But the "late return type" signifier was there only because we could not get the natural and simpler syntax in C++11!

I am hoping we are not retaining complexity only because we can.

"auto" is the new "static".

-- Gaby


> On Mar 4, 2015, at 8:41 AM, Faisal Vali <faisalv@gmail.com> wrote:
>
> On Wed, Mar 4, 2015 at 10:17 AM, Bjarne Stroustrup
> <bjarne@stroustrup.com> wrote:
>>
>>
>>> On 3/3/2015 10:45 PM, Gabriel Dos Reis wrote:
>>>
>>> Should we declare "auto f(auto) -> auto" an excess in sophism?
>>
>>
>>
>> and those two "auto"s can denote different and unrelated types?
>
> yes - the first 'auto' is a trailing return type signifier, the second
> 'auto' is an abbreviated template signifier (adds a template parameter
> to the function template's template parameter list) and is deduced
> from the supplied argument, the third auto is the placeholder for
> return type deduction and is deduced from a return statement within
> the body of the function - it  does not add a template parameter to
> the function template's template parameter list - and is quite
> different from the second auto in that way.
>
>>
>>>
>>> -- Gaby
>>>
>>>
>>>> On Mar 3, 2015, at 6:39 PM, Faisal Vali <faisalv@gmail.com> wrote:
>>>>
>>>> In the hopes of soliciting constructive feedback, if anyone has the
>>>> time or the interest to play with a patched up Clang that implements
>>>> enhanced auto deduction & abbreviated template syntax from the
>>>> concepts ts, here it is:
>>>>
>>>> https://github.com/faisalv/clang/tree/enhanced-auto-c%2B%2B1z .
>>>>
>>>> The last line of the README file tells you the subversion revision of
>>>> LLVM to use.
>>>>
>>>> For examples of test cases that successfully compile, please refer to:
>>>> --
>>>> https://github.com/faisalv/clang/blob/enhanced-auto-c%2B%2B1z/test/CXX/auto/cxx1z-auto-vars-generalized.cpp
>>>> --
>>>> https://github.com/faisalv/clang/blob/enhanced-auto-c%2B%2B1z/test/CXX/auto/cxx1z-abbreviated-templates.cpp
>>>>
>>>> I would certainly appreciate the feedback!
>>>>
>>>> 1) Enhanced-Auto Deduction:
>>>>
>>>> pair<auto...> f() { return make_pair([] { }, [] { }); }
>>>> vector<auto> v = vector<int>();
>>>>
>>>> 2) Abbreviated Template Syntax:
>>>> void f(auto) <=> template<class T> void f(T);
>>>>
>>>>
>>>> A few, perhaps not so obvious or interesting, technicalities:
>>>>
>>>> a)  The equivalence of templates when it comes to trailing return
>>>> types in function pointer parameters is based on the order of the
>>>> 'auto' that signifies a placeholder, not just the appearance of an
>>>> auto in the declaration-specifier of a parameter:
>>>>
>>>>    template<class R, class P> void f( R(P) );  // #1
>>>>    template<class P, class R> void f(  R(P) );  // #2 (order of
>>>> templ-params flipped)
>>>>
>>>>    template<class R, class P> void f( auto (P)->R); // equivalent to
>>>> #1, not abbreviated.
>>>>    void f(auto(auto));  // equivalent to #1
>>>>    void f(auto (auto) -> auto); // equivalent to #2
>>>>                                               // the trailing return
>>>> type auto
>>>>                                               // identifies a
>>>> template parameter
>>>>    template<class R> void f(R(auto); // equivalent to #1
>>>>    template<class P> void f(auto(P)); // equivalent to #2
>>>>
>>>> b) variadic auto
>>>>    Once an ellipsis is seen as part of the declarator, all contained
>>>> auto placeholders get transformed into parameter packs.
>>>>    void f(auto (*...)(auto) -> std::tuple<auto, std::pair<auto,
>>>> auto>...>);
>>>> Note there are 4 deducible template parameter packs above.
>>>>
>>>> c) multi abbreviated template declarations
>>>>   void f(auto), g(auto);
>>>> are allowed - the above declares two function templates.
>>>>
>>>> d) template aliases desugar their types *after* each auto has been
>>>> identified as a placeholder (and so must be deduced)
>>>>
>>>>    template<class T> using Int = int;
>>>>    Int<auto> i = 3; // Is an error, auto must be deducible.
>>>>
>>>> e) generic lambda's as default arguments work with abbreviated template
>>>> syntax:
>>>>  void f(int (*)(int) = [](auto a) { return a; }, auto b =
>>>> decltype(b){});
>>>>  f<float*>(); // OK
>>>>
>>>> f) variable templates and enhanced auto deduction interact as expected:
>>>>    template<class T> pair<auto...> (*vfp)(auto...) = [](T t1, T* t2)
>>>> { return make_pair(t1, t2); };
>>>>
>>>> Thanks!
>>>> Faisal Vali
>>>>
>>>> --
>>>>
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