Topic: RFC take 2: return type inference


Author: Matthew Woehlke <mwoehlke.floss@gmail.com>
Date: Fri, 29 Jan 2016 11:10:51 -0500
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As requested, I have separated anonymous struct return from inferred
return type. The new version of the proposal (just the inferred return
type part) is attached.

Abstract: this proposal adds an additional case to the use of "auto" as
the return type specified, where the type is not deduced, but inferred
from a previous declaration:

  int foo();
  auto foo() { ... } // return type is 'int' due to prior declaration

I'm... considering submitting this version for the February mailing. I'm
concerned, however, that without the anonymous struct return, the
rationale is... lame. I'd appreciate any feedback or suggestions in this
respect. (TBH, I would really personally feel better submitting the
prior version instead. While the two ideas are separable, from a
rationale standpoint, they support each other.)

Online version:
https://github.com/mwoehlke/cpp-proposals/blob/master/dxxxx-implicit-return-type.rst

(As of writing, the link is currently the old version. I don't expect to
push the new version to the repo until the second paper is also ready;
at that point, however, the link will point to the new version.)

--
Matthew

--

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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
  Implicit Return Type
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

:Document:  DXXXX (TBD)
:Date:      2016-01-29
:Project:   ISO/IEC JTC1 SC22 WG21 Programming Language C++
:Audience:  Evolution Working Group
:Author:    Matthew Woehlke (mwoehlke.floss@gmail.com)

=2E. raw:: html

  <style>
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=2E. role:: add
    :class: addition

=2E. role:: del
    :class: removal

Abstract
=3D=3D=3D=3D=3D=3D=3D=3D

This proposal recommends an enhancement to return type deduction to allow=
 the return type of a function definition to be inferred from a previous =
declaration of the same.

(Note: references made to the existing draft standard are made against N4=
567_.)

=2E. contents::


Rationale
=3D=3D=3D=3D=3D=3D=3D=3D=3D

The origin for this idea relates to multiple return values, and a desire =
to use anonymous structs to implement the same. This notion will be furth=
er explored in a different paper (for which this proposal would be a prer=
equisite). The obvious problem with anonymous struct returns involves sep=
arating the declaration of such a function from its definition; namely, h=
ow does one repeat the name of the return type? This lead to a more inter=
esting |--| and obvious in retrospect |--| question: since C++ functions =
(excluding templates) cannot be overloaded by their return value, **why i=
s it necessary to repeat the return value at all?**

Even in the case of return types that can be named, it may be that repeat=
ing the type name is excessively verbose or otherwise undesirable. Some m=
ight even call this a violation of the `Don't Repeat Yourself <https://en=
=2Ewikipedia.org/wiki/Don't_repeat_yourself>`_ principle, similar to some=
 of the issues that ``auto`` for variable declaration was introduced to s=
olve. (On the flip side, one could see the ability to elide the return ty=
pe as subject to many abuses, again in much the manner of ``auto``. Howev=
er, many language features can be abused; this should not prevent the add=
ition of a feature that would provide an important benefit when used corr=
ectly.)

We would be remiss not to note that this is already possible in simple ca=
ses using ``decltype`` and a sample invocation of the function. However, =
while this may be adequate in simple cases, it is nevertheless needlessly=
 verbose, and as the argument list grows longer, and/or gains arguments f=
or which providing a legal value is non-trivial, it can quickly become un=
wieldy and unfeasible.


Proposal
=3D=3D=3D=3D=3D=3D=3D=3D

Recent changes to the language have progressively relaxed the requirement=
s for how return types are specified. We began with trailing return type =
specification, and have progressed to inferred return types in certain ca=
ses.

This proposal is to continue this direction by adding an additional use o=
f ``auto`` as a return specifier, meaning "use the return type seen when =
this function was previously declared". The use of ``auto`` as the return=
 type specifier, with no trailing return type, and for a function that ha=
s been previously declared with a known return type, shall instruct the c=
ompiler to define the function using the return type from the previous de=
claration.

Naturally, "previous declaration" here means a declaration having the sam=
e name and argument list. This, for example, would remain illegal:

=2E. code:: c++

  int foo(int);
  float foo(float);

  auto foo(double input) // does not match any previous declaration
  {
    ...
    return result;
  }


Proposed Wording
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

(Proposed changes are specified relative N4567_.)

