[cfe-dev] Couldn't infer template argument '_BinaryOp' for std::max<int>

Krzysztof Parzyszek via cfe-dev cfe-dev at lists.llvm.org
Fri Jan 25 09:40:00 PST 2019


Hi,
I have this snippet of code and I'm trying to understand if I'm doing 
something wrong, or whether this is expected to work.

I want "max" to be the reduction function in transform_reduce, and so I 
passed std::max<int> as the argument, but it resulted in a compilation 
error (below). When I use an equivalent lambda, it works.

--- tr.cc ---
#include <numeric>
#include <vector>

struct S {
   int v;
};

int fred(std::vector<S> &v) {
   // Works
   return std::transform_reduce(v.begin(), v.end(), 0,
            [](const int &a, const int &b) { return std::max<int>(a, b); },
            [](const S &s) { return s.v; });

     // Fails
//  return std::transform_reduce(v.begin(), v.end(), 0,
//           std::max<int>,
//           [](const S &s) { return s.v; });
}
-------------


$ clang++ -std=c++17 -c tr.cc -stdlib=libc++


tr.cc:12:10: error: no matching function for call to 'transform_reduce'
   return std::transform_reduce(v.begin(), v.end(), 0,
          ^~~~~~~~~~~~~~~~~~~~~
/w/c/org/bin/../include/c++/v1/numeric:229:1: note: candidate template 
ignored:
       couldn't infer template argument '_BinaryOp'
transform_reduce(_InputIterator __first, _InputIterator __last,
^
/w/c/org/bin/../include/c++/v1/numeric:241:1: note: candidate function 
template
       not viable: requires 6 arguments, but 5 were provided
transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1,
^
/w/c/org/bin/../include/c++/v1/numeric:252:1: note: candidate function 
template
       not viable: requires 4 arguments, but 5 were provided
transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1,
^
1 error generated.


-Krzysztof


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