Adds function return type unification. (#6490)

## Description

The function return type unification was missing, leading to generic
functions not being resolved based on the return type.

## 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 <[email protected]>
Co-authored-by: IGI-111 <[email protected]>

sway-core/src/semantic_analysis/ast_node/expression/typed_expression/function_application.rs

@@ -61,6 +61,17 @@ pub(crate) fn instantiate_function_application(
         &function_decl.parameters,
     )?;
 
+    // unify function return type with current ctx.type_annotation().
+    engines.te().unify_with_generic(
+        handler,
+        engines,
+        function_decl.return_type.type_id,
+        ctx.type_annotation(),
+        &call_path_binding.span(),
+        "Function return type does not match up with local type annotation.",
+        None,
+    );
+
     let mut function_return_type_id = function_decl.return_type.type_id;
 
     let function_ident: IdentUnique = function_decl.name.clone().into();

test/src/e2e_vm_tests/test_programs/should_fail/bad_generic_annotation/test.toml

@@ -1,5 +1,11 @@
 category = "fail"
 
+# check: let _g: u32 = three_generics(true, "foo", 10);
+# nextln: $()Mismatched types.
+# nextln: $()expected: u32
+# nextln: $()found:    str.
+# nextln: $()help: Function return type does not match up with local type annotation.
+
 # check: let _g: u32 = three_generics(true, "foo", 10);
 # nextln: $()Mismatched types.
 # nextln: $()expected: u32

test/src/e2e_vm_tests/test_programs/should_fail/language/reexport/shadowing_in_reexporting_module/test.toml

@@ -188,4 +188,4 @@ category = "fail"
 #check: $()error
 #check: $()This path must return a value of type "u64" from function "items_2_trait_function", but it does not.
 
-#check: $()Aborting due to 40 errors.
+#check: $()Aborting due to 42 errors.

test/src/e2e_vm_tests/test_programs/should_fail/recursive_type_unification/test.toml

@@ -1,5 +1,12 @@
 category = "fail"
 
+# check: $()error
+# check: $()MyOption::Some::<V>(foo::<V>(value))
+# nextln: $()Mismatched types.
+# nextln: $()expected: V
+# nextln: $()found:    MyOption<V>.
+# nextln: $()Function return type does not match up with local type annotation.
+
 # check: $()error
 # check: $()MyOption::Some::<V>(foo::<V>(value))
 # nextln: $()Mismatched types.

test/src/e2e_vm_tests/test_programs/should_pass/language/function_return_type_unification/.gitignore

@@ -0,0 +1,2 @@
+out
+target

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

@@ -0,0 +1,13 @@
+[[package]]
+name = "core"
+source = "path+from-root-D73E65B1F4E48513"
+
+[[package]]
+name = "function_return_type_unification"
+source = "member"
+dependencies = ["std"]
+
+[[package]]
+name = "std"
+source = "path+from-root-D73E65B1F4E48513"
+dependencies = ["core"]

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

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

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

@@ -0,0 +1,29 @@
+script;
+
+trait Build {
+    fn build() -> Self;
+}
+
+impl Build for u32 {
+    fn build() -> Self {
+        31
+    }
+}
+
+impl Build for u64 {
+    fn build() -> Self {
+        63
+    }
+}
+
+fn produce<T>() -> T
+where T: Build,
+{
+    T::build()
+}
+
+fn main() -> bool {
+    let _:u32 = produce();
+
+    true
+}

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

@@ -0,0 +1,4 @@
+category = "run"
+expected_result = { action = "return", value = 1 }
+expected_result_new_encoding = { action = "return_data", value = "01" }
+validate_abi = false