Add a new section to [dcl.spec.auto] (7.1.6.4) as follows:

=2E. compound::
  :class: literal-block block-addition

  When a function is declared or defined using ``auto`` for the return ty=
pe, and a previous declaration or definition having a concrete return typ=
e exists, the return type shall be inferred to be the previously seen con=
crete type.
  [*Example:*

  .. parsed-literal::

    std::string f();
    auto f(); // OK, return type is std::string

  |--| *end example*]

Add a new section to [dcl.spec.auto] (7.1.6.4) as follows:

=2E. compound::
  :class: literal-block block-addition

  A template function redeclaration or specialization having a return typ=
e of ``auto`` shall match a previous declaration (or definition) if the f=
irst such declaration had a concrete return type. If the first such decla=
ration also had a return type of ``auto``, the declaration using return t=
ype deduction shall be matched instead.
  [*Example:*

  .. parsed-literal::

    template <typename T> T g(T t) { return t; } // #1
    template auto g(float); // matches #1

    template <typename T> auto g(T t) { return t; } // #2
    template <typename T> T g(T t) { return t; }
    template auto g(float); // matches #2

  |--| *end example*]


Discussion
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

What about template return types?
---------------------------------

In C++14, the following code is legal and produces two distinct templates=
:

=2E. code:: c++

  template <class T> int foo();
  template <class T> auto foo();

This obviously conflicts with the proposed feature. After discussion on `=
`std-proposals``, it was decided that the proposed feature should take pr=
ecedence in this case. It should also be noted that it is unclear how, or=
 even if, the second function can be invoked according to the current rul=
es of the language. (To this end, it may be desirable to simply forbid th=
e opposite ordering. However, we feel that this would be better addressed=
 separately, perhaps even as a DR.)

Must the declaration providing the concrete type be the first declaration=
?
-------------------------------------------------------------------------=
-

This question was originally brought up by Bengt Gustafsson. Specifically=
, for the sake of symmetry, it seems initially desirable to allow:

=2E. code:: c++

  int foo(); // specified return type
  auto foo() { return 42; } // return type inferred from prior declaratio=
n

  auto bar(); // forward declaration, type not yet known
  int bar(); // specify the return type as 'int'
  auto bar() { return 0; } // return type inferred from prior declaration=


To that end, earlier drafts of the proposal included the following propos=
ed change to [dcl.spec.auto]/13 (7.1.6.4.13):

=2E. compound::
  :class: literal-block

  Redeclarations or specializations of a function or function template wi=
th a declared return type that uses a placeholder type shall :del:`also u=
se that placeholder` :add:`use either that placeholder or a compatible co=
ncrete type`, not a deduced type. :add:`If the return type has previously=
 been deduced, a declaration using a concrete type shall use the deduced =
type.`
  [*Example:*

  .. parsed-literal::

    auto f();
    auto f() { return 42; } // return type is int
    auto f(); // OK
    :del:`int f(); // error, cannot be overloaded with auto f()`
    :add:`int f(); // OK, deduced type is also int`
    decltype(auto) f(); // error, auto and decltype(auto) don't match

    :add:`auto f(int);`
    :add:`int f(int); // OK, return type of f(int) is now int`
    :add:`float f(int); // error, redeclared with different return type`

However, upon further discussion, reservations were expressed, and the ge=
neral consensus seems to be that it is okay for the first declaration to =
"set in stone" if the return type will be known (and possibly later infer=
red), or deduced. Accordingly, absent the above change:

=2E. code:: c++

  auto bar();
  int bar(); // error, violates [dcl.spec.auto]/13
  auto bar() { return 0; } // okay, but return type is deduced, not infer=
red


Acknowledgments
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

We wish to thank everyone on the ``std-proposals`` forum, especially Beng=
t Gustafsson and Tim Song, for their valuable feedback and insights.


References
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=2E. _N4567: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n456=
7.pdf

* N4567_ Working Draft, Standard for Programming Language C++

  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4567.pdf

=2E. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..=
 .. ..

=2E. |--| unicode:: U+02014 .. em dash

=2E. kate: hl reStructuredText

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--------------010200050906090509090206--


.


