Error when a user tries to redefine a field used in a superclass.

lang_tests/instance_fields_overlap/test.som

@@ -0,0 +1,13 @@
+"
+VM:
+  status: error
+  stderr:
+    ...test.som', line 10, column 23:
+      test = test_super ( | a |
+    Field 'a' is already defined in a superclass.
+"
+
+test = test_super ( | a |
+  run = (
+  )
+)

lang_tests/instance_fields_overlap/test_super.som

@@ -0,0 +1,3 @@
+test_super = (
+    | a |
+)

src/lib/compiler/ast_to_instrs.rs

@@ -153,6 +153,15 @@ impl<'a, 'input> Compiler<'a, 'input> {
                     .iter()
                     .map(|(k, v)| (k.to_owned(), *v)),
             );
+            for var in ast_inst_vars {
+                let n = lexer.span_str(*var);
+                if inst_vars.contains_key(n) {
+                    return Err(vec![(
+                        *var,
+                        format!("Field '{}' is already defined in a superclass.", n),
+                    )]);
+                }
+            }
             inst_vars
         } else {
             HashMap::with_capacity(ast_inst_vars.len())