added test to demonstrate the issue

Author: burnpanck

.appveyor.yml

@@ -1,11 +1,21 @@
 os:
-- Visual Studio 2015
-- Visual Studio 2017
+  - Visual Studio 2017
+  - Visual Studio 2015
+
+environment:
+  matrix:
+    - STD: 17
+    - STD: 14
 
 build_script:
   - git submodule update --init --recursive
   - mkdir build
   - cd build
-  - cmake ..
+  - cmake -DCMAKE_CXX_STANDARD=%STD% -DCMAKE_CXX_STANDARD_REQUIRED=True ..
   - cmake --build .
   - C:\projects\function-ref\build\Debug\tests.exe
+
+matrix:
+  exclude:
+    - os: Visual Studio 2015
+      STD: 17

.travis.yml

@@ -5,6 +5,14 @@ sudo: false
 
 matrix:
   include:
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+      env: COMPILER=g++-7 CXX_STANDARD=17
     - compiler: gcc
       addons:
         apt:

CMakeLists.txt

@@ -20,6 +20,12 @@ include(add-tl)
 tl_add_library(function-ref ARCH_INDEPENDENT SOURCES 
                include/tl/function_ref.hpp)
 
+# make sure we see the C++ version, even in MSVC:
+# https://stackoverflow.com/questions/57102212/cannot-set-cplusplus-to-c17-standard-with-visual-studio-and-cmake
+if ((MSVC) AND (MSVC_VERSION GREATER_EQUAL 1914))
+  target_compile_options(function-ref INTERFACE "/Zc:__cplusplus")
+endif()
+
 # Prepare "Catch" library for other executables
 set(CATCH_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/test)
 add_library(Catch INTERFACE)
@@ -37,5 +43,8 @@ if(FUNCTION_REF_ENABLE_TESTS)
 
   target_link_libraries(tests Catch function-ref)
 
-  set_property(TARGET tests PROPERTY CXX_STANDARD 14)
+  message(STATUS "CMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}")
+  if(NOT CMAKE_CXX_STANDARD)
+    set_property(TARGET tests PROPERTY CXX_STANDARD 14)
+  endif()
 endif()

tests/issues.cpp

@@ -29,4 +29,20 @@ TEST_CASE("Issue #10") {
   int z = 12;
   auto f = [&](const std::vector<int> i) { return i[0] * z; };
   foo(f);
-}
+}
+
+#if __cplusplus >= 201703
+struct NonCopyNonMove {
+  NonCopyNonMove() = default;
+  NonCopyNonMove(const NonCopyNonMove &) = delete;
+  NonCopyNonMove(NonCopyNonMove &&) = delete;
+};
+
+TEST_CASE("Issue #20") {
+  auto f = []() { return NonCopyNonMove(); };
+  auto fr = tl::function_ref<NonCopyNonMove()>(f);
+
+  // silence warnings
+  (void)fr;
+}
+#endif