Merge pull request #4558 from roc-lang/specialization-sets-for-impl-opaques

Support custom abilities for opaques with immaterial lambda sets

Cargo.lock

@@ -4144,6 +4144,7 @@ dependencies = [
  "roc_error_macros",
  "roc_module",
  "roc_solve_problem",
+ "roc_tracing",
  "roc_types",
 ]
 

crates/compiler/solve/tests/solve_expr.rs

@@ -8320,6 +8320,50 @@ mod solve_expr {
         )
     }
 
+    #[test]
+    fn impl_ability_for_opaque_with_lambda_sets() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test" provides [isEqQ] to "./platform"
+
+                Q := [ F (Str -> Str), G ] has [Eq { isEq: isEqQ }]
+
+                isEqQ = \@Q q1, @Q q2 -> when T q1 q2 is
+                #^^^^^{-1}
+                    T (F _) (F _) -> Bool.true
+                    T G G -> Bool.true
+                    _ -> Bool.false
+                "#
+            ),
+        @"isEqQ : Q, Q -[[isEqQ(0)]]-> Bool"
+        );
+    }
+
+    #[test]
+    fn impl_ability_for_opaque_with_lambda_sets_material() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test" provides [main] to "./platform"
+
+                Q := ({} -> Str) has [Eq {isEq: isEqQ}]
+
+                isEqQ = \@Q f1, @Q f2 -> (f1 {} == f2 {})
+                #^^^^^{-1}
+
+                main = isEqQ (@Q \{} -> "a") (@Q \{} -> "a")
+                #      ^^^^^
+                "#
+            ),
+        @r###"
+        isEqQ : ({} -[[]]-> Str), ({} -[[]]-> Str) -[[isEqQ(2)]]-> [False, True]
+        isEqQ : ({} -[[6(6), 7(7)]]-> Str), ({} -[[6(6), 7(7)]]-> Str) -[[isEqQ(2)]]-> [False, True]
+        "###
+        print_only_under_alias: true
+        );
+    }
+
     #[test]
     fn infer_concrete_type_with_inference_var() {
         infer_queries!(indoc!(

crates/compiler/test_gen/src/gen_abilities.rs

@@ -1734,6 +1734,45 @@ mod eq {
         )
     }
 
+    #[test]
+    fn custom_eq_impl_for_fn_opaque() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                app "test" provides [main] to "./platform"
+
+                Q := ({} -> Str) has [Eq {isEq: isEqQ}]
+
+                isEqQ = \@Q _, @Q _ -> Bool.true
+
+                main = isEqQ (@Q \{} -> "a") (@Q \{} -> "a")
+                "#
+            ),
+            true,
+            bool
+        )
+    }
+
+    #[test]
+    #[ignore = "needs https://github.com/roc-lang/roc/issues/4557 first"]
+    fn custom_eq_impl_for_fn_opaque_material() {
+        assert_evals_to!(
+            indoc!(
+                r#"
+                app "test" provides [main] to "./platform"
+
+                Q := ({} -> Str) has [Eq {isEq: isEqQ}]
+
+                isEqQ = \@Q f1, @Q f2 -> (f1 {} == f2 {})
+
+                main = isEqQ (@Q \{} -> "a") (@Q \{} -> "a")
+                "#
+            ),
+            true,
+            bool
+        )
+    }
+
     #[test]
     fn derive_structural_eq() {
         assert_evals_to!(

crates/compiler/unify/Cargo.toml

@@ -29,3 +29,6 @@ path = "../can"
 
 [dependencies.roc_solve_problem]
 path = "../solve_problem"
+
+[dependencies.roc_tracing]
+path = "../../tracing"

crates/compiler/unify/src/unify.rs

@@ -1259,12 +1259,29 @@ fn extract_specialization_lambda_set<M: MetaCollector>(
     debug_assert!(member_rec_var.is_none());
 
     let member_uls = env.subs.get_subs_slice(member_uls_slice);
-    debug_assert_eq!(
-        member_uls.len(),
-        1,
-        "member signature lambda sets should contain only one unspecialized lambda set"
+    debug_assert!(
+        member_uls.len() <= 1,
+        "member signature lambda sets should contain at most one unspecialized lambda set"
     );
 
+    if member_uls.is_empty() {
+        // This can happen if the specialized type has a lambda set that is determined to be
+        // immaterial in the implementation of the specialization, because the specialization
+        // lambda set does not line up with one required by the ability member prototype.
+        // As an example, consider
+        //
+        //   Q := [ F (Str -> Str) ] has [Eq {isEq}]
+        //
+        //   isEq = \@Q _, @Q _ -> Bool.false
+        //
+        // here the lambda set of `F`'s payload is part of the specialization signature, but it is
+        // irrelevant to the specialization. As such, I believe it is safe to drop the
+        // empty specialization lambda set.
+        roc_tracing::info!(ambient_function=?env.subs.get_root_key_without_compacting(specialization_lset.ambient_function), "ambient function in a specialization has a zero-lambda set");
+
+        return merge(env, ctx, Content::LambdaSet(specialization_lset));
+    }
+
     let Uls(_, member, region) = member_uls[0];
 
     let mut outcome: Outcome<M> = merge(env, ctx, Content::LambdaSet(specialization_lset));