$defined(x[0] = val) causes an error instead of returning false when a type does not have []= defined #2454

test/test_suite/compile_time_introspection/defined_subscript_assign.c3

@@ -0,0 +1,14 @@
+fn int main(String[] args)
+{
+	MyArray a = {{1, 4, 3, 6, 5}};
+	$assert(!$defined(a[0] = 2));
+	return 0;
+}
+
+struct MyArray
+{
+	int[] val;
+}
+
+fn int MyArray.get(self, usz idx) @operator([]) => self.val[idx];
+fn usz MyArray.len(self) @operator(len) => self.val.len;