From 5ff7c39ec15b5ef7cdc9e663d7d6e858dd73567b Mon Sep 17 00:00:00 2001
From: Daniel Frederico Lins Leite <xunilrj@hotmail.com>
Date: Tue, 29 Oct 2024 11:21:02 +0000
Subject: [PATCH] return type checked trait constraints (#6604)

## Description

This PR fixes https://github.com/FuelLabs/sway/issues/6382.

The issue was that `TypeParameter::type_check_trait_constraints(...)`
mutates the type inside the `TypeEngine`, but it was not returning the
new type-checked trait constraints.

## Checklist

- [ ] I have linked to any relevant issues.
- [ ] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [ ] I have added tests that prove my fix is effective or that my
feature works.
- [ ] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [ ] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [ ] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: IGI-111 <igi-111@protonmail.com>
---
 .../type_system/ast_elements/type_parameter.rs |  6 ++++--
 .../generic_where_in_impl_self/src/main.sw     | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/sway-core/src/type_system/ast_elements/type_parameter.rs b/sway-core/src/type_system/ast_elements/type_parameter.rs
index fb008336f4d..2b2ca658d23 100644
--- a/sway-core/src/type_system/ast_elements/type_parameter.rs
+++ b/sway-core/src/type_system/ast_elements/type_parameter.rs
@@ -213,7 +213,7 @@ impl TypeParameter {
             // Type check trait constraints only after type checking all type parameters.
             // This is required because a trait constraint may use other type parameters.
             // Ex: `struct Struct2<A, B> where A : MyAdd<B>`
-            for type_param in &new_type_params {
+            for type_param in new_type_params.iter_mut() {
                 TypeParameter::type_check_trait_constraints(handler, ctx.by_ref(), type_param)?;
             }
 
@@ -327,7 +327,7 @@ impl TypeParameter {
     fn type_check_trait_constraints(
         handler: &Handler,
         mut ctx: TypeCheckContext,
-        type_parameter: &TypeParameter,
+        type_parameter: &mut TypeParameter,
     ) -> Result<(), ErrorEmitted> {
         let type_engine = ctx.engines.te();
 
@@ -372,6 +372,8 @@ impl TypeParameter {
             },
         );
 
+        type_parameter.trait_constraints = trait_constraints_with_supertraits;
+
         // Insert the trait constraints into the namespace.
         type_parameter.insert_into_namespace_constraints(handler, ctx.by_ref())?;
 
diff --git a/test/src/e2e_vm_tests/test_programs/should_pass/language/generic_where_in_impl_self/src/main.sw b/test/src/e2e_vm_tests/test_programs/should_pass/language/generic_where_in_impl_self/src/main.sw
index 4c3e852777f..474043af251 100644
--- a/test/src/e2e_vm_tests/test_programs/should_pass/language/generic_where_in_impl_self/src/main.sw
+++ b/test/src/e2e_vm_tests/test_programs/should_pass/language/generic_where_in_impl_self/src/main.sw
@@ -27,9 +27,27 @@ impl<K> CallTrait<K> where K: Trait {
     }
 }
 
+// https://github.com/FuelLabs/sway/issues/6382
+trait Devour<T> { fn eat(t: T); }
+struct Brain { }
+struct Zombie { }
+
+impl Devour<Brain> for Zombie {
+    fn eat(_b: Brain) { }
+}
+
+fn feed<T, U>(t: T) where U: Devour<T> {
+    U::eat(t);
+}
+
 fn main() -> bool {
     let _  = call_trait(1);
     let ct = CallTrait::<u64> {};
     assert(ct.call_trait(1) == 42);
+
+    // https://github.com/FuelLabs/sway/issues/6382
+    let b = Brain{};
+    feed::<Brain, Zombie>(b);
+
     true
 }