Fixes disagreement between operator<< available for SFINAE and actually usable

[thecppzoo]

Description

Originally I reported a bug, 1403 about how user code provides an insertion overload (an overload for operator<<) for an std type in the global namespace, however, the code fails to compile in Clang at the point Catch tries to use it, at ReusableStringStream::operator<<. This happens because the SFINAE tests for insertion availability happen after using ::operator<<, thus the global overload is available, which activates the Reusable..., but when actually attempted to use them at ReusableStringStream::operator<< they are not available because:

1. there hasn't been the using ::operator<< at that point
2. at the namespace Catch there has been one overload for operator<<(std::ostream &, SourceLineInfo const &) that makes the global overloads not available.

This is not a strict bug because the user is providing an overload for types in std in the global namespace, which is not "the same namespace" as indicated in the Catch documentation.

However, adding to the std namespace overloads of operator<< are undefined behavior, and thus the only other choice for a user who wants to allow catch to stream-insert std types would be to put the overloads for std types into Catch, which then are inconvenient to use from the global namespace or the user namespaces. It seems the code tries to use operator<< global overloads, which works in GCC and perhaps other compilers that have broken two-phase lookup, but does not work in Clang which does it correctly in this case.

With the fix presented, users will now be able to provide inserters in either the same namespace as their types (this does not change) or the global namespace, and Catch2 would be able to consistently test for insertion and use the given insertion operations.

The fix is simple, to hoist the using ::operator<< directive to right after introducing an overload into Catch namespace, which is the point at which the global overloads become non-available.

I am thankful to Github user @mpark for his help elucidating ADL.

GitHub Issues

1403
#Closes 1403

[codecov]

Codecov Report

Merging #1405 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1405      +/-   ##
==========================================
- Coverage   80.14%   80.11%   -0.03%     
==========================================
  Files         119      119              
  Lines        3389     3389              
==========================================
- Hits         2716     2715       -1     
- Misses        673      674       +1

[horenmar]

Nice work on debugging the issue. 👍

I am going to add a test before merging it in though.