Relax input type requirements for thrust::merge

[jrhemstad]

thrust::merge states this requirement:

InputIterator2 and InputIterator1 have the same value_type

https://thrust.github.io/doc/group__merging_ga3d2776685a00dca265399f411784acec.html

cppreference for std::merge has a slightly more relaxed requirement:

The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​

https://en.cppreference.com/w/cpp/algorithm/merge

However, I can't find this requirement anywhere in the standard, so I don't know where cppreference is getting that from

• Current https://eel.is/c++draft/alg.merge
• C++17 https://timsong-cpp.github.io/cppwp/n4659/alg.merge

Furthermore, none of gcc/clang/msvc seem to actually require this: https://godbolt.org/z/v1WcW5scv

The Thrust requirement stems from the fact that it loads elements from both input ranges into a single thread local, register array, thus requiring them to be the same type: https://github.com/NVIDIA/thrust/blob/7afdb87ad0e0a213a6f7e50548cbabc7e19f400f/thrust/system/cuda/detail/merge.h#L480-L485

A cursory look through the algorithm makes it seem like it wouldn't be too difficult to instead use two distinct storage arrays for each input range.

[jrhemstad]

Synced with @allisonvacanti offline and she agrees this requirement can be relaxed.