Type inference of a function parameter

[Mike Parker via Digitalmars-d-learn]

On Wednesday, 29 July 2015 at 20:20:47 UTC, Adel Mamin wrote:
> Hello,
>
> Why dmd cannot inference the type of 'arr' in my_func() 
> parameter?
> test.d:
>
> import std.stdio;
>
> void my_func(auto arr)
> {
>   writeln(arr);
> }
>
> void main()
> {
>   auto arr = new int[5];
>
>   arr = [1, 2, 3, 4, 5];
>
>   my_func(arr);
> }
>
>> dmd test.d
> test.d(3): Error: undefined identifier arr

Consider what would happen if my_func is compiled in a library. 
How would the compiler know what type arr is if there is no code 
around that is calling it? In that case, it would be unable to 
compile the function. Furthermore, what happens if my_func is 
called with multiple types? A function has one instance and one 
instance only, so arr can only ever be one type.

With templates, the code is always available, so the compiler can 
always know what the type is inside the function. And you can 
have multiple instances of any template, so it's possible for arr 
to be different types.