ambiguous call to begin() when using CAPTURE

[jwdevel]

Describe the bug
I am using EASTL's fixed_vector in a test.
When I try to CAPTURE() the fixed_vector, I get a compile error due to ambiguity of begin(), when trying to evaluate Catch::is_range::value:

../../3rdparty/catch2/single_include/catch2/catch.hpp|1979 col 41| error: call of overloaded ‘begin(eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>)’ is ambiguous
             !std::is_same<decltype(begin(std::declval<T>())), not_this_one>::value &&
                                    ~~~~~^~~~~~~~~~~~~~~~~~~

(full error below)

Expected behavior
I would expect the appropriate begin() to be chosen.
In this case, it would be: eastl::begin

Theorizing

I imagine this has to do with some subtlety of ADL, but I am not sure.

Perhaps eastl::begin is a candidate because the type is from that namespace, and std::begin is an equally-good candidate because this is being evaluated within std::is_same ?
Or maybe it is due to this catch2 code.

Note that other cases outside of Catch2 work fine, such as:

for (auto i : myvec) {
    //...
}

Though I know the rules for range-based for are a bit specialized, so maybe not the best example...

Reproduction steps

Nothing special with my setup; once you have EASTL and Catch2 set up, this will produce the error:

#include <EASTL/fixed_vector.h>
#include <catch2/catch.hpp>

SCENARIO("bug repro") {
    eastl::fixed_vector<int, 2, false> vec;
    CAPTURE(vec);
}

Platform information:

• C++14
• compiler: gcc (Debian 8.3.0-6) 8.3.0
• Catch2 version: 2d172dc
• OS: Debian 4.19.67-2

Additional context

Full error output:

In file included from tests/main/src/box.cpp:11:
../../3rdparty/catch2/single_include/catch2/catch.hpp: In instantiation of ‘const bool Catch::is_range<eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false> >::value’:
../../3rdparty/catch2/single_include/catch2/catch.hpp|2013 col 70| required by substitution of ‘template<class R> struct Catch::StringMaker<R, typename std::enable_if<(Catch::is_range<T>::value && (! Catch::Detail::IsStreamInsertable<T>::value))>::type> [with R = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>]’
../../3rdparty/catch2/single_include/catch2/catch.hpp|1625 col 121| required from ‘std::__cxx11::string Catch::Detail::stringify(const T&) [with T = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>; std::__cxx11::string = std::__cxx11::basic_string<char>]’
../../3rdparty/catch2/single_include/catch2/catch.hpp|2625 col 58| required from ‘void Catch::Capturer::captureValues(size_t, const T&) [with T = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>; size_t = long unsigned int]’
tests/main/src/box.cpp|263 col 4| required from here
../../3rdparty/catch2/single_include/catch2/catch.hpp|1979 col 41| error: call of overloaded ‘begin(eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>)’ is ambiguous
             !std::is_same<decltype(begin(std::declval<T>())), not_this_one>::value &&
                                    ~~~~~^~~~~~~~~~~~~~~~~~~
../../3rdparty/catch2/single_include/catch2/catch.hpp|1973 col 18| note: candidate: ‘Catch::not_this_one Catch::begin(...)’
     not_this_one begin( ... );
                  ^~~~~
In file included from /usr/include/c++/8/string:51,
                 from /usr/include/c++/8/stdexcept:39,
                 from /usr/include/c++/8/array:39,
                 from pu/include/pu/box.h:4,
                 from tests/main/src/box.cpp:1:
/usr/include/c++/8/bits/range_access.h|58 col 5| note: candidate: ‘decltype (__cont.begin()) std::begin(const _Container&) [with _Container = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>; decltype (__cont.begin()) = const pu::Box<glm::vec<2, float, (glm::qualifier)0> >*]’
     begin(const _Container& __cont) -> decltype(__cont.begin())
     ^~~~~
In file included from ../../3rdparty/EASTL/include/EASTL/vector.h:39,
                 from ../../3rdparty/EASTL/include/EASTL/fixed_vector.h:16,
                 from pu/include/pu/box.h:8,
                 from tests/main/src/box.cpp:1:
../../3rdparty/EASTL/include/EASTL/iterator.h|1074 col 34| note: candidate: ‘decltype (container.begin()) eastl::begin(const Container&) [with Container = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>; decltype (container.begin()) = const pu::Box<glm::vec<2, float, (glm::qualifier)0> >*]’
   EA_CPP14_CONSTEXPR inline auto begin(const Container& container) -> decltype(container.begin())
                                  ^~~~~

In file included from tests/main/src/box.cpp:11:
../../3rdparty/catch2/single_include/catch2/catch.hpp|1980 col 39| error: call of overloaded ‘end(eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>)’ is ambiguous
             !std::is_same<decltype(end(std::declval<T>())), not_this_one>::value;
                                    ~~~^~~~~~~~~~~~~~~~~~~
../../3rdparty/catch2/single_include/catch2/catch.hpp|1974 col 18| note: candidate: ‘Catch::not_this_one Catch::end(...)’
     not_this_one end( ... );
                  ^~~
In file included from /usr/include/c++/8/string:51,
                 from /usr/include/c++/8/stdexcept:39,
                 from /usr/include/c++/8/array:39,
                 from pu/include/pu/box.h:4,
                 from tests/main/src/box.cpp:1:
/usr/include/c++/8/bits/range_access.h|78 col 5| note: candidate: ‘decltype (__cont.end()) std::end(const _Container&) [with _Container = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>; decltype (__cont.end()) = const pu::Box<glm::vec<2, float, (glm::qualifier)0> >*]’
     end(const _Container& __cont) -> decltype(__cont.end())
     ^~~
In file included from ../../3rdparty/EASTL/include/EASTL/vector.h:39,
                 from ../../3rdparty/EASTL/include/EASTL/fixed_vector.h:16,
                 from pu/include/pu/box.h:8,
                 from tests/main/src/box.cpp:1:
../../3rdparty/EASTL/include/EASTL/iterator.h|1092 col 34| note: candidate: ‘decltype (container.end()) eastl::end(const Container&) [with Container = eastl::fixed_vector<pu::Box<glm::vec<2, float, (glm::qualifier)0> >, 4, false>; decltype (container.end()) = const pu::Box<glm::vec<2, float, (glm::qualifier)0> >*]’
   EA_CPP14_CONSTEXPR inline auto end(const Container& container) -> decltype(container.end())

[horenmar]

Yeah, this is not really possible to fix, unless you make EASTL live in std and use it as the standard library in your code.

To exposit a bit, the problem is caused by mix of ADL, unconstrained templates and common idioms in generic code. As an example, the idiom to swap two instances of a generic T is this:

using std::swap;
swap(a, b);

This works by bringing in std::swap, which has a reasonable templated fallback implementation that works with any copyable/moveable type, and then it allows for ADL to find a specialized swap implementation for the concrete type of a and b. However, if the swap implementation found by ADL is not a better match (for example, if it is also a template), then you end up with ambiguous call, like in this example.

---

You are running into the same problem, but with begin and end.

[horenmar]

There should be a workaround though, if you explicitly specialize Catch::is_range for EASTL containers, you should avoid the compilation error because there will be no need for the generic range check.