Author: Moritz Klammler <moritz.klammler@gmail.com>
Date: Fri, 29 Jan 2016 18:09:58 +0100
Raw View
I didn't participate in the prior discussion so I'm sorry if I repeat
something that you have already discussed.  Anyway, my first thought
when reading this example from your proposal

    int foo(int);
    float foo(float);

    auto foo(double input) // does not match any previous declaration
    {
      ...
      return result;
    }

was: Oh, this could be a nasty source of bugs.  Let me re-write your
example a little.

    float foo(double);

    auto foo(double x)
    {
      return x;
    }

Looking only at the definition, I could reasonably expect that `foo` has
a `return` type of `double` when actually it narrows it to a `float`.
This reminds me of the various gotchas you can run into when trying to
override `virtual` member functions and fail to match the signature
exactly.

If you think that this potential downside is overcompensated by the
gains, I think you should include a paragraph into your proposal to
mollify people who think like I did.

Did you consider adding an analogue to the `override` contextual keyword
such that one could write

    float foo(double);

    auto foo(double x) overlook
    {
      return x;
    }

which would cause the compiler to refuse the code if there was no
previous declaration of `auto foo(double)`?  It would also hint the
human reader that it is not `auto` type deduction that is going on here
and they should probably go and look for the preceding declaration.

You'll probably be able to come up with a better word than `overlook`...



Matthew Woehlke <mwoehlke.floss@gmail.com> writes:

> As requested, I have separated anonymous struct return from inferred
> return type. The new version of the proposal (just the inferred return
> type part) is attached.
>
> Abstract: this proposal adds an additional case to the use of "auto"
> as the return type specified, where the type is not deduced, but
> inferred from a previous declaration:
>
>   int foo();
>   auto foo() { ... } // return type is 'int' due to prior declaration
>
> I'm... considering submitting this version for the February
> mailing. I'm concerned, however, that without the anonymous struct
> return, the rationale is... lame. I'd appreciate any feedback or
> suggestions in this respect. (TBH, I would really personally feel
> better submitting the prior version instead. While the two ideas are
> separable, from a rationale standpoint, they support each other.)
>
> Online version:
> https://github.com/mwoehlke/cpp-proposals/blob/master/dxxxx-implicit-return-type.rst
>
> (As of writing, the link is currently the old version. I don't expect
> to push the new version to the repo until the second paper is also
> ready; at that point, however, the link will point to the new
> version.)

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Author: Matthew Woehlke <mwoehlke.floss@gmail.com>
Date: Fri, 29 Jan 2016 12:59:32 -0500
Raw View
On 2016-01-29 12:09, Moritz Klammler wrote:
> I didn't participate in the prior discussion so I'm sorry if I repeat
> something that you have already discussed.  Anyway, my first thought
> when reading this example from your proposal
>
>     int foo(int);
>     float foo(float);
>
>     auto foo(double input) // does not match any previous declaration
>     {
>       ...
>       return result;
>     }
>
> was: Oh, this could be a nasty source of bugs.  Let me re-write your
> example a little.
>
>     float foo(double);
>
>     auto foo(double x)
>     {
>       return x;
>     }
>
> Looking only at the definition, I could reasonably expect that `foo` has
> a `return` type of `double` when actually it narrows it to a `float`.

1. Your compiler should warn you of this.
2. Don't do that?

I'm not really expecting people to use this for simple return types (in
fact, the original motivating case was for anonymous structs), but for
types that are a pain to write (i.e. 30+ characters long). As mentioned
in the paper, it can be abused, but so can 'auto' for variable
declarations. In many of the same ways, in fact.

> Did you consider adding an analogue to the `override` contextual keyword
> such that one could write
>
>     float foo(double);
>
>     auto foo(double x) overlook
>     {
>       return x;
>     }
>
> which would cause the compiler to refuse the code if there was no
> previous declaration of `auto foo(double)`?

No, but it's an interesting idea. Given the time constraints, I don't
know if I'll be able to add that or not... We'll see.

> You'll probably be able to come up with a better word than `overlook`...

There are only two hard problems in programming: cache invalidation,
naming things, and off-by-one errors. This, alas, falls into the second
category. (IOW: suggestions welcomed :-).)

--
Matthew

--

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