Re: [Boost-bugs] [Boost C++ Libraries] #10133: boost 1.55 breaks fusion map arrays

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #10133: boost 1.55 breaks fusion map arrays
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-02-21 14:35:49

#10133: boost 1.55 breaks fusion map arrays
-------------------------------+------------------------
  Reporter:  steven.seeger@…   |      Owner:  djowel
      Type:  Bugs              |     Status:  new
 Milestone:  To Be Determined  |  Component:  fusion
   Version:  Boost 1.55.0      |   Severity:  Regression
Resolution:                    |   Keywords:
-------------------------------+------------------------

Comment (by Kohei Takahashi <flast@…>):

 I believe that it is not a fusion's bug.

 The constructor of `map` will take their arguments by Universal
 References, like `template <class... T> map(T &&...)`, to accept any types
 and any value categories, i.e. perfect forwarding.
 However, compiler cannot deduce the argument type from brace-enclosed
 initializer, see below.

 n3337 14.8.2.1 [temp.deduct.call] p.1 sais:

   If removing references
   and cv-qualifiers from P gives std::initializer_list<P 0 > for some P 0
 and the argument is an initializer
   list (8.5.4), then deduction is performed instead for each element of
 the initializer list, taking P 0 as a function
   template parameter type and the initializer element as its argument.
 Otherwise, an initializer list argument
   causes the parameter to be considered a non-deduced context (14.8.2.5).

 and 14.8.2.5 [temp.deduct.type] p.4 sais:

   If a template parameter is used only in non-deduced
   contexts and is not explicitly specified, template argument deduction
 fails.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/10133#comment:2>
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