diff --git a/src/library/io_state.cpp b/src/library/io_state.cpp
index 5bf587e97..f5223ff79 100644
--- a/src/library/io_state.cpp
+++ b/src/library/io_state.cpp
@@ -20,6 +20,12 @@ io_state::io_state(options const & opts, formatter const & fmt):
     m_regular_channel(std::make_shared<stdout_channel>()),
     m_diagnostic_channel(std::make_shared<stderr_channel>()) {
 }
+io_state::io_state(io_state const & ios, std::shared_ptr<output_channel> const & r, std::shared_ptr<output_channel> const d):
+    m_options(ios.m_options),
+    m_formatter(ios.m_formatter),
+    m_regular_channel(r),
+    m_diagnostic_channel(d) {
+}
 
 io_state::~io_state() {}
 
diff --git a/src/library/io_state.h b/src/library/io_state.h
index 8a6d490ff..ebe31c69a 100644
--- a/src/library/io_state.h
+++ b/src/library/io_state.h
@@ -25,6 +25,7 @@ class io_state {
 public:
     io_state(formatter const & fmt);
     io_state(options const & opts, formatter const & fmt);
+    io_state(io_state const & ios, std::shared_ptr<output_channel> const & r, std::shared_ptr<output_channel> const d);
     ~io_state();
 
     options get_options() const { return m_options; }