Re: C++ PATCH to add auto return type deduction with -std=c++1y

[Marc Glisse <marc dot glisse at inria dot fr>]

On Sun, 25 Mar 2012, Basile Starynkevitch wrote:

> On Sun, 25 Mar 2012 12:17:10 +0200 (CEST)
> Marc Glisse <marc.glisse@inria.fr> wrote:
>
> > On Sun, 25 Mar 2012, Jason Merrill wrote:
> >
> > > As I mentioned in my patch to add -std=c++1y, I've been working on a proposal
> > > for the next standard to support return type deduction for normal functions,
> > > not just lambdas.  This patch implements that proposal.
> >
> > Nice. I like the way you seem to be going for the basic, uncontroversial
> > version (extensions can always be discussed later), instead of trying to
> > figure out something universal.
> >
> > If I understand correctly, you pick the first return statement for type
> > deduction, and other returns (if any) are only checked afterwards for
> > exact consistency [...]
>
> I am not a C++ or a GCC front-end expert, but I am not sure it is the right approach for
> functions starting with
>   if (!p) return nullptr;
> where p is a formal argument.
>
> Or perhaps I am misunderstanding what Marc is saying.
>
> I would rather suggest using all the return statements to infer the type of the function,
> not only the first one.

First, note that I only explained my understanding from a quick look at 
the testsuite examples and a couple minutes playing with it, the actual 
proposal may well be very different. However:

- a first goal is simple functions, with a single return statement (which 
may even often be the only statement).
- inferring a common type is very hard to define (std::common_type has a 
number of funny properties).
- since he rejects code that has several returns with different types, any 
kind of inference you later manage to add will be 100% compatible.

Yes, it has limitations (it is because I hit them that I guessed the 
"first return statement" rule), but it never does the wrong thing, at 
worst it rejects code that is already invalid today. I agree that 
inference from all returns would be great, later...

--
Marc Glisse