Library Typedefs are fundamentally broken

[Wyatt via Digitalmars-d]

On Saturday, 20 September 2014 at 04:52:58 UTC, Andrei 
Alexandrescu wrote:
>
> alias A = Typedef!float;
> alias B = Typedef!float;
>
> By basic language rules, A and B are identical. Making them 
> magically distinct would be surprising...
>
Hold up.  See, "Making them magically distinct would be 
surprising" is really the sticking point for me because in my 
experience it rings false.

When I reach for a typedef, I expect these things to NOT be 
identical.  More to the point, I'm explicitly stating: "This 
thing is not like any other thing."  That's the fundamental 
reason any sort of typedef exists in my world.  That the 
idiomatic library Typedef doesn't actually give these semantics 
unless I do extra stuff? _That_ is surprising and inconvenient.  
I'm personally having difficulty coming up with a situation where 
the current default behaviour is even useful.

You can make the argument that it's not that much of a burden.  
And on a cursory read, sure, that makes enough sense.  But it's 
still there and still acts as positive punishment.  We tend to 
tout type safety as a major feature, which builds the expectation 
that getting it for the common case is _completely trivial_ and 
in-line with the pragmatic approach taken by the rest of the 
language.  Adding a gotcha like this makes it less likely to be 
used correctly.

And I think it needs to be stressed: No one is arguing that the 
current behaviour shouldn't be possible at all; just that it's an 
unusual special case that makes for a warty default.  If you want 
two distinctly-named types to hash the same, give them a common 
cookie and be on your merry way.  Meanwhile, the issues with e.g. 
Typedef in templates disappear because it makes introducing a 
unique type the trivial default.

Anecdotally, I was explaining how neat it is that D has library 
Typedef to an engineer on my team and he commented that he'd 
never expect a word like "alias" to be associated with defining a 
distinct type, suggesting its use here is a misfeature.  He also 
called the cookie parameter a wart (unprompted).

-Wyatt