Fix trait interface method name resolution during typechecking of impl

sway-core/src/semantic_analysis/ast_node/declaration/impl_trait.rs

@@ -522,7 +522,33 @@ fn type_check_trait_implementation(
                 let method = decl_engine.get_trait_fn(decl_ref);
                 let name = method.name.clone();
                 method_checklist.insert(name.clone(), method);
-                interface_item_refs.insert((name, self_type), item.clone());
+
+                // After monomorphization (and typechecking trait implementation
+                // works on a monomorphized version of a trait declaration), the
+                // newly produced [decl_id]s for interface methods can only be
+                // traced back to the original (non-monomorphized) trait
+                // declaration, which means that during [decl_id] substitutions
+                // of trait interface dummy functions for the corresponding
+                // interface method _implementations_, we need to refer to the
+                // original [decl_id]s of the non-monomorphized trait
+                // declaration.
+                let interface_item_parents = decl_engine.find_all_parents(engines, decl_ref.id());
+                match interface_item_parents.len() {
+                    0 => { interface_item_refs.insert((name, self_type), item.clone()); },
+                    1 => match interface_item_parents[0] {
+                            AssociatedItemDeclId::TraitFn(parent_decl_id) => {
+                                let parent_interface_item =
+                                    TyTraitInterfaceItem::TraitFn(DeclRef::new(name.clone(), parent_decl_id, decl_ref.span()));
+                                interface_item_refs.insert((name, self_type), parent_interface_item);
+                            }
+                            _ => return Err(handler.emit_err(CompileError::Internal(
+                                    "A trait interface method's parent is expected to be another trait interface method",
+                                    name.span())))
+                         }
+                    _ => return Err(handler.emit_err(CompileError::Internal(
+                            "A trait interface method is expected to have zero parents or one parent in the Declaration Engine",
+                            name.span())))
+                }
             }
             TyTraitInterfaceItem::Constant(decl_ref) => {
                 let constant = decl_engine.get_constant(decl_ref);

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

@@ -0,0 +1,3 @@
+[[package]]
+name = "name_resolution_after_monomorphization"
+source = "member"

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

@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <contact@fuel.sh>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "name_resolution_after_monomorphization"
+implicit-std = false
+
+[dependencies]

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

@@ -0,0 +1,26 @@
+script;
+
+trait Eq<T> {
+    fn eq(self, other: Self) -> bool;
+} {
+    fn neq(self, other: Self) -> bool {
+        __eq((self.eq(other)), false)
+    }
+}
+
+impl Eq<u64> for u64 {
+    fn eq(self, other: Self) -> bool {
+        __eq(self, other)
+    }
+}
+
+fn main() -> u64 {
+    // block const evaluation for `x` (it does not currently support asm-blocks)
+    let x = asm(x: 42u64) { x: u64 };
+    let y = 1u64;
+    if x.neq(y) {
+        2
+    } else {
+        101
+    }
+}

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

@@ -0,0 +1,4 @@
+category = "run"
+expected_result = { action = "return", value = 2 }
+validate_abi = false
+expected_warnings = 1