[C++] Build error with g++ 7.5.0

[kou]

Describe the bug, including details regarding any error messages, version, and platform.

https://dev.azure.com/ursacomputing/crossbow/_build/results?buildId=68969&view=logs&j=0da5d1d9-276d-5173-c4c4-9d4d4ed14fdb&t=6c939d89-0d1a-51f2-8b30-091a7a82e98c&l=1571

/arrow/cpp/src/arrow/compute/kernels/pivot_internal.cc: In static member function ‘static arrow::Result<std::unique_ptr<arrow::compute::internal::PivotWiderKeyMapper> > arrow::compute::internal::PivotWiderKeyMapper::Make(const arrow::DataType&, const arrow::compute::PivotWiderOptions*, arrow::compute::ExecContext*)’:
/arrow/cpp/src/arrow/compute/kernels/pivot_internal.cc:153:10: error: could not convert ‘instance’ from ‘std::unique_ptr<arrow::compute::internal::ConcretePivotWiderKeyMapper, std::default_delete<arrow::compute::internal::ConcretePivotWiderKeyMapper> >’ to ‘arrow::Result<std::unique_ptr<arrow::compute::internal::PivotWiderKeyMapper> >’
   return instance;
          ^~~~~~~~

g++ 7.5.0 is used by test-r-rstudio-r-base-4.1-opensuse155.

Component(s)

C++

[kou]

Issue resolved by pull request 46028
#46028