When I use typedef or using for an int, how much is it still an int?

Question

I tried the following:

using Idx = int;
array<Value, N> arr;

for(Idx i = 0; i < N; i ++){
  arr[i].doSomething();
}

I expected that the compiler would issue a warning or an error when I try to use Idx as if it was an int. But it does not.

So, when I use using or typedef to alias type A as a B, are variables of type B still also of type A, and vice versa? So that no type safety can be achieved by renaming a type when it looks the same but has a different meaning.

(This is related to the following question I recently asked: How to make types for indexing)

Answer 1

An alias declaration or typedef just makes a new name for the aliased type. Idx is not a different type from int in this case; they can be used interchangeably with no difference in semantics.

From [dcl.typedef] (emphasis mine):

A name declared with the typedef specifier becomes a typedef-name. Within the scope of its declaration, a typedef-name is syntactically equivalent to a keyword and names the type associated with the identifier in the way described in Clause 8. A typedef-name is thus a synonym for another type. A typedef-name does not introduce a new type the way a class declaration or enum declaration does.