libstdc++: Add static assertions for std::function requirements

Add static assertions to std::function, so that more user-friendly
diagnostics are given when trying to store a non-copyable target object.

These preconditions were added as "Mandates:" by LWG 2447, but I'm
committing them separately from implementing that, to allow just this
change to be backported more easily.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/std_function.h (function::function(F)): Add
	static assertions to check constructibility requirements.

diff --git a/libstdc++-v3/include/bits/std_function.h b/libstdc++-v3/include/bits/std_function.h
index fb86ff1c5f84ebdc487c260ad8c9fa85dd11ca83..e081cd81ef48deb33e44fbb34c997b774d094847 100644 (file)
--- a/libstdc++-v3/include/bits/std_function.h
+++ b/libstdc++-v3/include/bits/std_function.h
@@ -422,6 +422,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        function(_Functor __f)
        : _Function_base()
        {
+         static_assert(is_copy_constructible<_Functor>::value,
+             "std::function target must be copy-constructible");
+         static_assert(is_constructible<_Functor, _Functor>::value,
+             "std::function target must be constructible from the "
+             "constructor argument");
+
          using _My_handler = _Function_handler<_Res(_ArgTypes...), _Functor>;
 
          if (_My_handler::_M_not_empty_function(__f))