add a typechecking test

numbat/src/typechecker/tests/type_checking.rs

@@ -190,6 +190,20 @@ fn recursive_functions() {
     ));
 }
 
+#[test]
+fn function_definitions_with_local_variables() {
+    assert_successful_typecheck("fn f(x: A) -> C = x * y where y: B = b");
+    assert_successful_typecheck(
+        "fn f(x: A) -> C = y * z where
+                                    y = x * 2
+                                    z = b * 2",
+    );
+    assert!(matches!(
+        get_typecheck_error("fn f(x: A) = y where y = x + b"),
+        TypeCheckError::IncompatibleDimensions(_)
+    ));
+}
+
 #[test]
 fn generics_basic() {
     assert_successful_typecheck(