Added gcc-7

[denzor200]

[coveralls]

Pull Request Test Coverage Report for Build 804969193

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 804489526:	0.0%
Covered Lines:	325
Relevant Lines:	325

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3836890985

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 2983245262:	0.0%
Covered Lines:	337
Relevant Lines:	337

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💛 - Coveralls

[denzor200]

Needs more informative error message in core17_generated.hpp

[denzor200]

@apolukhin i'm finished

[denzor200]

/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../../include/c++/5.5.0/tuple:807:33: error: implicit instantiation of undefined template 'std::tuple_size<my_struct>'
    : integral_constant<size_t, tuple_size<_Tp>::value> { };
                                ^
./boost/pfr/detail/core17_generated.hpp:57:9: note: in instantiation of template class 'std::tuple_size<const my_struct>' requested here
  auto& [a,b,c] = val; // ====================> Boost.PFR: User-provided type is not a SimpleAggregate.

another day, troubles are the same

[denzor200]

/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../../include/c++/5.5.0/tuple:807:33: error: implicit instantiation of undefined template 'std::tuple_size<my_struct>'
    : integral_constant<size_t, tuple_size<_Tp>::value> { };
                                ^
./boost/pfr/detail/core17_generated.hpp:57:9: note: in instantiation of template class 'std::tuple_size<const my_struct>' requested here
  auto& [a,b,c] = val; // ====================> Boost.PFR: User-provided type is not a SimpleAggregate.

another day, troubles are the same

i suppose, i know how to fix it https://godbolt.org/z/hWzsojGe6

[denzor200]

Closed because it's not actual. Actual PR here: https://github.com/boostorg/pfr/pull/116