Make Catch_global_namespace_dummy a complete type

[AndrewPaxie]

Description

Compile the unit tests for the Trompeloeil mock object library,
in this particular combination of C++ Standard Library and compiler,

Operating system: Ubuntu 17.10 (Artful Aardvark)
Compiler: g++-7 (from package g++-7 7.2.0-8ubuntu3)
Standard C++ Library: libc++ (from package libc++-dev:amd64 3.9.1-3)
Catch version: v2.0.1

Expected result: The unit tests compile and pass.

Actual result: The compilation fails with an error message,

/usr/include/c++/v1/type_traits:2928:50:
error: invalid use of incomplete type ‘struct Catch_global_namespace_dummy’
   decltype(__is_constructible_test(declval<_Tp>(), declval<_Args>()...))
            ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Other combinations that fail: any other supported version of g++ with libc++.

Other combinations that succeed:

• Any supported version of g++ with libstd++-v3.
• Any supported version of clang++ with libstdc++-v3.
• Any supported version of clang++ with libc++.

Steps to reproduce

# Assume packages g++-7, libc++-dev are installed

$ mkdir trompeloeil-scratch
$ git clone https://github.com/AndrewPaxie/trompeloeil trompeloeil-scratch
$ cd trompeloeil-scratch
$ git checkout c++11

$ wget https://raw.githubusercontent.com/catchorg/Catch2/v2.0.1/single_include/catch.hpp

$ g++-7 -std=c++14 -nostdinc++ \
-isystem/usr/include/c++/v1 \
-ftemplate-backtrace-limit=0 \
-Wall -Wextra -Wpedantic -Wshadow \
-I . -I ./include \
-o compiling_tests \
test/compiling_tests.cpp test/compiling_tests_11.cpp test/compiling_tests_14.cpp \
-nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc

Extra information

Supposed cause

In include/internal/catch_tostring.h there is the code

// We need a dummy global operator<< so we can bring it into Catch namespace later
struct Catch_global_namespace_dummy;
std::ostream& operator<<(std::ostream&, Catch_global_namespace_dummy);

namespace Catch {
    // Bring in operator<< from global namespace into Catch namespace
    using ::operator<<;

Suggested fix

When Catch_global_namespace_dummy is changed to

struct Catch_global_namespace_dummy {};

the Trompeloeil unit test code compiles with success.

[horenmar]

Interesting.

As far as I know, there is no problem with making it complete, so I am going to fix it soonish.