All branches in an `if` must have the same type!
     Tip: You can define a custom implementation of `Encoding` for `F`.
     "###
     );
+
+    test_report!(
+        duplicate_ability_in_has_clause,
+        indoc!(
+            r#"
+            f : a -> {} | a has Hash & Hash
+
+            f
+            "#
+        ),
+    @r###"
+    ── DUPLICATE BOUND ABILITY ─────────────────────────────── /code/proj/Main.roc ─
+
+    I already saw that this type variable is bound to the `Hash` ability
+    once before:
+
+    4│      f : a -> {} | a has Hash & Hash
+                                       ^^^^
+
+    Abilities only need to bound to a type variable once in a `has` clause!
+    "###
+    );
 }