From ea45295200abb6b9b498e97fbef37567315dddbe Mon Sep 17 00:00:00 2001
From: Ed Catmur <edward.catmur@mavensecurities.com>
Date: Tue, 18 Jan 2022 14:02:11 +0000
Subject: [PATCH] Check that lazy_ostream argument is printable

The operator() is not instantiated till later, which makes it
difficult to locate an erroneous usage of <<.  Place a
static_assert in the ctor (copied from print_helper.hpp) to check
at construction time, which will point back to the << site.
---
 include/boost/test/utils/lazy_ostream.hpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/boost/test/utils/lazy_ostream.hpp b/include/boost/test/utils/lazy_ostream.hpp
index 2d1edbed8e..7d62fd46b1 100644
--- a/include/boost/test/utils/lazy_ostream.hpp
+++ b/include/boost/test/utils/lazy_ostream.hpp
@@ -15,6 +15,10 @@
 #include <boost/test/detail/config.hpp>
 #include <boost/test/tools/detail/print_helper.hpp>
 
+// Boost
+#include <boost/static_assert.hpp>
+#include <boost/type_traits/has_left_shift.hpp>
+
 // STL
 #include <iosfwd>
 
@@ -75,6 +79,8 @@ class lazy_ostream_impl : public lazy_ostream {
     , m_prev( prev )
     , m_value( value )
     {
+        BOOST_STATIC_ASSERT_MSG( (boost::has_left_shift<std::ostream,T>::value),
+                                "Type has to implement operator<< to be printable");
     }
 
     std::ostream&   operator()( std::ostream& ostr ) const BOOST_OVERRIDE