Features: Add cxx_extern_templates.

Help/variable/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -52,6 +52,11 @@ The features known to this version of CMake are:
 
   .. _N2437: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2437.pdf
 
+``cxx_extern_templates``
+  Extern templates, as defined in N1987_.
+
+  .. _N1987: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1987.htm
+
 ``cxx_final``
   Override control ``final`` keyword, as defined in N2928_.
 

Modules/Compiler/GNU-CXX-FeatureTests.cmake

@@ -53,4 +53,6 @@ set(GNU43_CXX11 "${_oldestSupported} && __cplusplus >= 201103L")
 set(_cmake_feature_test_cxx_decltype "${GNU43_CXX11}")
 set(_cmake_feature_test_cxx_rvalue_references "${GNU43_CXX11}")
 set(_cmake_feature_test_cxx_static_assert "${GNU43_CXX11}")
+# TODO: Should be supported since GNU 3.4?
+set(_cmake_feature_test_cxx_extern_templates "${_oldestSupported} && __cplusplus >= 201103L")
 set(_oldestSupported)

Source/cmMakefile.cxx

@@ -50,6 +50,7 @@
   F(cxx_delegating_constructors) \
   F(cxx_deleted_functions) \
   F(cxx_explicit_conversions) \
+  F(cxx_extern_templates) \
   F(cxx_final) \
   F(cxx_inheriting_constructors) \
   F(cxx_lambdas) \

Tests/CompileFeatures/cxx_extern_templates.cpp

@@ -0,0 +1,12 @@
+
+template<typename T>
+void someFunc()
+{
+}
+
+extern template void someFunc<int>();
+
+void otherFunc()
+{
+  someFunc<int>();
+}