Re: C++ PATCHes to xvalue handling

[Sudakshina Das <sudi dot das at arm dot com>]

On 23/05/18 18:21, Jason Merrill wrote:
> The first patch implements the adjustments from core issues 616 and
> 1213 to the value category of subobjects of class prvalues: they were
> considered prvalues themselves, but that was kind of nonsensical.  Now
> they are considered xvalues.  Along with this, I've removed the
> diagnostic distinction between xvalues and prvalues when trying to use
> one or the other as an lvalue; the important thing is that they are
> rvalues.
>
> The second patch corrects various issues with casts and xvalues/rvalue
> references: we were treating an xvalue operand to dynamic_cast as an
> lvalue, and we were objecting to casts from prvalue to rvalue
> reference type.

With the second patch:
commit f7d2790049fd1e59af4b69ee12f7c101cfe4cdab
Author: jason <jason@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed May 23 17:21:39 2018 +0000

        Fix cast to rvalue reference from prvalue.

        * cvt.c (diagnose_ref_binding): Handle rvalue reference.
        * rtti.c (build_dynamic_cast_1): Don't try to build a reference to
        non-class type.  Handle xvalue argument.
        * typeck.c (build_reinterpret_cast_1): Allow cast from prvalue to
        rvalue reference.
        * semantics.c (finish_compound_literal): Do direct-initialization,
        not cast, to initialize a reference.

    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@260622 
138bc75d-0d04-0410-961f-82ee72b054a4

I have observed the following failure in Spec2017 while building 
510.parest_r on aarch64-none-linux-gnu

aarch64-none-linux-gnu-g++ -c -o 
source/numerics/matrices.all_dimensions.o -DSPEC -DNDEBUG -Iinclude -I. 
-DSPEC_AUTO_SUPPRESS_OPENMP  -mcpu=cortex-a57+crypto -Ofast 
-fomit-frame-pointer     -fpermissive    -DSPEC_LP64 
source/numerics/matrices.all_dimensions.cc

source/numerics/matrices.all_dimensions.cc: In static member function 
'static void dealii::MatrixTools::apply_boundary_values(const 
std::map<unsigned int, double>&, dealii::BlockSparseMatrix<number>&, 
dealii::BlockVector<number>&, dealii::BlockVector<number>&, bool)':

source/numerics/matrices.all_dimensions.cc:469:50: error: lvalue 
required as unary '&' operand

        [this_sparsity.get_rowstart_indices()[row]];

                                                  ^

source/numerics/matrices.all_dimensions.cc:472:55: error: lvalue 
required as unary '&' operand

           [this_sparsity.get_rowstart_indices()[row]+1],

                                                       ^

source/numerics/matrices.all_dimensions.cc:474:55: error: lvalue 
required as unary '&' operand

           [this_sparsity.get_rowstart_indices()[row+1]],

                                                       ^

source/numerics/matrices.all_dimensions.cc:479:49: error: lvalue 
required as unary '&' operand

       [this_sparsity.get_rowstart_indices()[row]],

                                                 ^

source/numerics/matrices.all_dimensions.cc:481:51: error: lvalue 
required as unary '&' operand

       [this_sparsity.get_rowstart_indices()[row+1]],

                                                   ^

source/numerics/matrices.all_dimensions.cc:510:50: error: lvalue 
required as unary '&' operand

          [this_sparsity.get_rowstart_indices()[0]]);

Sudi

> Tested x86_64-pc-linux-gnu, applying to trunk.