Internal quality is external quality haha. If your types are wrong your program is wrong, and if it just happens to work now wait until more people use it, or you refactor it, or you change the way it works. Your users are basically just fuzzing machines haha.
Your statement that "nobody ever cared to use tests to replicate what the type system does" is trivially false, have you ever tested what happens when you pass "null" into a function that probably shouldn't take null? If so, you just did. Some type systems allow you to even specify valid subranges of types, and combined with algebraic data types, it's effectively impossible to misuse.
Your statement that "nobody ever cared to use tests to replicate what the type system does" is trivially false, have you ever tested what happens when you pass "null" into a function that probably shouldn't take null? If so, you just did. Some type systems allow you to even specify valid subranges of types, and combined with algebraic data types, it's effectively impossible to misuse.