type awkwardness with parallel_reduce

[paciorek]

I'm not sure how this fits into general type questions in nCompiler.

If I use +, this is fine, albeit with the internal accumulator variable being a double. If I use min/max (i.e., pair{min,max}), this fails during C++ compilation:

 nc <- nClass(
    Cpublic = list(
      go = nFunction(
        fun = function(x = 'integerVector') {
          y <- parallel_reduce('pairmin', x, 100)
          return(y)
        },
        returnType = 'integerScalar'
      )
    )
  )
  Cnc <- nCompile(nc)

In file included from nCompiler_units_6.cpp:16:
/tmp/RtmpjUETF2/nCompiler_generatedCode/nClass_10_c_.h: In member function ‘void nClass_10::parallel_reduce_body::operator()(const tbb::blocked_range<int>&)’:
/tmp/RtmpjUETF2/nCompiler_generatedCode/nClass_10_c_.h:29:72: error: no matching function for call to ‘min(Eigen::Tensor<int, 1>::Scalar&, double&)’
   29 | for(i__ = r__.begin(); i__ != r__.end(); ++i__) flex_(val__) = std::min(x[(i__) - 1], val__);
      |                                                                ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/13/bits/specfun.h:43,
                 from /usr/include/c++/13/cmath:3699,
                 from /system/linux/lib/R-24.04/4.5.0/x86_64/site-library/Rcpp/include/Rcpp/platform/compiler.h:100,
                 from /system/linux/lib/R-24.04/4.5.0/x86_64/site-library/Rcpp/include/Rcpp/r/headers.h:66,
                 from /system/linux/lib/R-24.04/4.5.0/x86_64/site-library/Rcpp/include/RcppCommon.h:30,
                 from /accounts/vis/paciorek/R/x86_64-pc-linux-gnu-library-ubuntu-24.04/4.5/nCompiler/include/nCompiler/nCompiler_omnibus_pre_Rcpp.h:26:
/usr/include/c++/13/bits/stl_algobase.h:233:5: note: candidate: ‘template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)’
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/13/bits/stl_algobase.h:233:5: note:   template argument deduction/substitution failed:
/tmp/RtmpjUETF2/nCompiler_generatedCode/nClass_10_c_.h:29:72: note:   deduced conflicting types for parameter ‘const _Tp’ (‘int’ and ‘double’)
   29 | for(i__ = r__.begin(); i__ != r__.end(); ++i__) flex_(val__) = std::min(x[(i__) - 1], val__);
      |                                                                ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/13/bits/stl_algobase.h:281:5: note: candidate: ‘template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)’
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/13/bits/stl_algobase.h:281:5: note:   template argument deduction/substitution failed:
/tmp/RtmpjUETF2/nCompiler_generatedCode/nClass_10_c_.h:29:72: note:   deduced conflicting types for parameter ‘const _Tp’ (‘int’ and ‘double’)
   29 | for(i__ = r__.begin(); i__ != r__.end(); ++i__) flex_(val__) = std::min(x[(i__) - 1], val__);
      |                                                                ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~