Compile error under MSVC 2015/2017 if <thread> included in same file as "doctest.h"

[lewissbaker]

The following snippet fails to compile with Visual Studio 2017.1 and single-header doctest.h from v1.2.0.
I also see the same compiler error under VS 2015.3.

#include <thread>
#include "doctest.h"

It fails with the following compile error:

c:\path\to\doctest.h(830): error C2248: 'std::thread::id::id': cannot access private member declared in class 'std::thread::id'
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.10.25017\include\thread(165): note: see declaration of 'std::thread::id::id'
c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.10.25017\include\thread(149): note: see declaration of 'std::thread::id'
c:\path\to\doctest.h(836): note: see reference to class template instantiation 'doctest::detail::has_insertion_operator_impl::has_insertion_operator<T>' being compiled
        with
        [
            T=char
        ]
c:\path\to\doctest.h(1828): note: see reference to class template instantiation 'doctest::detail::has_insertion_operator<T>' being compiled
        with
        [
            T=char
        ]
c:\path\to\doctest.h(1833): note: see reference to class template instantiation 'doctest::detail::StringStream<T>' being compiled
        with
        [
            T=char
        ]
c:\path\to\doctest.h(1863): note: see reference to function template instantiation 'void doctest::detail::toStream<T>(std::ostream *,const T &)' being compiled
        with
        [
            T=char
        ]
c:\path\to\doctest.h(1862): note: while compiling class template member function 'void doctest::detail::ContextBuilder::Capture<char>::toStream(std::ostream *) const'
c:\path\to\doctest.h(1869): note: see reference to class template instantiation 'doctest::detail::ContextBuilder::Capture<char>' being compiled

However, if I add #include <ostream> before including "doctest.h" it compiles fine.

It seems like it's missing the declaration of operator<<(std::ostream&, char) from <ostream>.

This means that when it's instantiating doctest::detail::has_insertion_operator<char> it's only finding the operator<<(std::ostream&, std::thread::id) overload and thus trying to convert the char value to a std::thread::id via the private constructor, which takes an unsigned int.

The doctest.h header includes standard headers inside a section guarded by #if defined(DOCTEST_CONFIG_IMPLEMENT) || !defined(DOCTEST_SINGLE_HEADER).

This means that if you're using the single-header version then unless you have defined either the DOCTEST_CONFIG_IMPLEMENTATION_IN_DLL or the DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN macro then doctest.h won't pull in any of the standard headers that define the built-in ostream operators.

I think doctest.h needs to #include <ostream> to get visibility of the std::ostream& operator<<(std::ostream&, char) overload as it's required by the ContextBuilder::Chunk type, which instantiates Capture<char>, which instantiates detail::toStream<T>(std::ostream&, const char&), which instantiates detail::StringStream<char>, which instantiates detail::has_insertion_operator<char>, which tries to sink a char value into a std::ostream.

Or am I missing something in how I'm using the library?

I've tried defining DOCTEST_CONFIG_USE_IOSFWD but this doesn't fix the problem (<iosfwd> doesn't declare operator<< overloads under MSVC).