String foo = "bar"; String baz = foo;

The `String`s can be completely avoided in languages with type inference because it's obvious that a string literal is a string.

or auto&&. Did you really intend to make a copy?

If it was instead

    String foo = "bar";
    auto baz = foo;

I remember my first attempts at programming and being annoyed that I can't add a string and an int; ever since that little bit of housekeeping of using types made sense to me and I can clearly see how it eliminates entire classes of errors.

I find it more painful to read code that has too many type annotations. I also find it painful to read code that has too few, so I'd argue there's a bit of an art to it.

But languages that have type inference but allow type annotations at least allow you to try to hit that balance.

Type inference doesn't make these errors go away.

And about your other point, it's unfair to look at just a simple case of writing "string" or not as the only thing inference provides. Although I'd argue that leaving out types where possible helps readability-- it's really the more elaborate cases or intermediate steps during a longer transformation that inference helps with. Not to mention the fact that inference in closures is also really nice.

Generally, I prefer being able to read a line of code and understanding exactly what it does. If I need an IDE and have to repeatedly try to find the definition of something then, in my opinion, that's wasting my time.

C++'s 'auto' is really useful but it's over-used IMO. I think that there's a belief that if you're not using 'auto' everywhere then you're not writing 'modern' C++. Just becuase your code compiles doesn't necessarily mean it's correct.

I'd never have believed it myself, but find myself using acronyms instead of variable names when the type allows it.

    void foo(MyType mt, const MyOtherType& mot);

However, it maybe doesn't work that well with things like class member names. YMMV

    using B64String = std::string;

    B64String encode(const std::string& text);
    std::string decode(const B64String b64);

The fact C++ doesn't have Numeric but instead has int and long and unsigned and long long and float and double all off on their own is another problem: The compiler knows enough about them to have complex promotion rules but doesn't know enough to allow me to refer to all of them under one name in my code.

> I can have an array of T, but I can't specify that T is Numeric?

Sure you can. If you have C++20 concepts:

  template<class T>
  concept Numeric = std::integral<T> || std::floating_point<T>;
  
  template<Numeric T>
  T twice(T x)
  { return x + x; }

  template<class T>
  typename std::enable_if<
    std::is_arithmetic<T>::value,
    T>::type twice(T x)
  { return x + x; }

    Numeric auto twice(Numeric auto x)
    { return x + x; }

And imagine what would happen when you get a few more 'auto' variables in the return expression. Suddenly your return type will depend on the implementation of your callees. And the code can then quickly become impossible to understand.

auto is overused.

Why would that be a problem ? It's super common in templates and has never troubled me the least

1. "My return type is whatever I happen to return" circumvents the ability of the type checker to ensure correctness.

2. More generally, the purpose of a specification (a function declaration in this case) is to declare what is required of a compliant implementation, and to provide a way to check the validity of that implementation. But when you make the types all become auto-deduced, you're basically reducing the specification to a ~ shoulder shrug "it does whatever it does" ~.

3. Moreover, as I alluded to in the comment, it quickly becomes near-impossible to meaningfully separate the definition from the declaration, whether that's because you want to hide it or because you want to compile it separately. Simply put, you lose modularity. It seems like a minor thing when (as in the example) the return value doesn't depend on types inferred from other callees' return values, but as soon as that ceases to be true, you suddenly tie together the implementations of multiple functions. At that point, your functions lose much of their power to abstract away anything, since as soon as you change the return expression for one function, it has the potential to break code (up to and including causing compilation errors) in the the entire chain of callers. (!)

4. Templates end up getting re-instantiated far more often than they need to be (which can slow down both the compilation and the runtime efficiency). You almost certainly don't want '0' and '(size_t)0' to result in duplicate instantiations when dealing with sizes, for instance.

5. Issue #4 can also result warnings/errors/bugs, since now you have a function that returns a different concrete type than you likely intended, which can result in everything from spurious warnings (signed/unsigned casts, for instance) to actual bugs (later truncation of other variables whose types were inferred incorrectly as a result).

6. The code becomes difficult for a human to read too. You now no longer have any idea what types some variables are supposed to be. Not only does this hamper your ability to cross-check the correctness of the implementation itself (just as with the declarations, in #1) but unless your function is trivial, this quickly makes it harder to even understand what the code is doing in the first place, never mind what it's supposed to do.

7. Proxy types become impossible to implement, since they won't undergo the intended conversions anymore.

All this just to reduce keystrokes might be a common trade-off, but a poor one. I can come up with more reasons, but hopefully this gets the point across.

> I can have an array of T, but I can't specify that T is Numeric?

This is what type-traits and 'concepts' are for, right?

> The compiler knows enough about them to have complex promotion rules but doesn't know enough to allow me to refer to all of them under one name in my code.

This is what std::is_integral gives you.

https://en.cppreference.com/w/cpp/types/is_integral