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Why is the new auto keyword from C++11 or C23 dangerous?

In older C and C++ standards, the `auto` keyword simply meant automatic storage duration. As in the compiler automatically handles where the variable is stored, typically on the stack or in a register. And it was a pretty useless keyword since it can only be used at local scope, where all variables default to automatic storage duration anyway.

The C++11 committee decided to change the meaning of this keyword so that during declaration, the type is picked based on the initializer(s) provided. For example `auto i=0;` will result in `int` because the integer constant `0` is of type `int`.

As I understand it, the main rationale was to get rid of cumbersome declarations in `for` loops in particular.

    for(auto i = cont.begin(); ...

is admittedly easier for the eye than

    for(std::vector<std::string>::iterator i = cont.begin(); ...

However, veteran programmers seem to raise concerns about `auto` being unsafe. It seems to be a topic where there's plenty of personal opinions as seen over at SO: [How much is too much with C++11 auto keyword?](https://stackoverflow.com/questions/6434971/how-much-is-too-much-with-c11-auto-keyword) Some people just happily encourage "go for it everywhere". Others, including various well-known C++ gurus, speak in favour of using it with caution.

Now C too is adapting the same functionality of `auto` as C++,
as per C23. 

**What exactly is dangerous with the `auto` keyword?**