Code behaves incorrectly if it is compiled in std.functional

[Artur Skawina via Digitalmars-d]

On 06/07/15 18:49, Timon Gehr via Digitalmars-d wrote:
> This is valid C:
> 
> int main(){
>    const auto int x=2;
>    return 0;
> }
> 
> This is not valid C:
> 
> int main(){
>    auto auto int x=2;
>    return 0;
> }
> 
> What is the problem?

The problem is the apparently common misunderstanding that
'auto' is a thing in D. It is not; it's just a grammar hack.


The (very simplified) rule for declarations in C/C++/D is

   <storage_class> <type> lhs = rhs

In C, you can omit the /storage_class/ and it then defaults to
'auto'. Obviously, the 'auto' keyword is redundant and nobody
actually uses it.

In D, you can omit the /type/ and it's then propagated from the
/rhs/ expression. The compiler already knows the type that 'rhs'
evaluates to.

But you can not omit both the /type/ and the /storage_class/ as
'lhs=rhs' would be indistinguishable from an assignment. In some
contexts for the compiler, but, more importantly, for the human.
Hence 'auto lhs = rhs'.

[The exceptions are either because of backward C compatibility
 (function args) or no need for such compatibility (foreach)]


Still, 'auto' isn't as bad as 'static', which D redefined to mean
something different than in C/C++, and did this so subtly that
the C version will still compile, giving unexpected results without
even a warning.

artur