Compiler workarounds

[JohanMabille]

This issue is meant for tracking the workarounds we have implemented around compiler bugs

MSVC 2015: bug with std::enable_if and invalid types

std::enable_if evaluates its second argument, even if the condition is false. This is the reason for the get_xfunction_type_t workaround which adds a level of indirection for the second type to always be a valid type (Original issue #80, fixed in PR #148).

MSVC 2015: math functions not fully qualified

fma class is ambiguous if not fully qualified. See #81.

GCC-4.9 and clang < 3.8: constexpr std::min and std::max

std::min and std::max are not constexpr in these compiler. In xio.hpp, we define a XTENSOR_MIN macro before its usage and undefine it right after.

clang < 3.8 matching initializer_list with static arrays.

Old versions of clang don't handle overload resolution with braced initializer lists correctly: braced initializer lists are not properly matched to static arrays. This prevent compile-time detection of the length of a braced initializer list.

A consequence is that we need to use stack-allocated shape types in these cases.

GCC-6: std::isnan and std::isinf.

We are not directly using std::isnan or std::isinf in xmath as a workaround to the following bug in GCC-6 for the following reason.

C++11 requires that the <cmath> header declares bool std::isnan(double) and bool std::isinf(double).
C99 requires that the <math.h> header declares int ::isnan(double) and int ::isinf(double).
These two definitions would clash when importing both headers and using namespace std.

As of version 6, gcc detects whether the obsolete functions are present in the C <math.h> header and uses them if they are, avoiding the clash. However, this means that the function might return int instead
of bool as C++11 requires, which is a bug.