resolve c2146 on vs15 with area_result declaration

[lpranam]

Visual Studio 2015 C++14 (Default) we encountered C2146 stating
error C2146: syntax error: missing ',' before identifier 'result_type'

reference:
https://ci.appveyor.com/project/lpranam/astronomy-il73i/branch/develop/job/uo7pirc0s4v9xpb2

[awulkiew]

typename can't be used for base classes, see the failing minimal test:

./boost/geometry/algorithms/area_result.hpp:50:7: error: keyword ‘typename’ not allowed in this context (the base class is implicitly a type)

So the solution you proposed is incorrect. We could think of some workaround if we were sure that this is e.g. a bug in the compiler. But before that I'd like to investigate something different.

Recently I modified the area() algorithm to use "umbrella strategies":
#726
so strategies are used differently now.

What Boost.Geometry headers do you include?
How is boost/geometry/algorithms/area_result.hpp included in your code?
Do you use area?
Is area_result actually instantiated and if it is then where?
Is the problem still there when you include the whole Boost.Geometry library?

[awulkiew]

@lpranam ^

[lpranam]

@awulkiew I am yet to try your suggested solutions. Will update you soon :)

[lpranam]

So the solution you proposed is incorrect. We could think of some workaround if we were sure that this is e.g. a bug in the compiler. But before that I'd like to investigate something different.

Recently I modified the area() algorithm to use "umbrella strategies":
#726
so strategies are used differently now.

What Boost.Geometry headers do you include?

Thee are all the headers we have used

<boost/geometry/core/cs.hpp>
<boost/geometry/geometries/point.hpp>
<boost/geometry/algorithms/transform.hpp>
<boost/geometry/algorithms/equals.hpp>
<boost/geometry/strategies/strategy_transform.hpp>
<boost/geometry/arithmetic/cross_product.hpp>
<boost/geometry/arithmetic/dot_product.hpp>

How is boost/geometry/algorithms/area_result.hpp included in your code?
Do you use area?

No we do not use area or the related header anywhere is the code.

Is area_result actually instantiated and if it is then where?

No it is not instatiated

Is the problem still there when you include the whole Boost.Geometry library?

yes, I tried including the entire library(commit) and the result is still the same

@awulkiew ^

[gopi487krishna]

A temporary solution in fact could be to provide an indirection for derivation. What i mean by that is using multilevel inheritance we can separate the typename qualification . A working example could be something like this

But as i said before this is just a temporary solution although it doesn't have any costs associated with it 🤔

[lpranam]

@awulkiew a friendly ping. Do we have any solution for this?

[awulkiew]

@lpranam Not yet. The solution you proposed does not compile with standard-conforming compilers so I can't merge it. The solution proposed by @gopi487krishna looks overly complicated to me. If you want me to propose a workaround you'll have to wait because I have to test it with VS2015 which I don't have installed right now and I'm currently busy with something else.

But maybe something like this would work. Could you try to compile it with VS2015?

template
<
    typename Geometry,
    typename Strategy,
    bool IsUmbrella = strategies::detail::is_umbrella_strategy<Strategy>::value
>
struct area_result
{
    typedef decltype(std::declval<Strategy>().area(std::declval<Geometry>())) strategy_type;
    typedef typename strategy_type::template result_type<Geometry>::type type;
};

[lpranam]

@awulkiew your proposed solution works. Though there seems to be some problem in CI which is unrelated to the code.

[awulkiew]

Ok, let's give it a try. Thanks!