Features: Add cxx_extended_friend_declarations.

Help/variable/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -82,6 +82,11 @@ The features known to this version of CMake are:
 
   .. _N2437: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2437.pdf
 
+``cxx_extended_friend_declarations``
+  Extended friend declarations, as defined in N1791_.
+
+  .. _N1791: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1791.pdf
+
 ``cxx_extern_templates``
   Extern templates, as defined in N1987_.
 

Modules/Compiler/GNU-CXX-FeatureTests.cmake

@@ -16,6 +16,7 @@ set(_cmake_feature_test_cxx_thread_local "${GNU48_CXX11}")
 set(GNU47_CXX11 "${_oldestSupported} && __cplusplus >= 201103L")
 set(_cmake_feature_test_cxx_alias_templates "${GNU47_CXX11}")
 set(_cmake_feature_test_cxx_delegating_constructors "${GNU47_CXX11}")
+set(_cmake_feature_test_cxx_extended_friend_declarations "${GNU47_CXX11}")
 set(_cmake_feature_test_cxx_final "${GNU47_CXX11}")
 set(_cmake_feature_test_cxx_noexcept "${GNU47_CXX11}")
 set(_cmake_feature_test_cxx_nonstatic_member_init "${GNU47_CXX11}")

Source/cmMakefile.cxx

@@ -56,6 +56,7 @@
   F(cxx_deleted_functions) \
   F(cxx_enum_forward_declarations) \
   F(cxx_explicit_conversions) \
+  F(cxx_extended_friend_declarations) \
   F(cxx_extern_templates) \
   F(cxx_final) \
   F(cxx_generalized_initializers) \

Tests/CompileFeatures/cxx_extended_friend_declarations.cpp

@@ -0,0 +1,25 @@
+
+template <typename T>
+struct B
+{
+  B() : m_i(42) {}
+private:
+  int m_i;
+  friend T;
+};
+
+struct A
+{
+  template<typename T>
+  int getBValue(B<T> b)
+  {
+    return b.m_i;
+  }
+};
+
+void someFunc()
+{
+  A a;
+  B<A> b;
+  a.getBValue(b);
+}