Fixes insert_trait_implementaion for nested generics. (#6827)

## Description
The unify_checker was returning false when comparing `MyOption<T>` with
`T`. And filter_by_type_inner type substitution when adding methods from
`MyOption<T>` to `MyOption<MyOption<T>>` was replacing MyOption<T> in
its own type, ending up by inserting into type
`MyOption<MyOption<MyOption<T>>>` instead of `MyOption<MyOption<T>>`.

Fixes #6825

## Checklist

- [x] I have linked to any relevant issues.
- [x] 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)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] 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).
- [x] I have requested a review from the relevant team or maintainers.

Co-authored-by: Joshua Batty <joshpbatty@gmail.com>
Co-authored-by: Sophie Dankel <47993817+sdankel@users.noreply.github.com>

Author: 3 people

sway-core/src/semantic_analysis/namespace/trait_map.rs

@@ -933,11 +933,6 @@ impl TraitMap {
                         *map_type_id,
                         *type_id,
                     );
-                    type_id.subst(&SubstTypesContext::new(
-                        engines,
-                        &type_mapping,
-                        matches!(code_block_first_pass, CodeBlockFirstPass::No),
-                    ));
                     let trait_items: TraitItems = map_trait_items
                         .clone()
                         .into_iter()

sway-core/src/type_system/unify/unify_check.rs

@@ -510,7 +510,8 @@ impl<'a> UnifyCheck<'a> {
                     // any type can be coerced into a generic,
                     // except if the type already contains the generic
                     (_e, _g @ UnknownGeneric { .. }) => {
-                        !OccursCheck::new(self.engines).check(right, left)
+                        matches!(self.mode, ConstraintSubset)
+                            || !OccursCheck::new(self.engines).check(right, left)
                     }
 
                     (Alias { ty: l_ty, .. }, Alias { ty: r_ty, .. }) => {

test/src/e2e_vm_tests/test_programs/should_pass/language/nested_generics/Forc.lock

@@ -0,0 +1,13 @@
+[[package]]
+name = "core"
+source = "path+from-root-9C31758901D851EE"
+
+[[package]]
+name = "nested_generics"
+source = "member"
+dependencies = ["std"]
+
+[[package]]
+name = "std"
+source = "path+from-root-9C31758901D851EE"
+dependencies = ["core"]

test/src/e2e_vm_tests/test_programs/should_pass/language/nested_generics/Forc.toml

@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "nested_generics"
+
+[dependencies]
+std = { path = "../../../../reduced_std_libs/sway-lib-std-assert" }

test/src/e2e_vm_tests/test_programs/should_pass/language/nested_generics/json_abi_oracle.json

@@ -0,0 +1,25 @@
+{
+  "configurables": [],
+  "functions": [
+    {
+      "attributes": null,
+      "inputs": [],
+      "name": "main",
+      "output": {
+        "name": "",
+        "type": 0,
+        "typeArguments": null
+      }
+    }
+  ],
+  "loggedTypes": [],
+  "messagesTypes": [],
+  "types": [
+    {
+      "components": null,
+      "type": "bool",
+      "typeId": 0,
+      "typeParameters": null
+    }
+  ]
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/nested_generics/json_abi_oracle_new_encoding.json

@@ -0,0 +1,23 @@
+{
+  "concreteTypes": [
+    {
+      "concreteTypeId": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903",
+      "type": "bool"
+    }
+  ],
+  "configurables": [],
+  "encodingVersion": "1",
+  "functions": [
+    {
+      "attributes": null,
+      "inputs": [],
+      "name": "main",
+      "output": "b760f44fa5965c2474a3b471467a22c43185152129295af588b022ae50b50903"
+    }
+  ],
+  "loggedTypes": [],
+  "messagesTypes": [],
+  "metadataTypes": [],
+  "programType": "script",
+  "specVersion": "1"
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/nested_generics/src/main.sw

@@ -0,0 +1,74 @@
+script;
+
+enum MyOption<T> {
+    Some: T,
+    None: (),
+}
+
+impl<T> MyOption<T> {
+    fn new() -> Self {
+        Self::None
+    }
+
+    fn is_none(self) -> bool {
+        true
+    }
+}
+
+fn generic_arg_in_function_method_call<T>() {
+    let o: MyOption<u64> = MyOption::None;
+    let _ = o.is_none();
+    
+    let o: MyOption<MyOption<u64>> = MyOption::None;
+    let _ = o.is_none();
+
+    let o: MyOption<T> = MyOption::None;
+    let _ = o.is_none();
+    
+    let o: MyOption<MyOption<T>> = MyOption::None;
+    let _ = o.is_none();
+    
+    let _ = MyOption::is_none(o);
+}
+
+fn generic_arg_in_function_associated_function_call<T>() {
+    let _ = MyOption::<u64>::new();
+    let o: MyOption<u64> = MyOption::new();
+    
+    let _ = MyOption::<MyOption<u64>>::new();
+    let o: MyOption<MyOption<u64>> = MyOption::new();
+
+    let _ = MyOption::<T>::new();
+    let o: MyOption<T> = MyOption::new();
+    
+    let _ = MyOption::<MyOption<T>>::new();
+}
+
+struct S<T> { }
+
+impl<T> S<T> {
+    fn generic_arg_in_type() {
+        let o: MyOption<u64> = MyOption::None;
+        let _ = o.is_none();
+        
+        let o: MyOption<MyOption<u64>> = MyOption::None;
+        let _ = o.is_none();
+    
+        let o: MyOption<T> = MyOption::None;
+        let _ = o.is_none();
+        
+        let o: MyOption<MyOption<T>> = MyOption::None;
+        let _ = o.is_none();
+        
+        let _ = MyOption::is_none(o);
+    }
+}
+
+pub fn main() ->  bool {
+    generic_arg_in_function_method_call::<(())>();
+    S::<()>::generic_arg_in_type();
+
+    generic_arg_in_function_associated_function_call::<()>();
+
+    true
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/nested_generics/test.toml

@@ -0,0 +1,5 @@
+category = "run"
+expected_result = { action = "return", value = 1 }
+expected_result_new_encoding = { action = "return_data", value = "01" }
+validate_abi = true
+expected_warnings = 30