Added template argument deduction for not_null (#689)

* Added template  argument deduction for not_null

This allows compilers with c++17 support to infer template
instantiation types when calling not_null constructor:

    int foo(not_null<const int*> x);

    int main()
    {
        int t = 0;
        not_null x{ &t };
        return foo(not_null{ &t });
    }

* replaced deduction guides by a simple constructor

* Updated tests

* fixed check for availability of std::byte

* testing c++1z on clang

* fixed cmakelists extra endif

* include cstddef header for clang and gcc in pointers

* fixed a typo

* fix missing nullptr_t type

* fixed typo in CMakeLists.tst

* change approach to c++17 testing, step one: revert cmake testing, update clang5.0 package

removed using latest c++ due to clang5.0 failing,
update clang5.0 travis config to use llvm-toolchain-trusty-5.0

* addressed comments

4 files changed, 80 insertions, 4 deletions

diff --git a/.travis.yml b/.travis.yml
index 647a1c1..d2e4d9d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -128,7 +128,7 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-5.0
-            - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main'
+            - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-5.0 main'
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - env: COMPILER=clang++-5.0 BUILD_TYPE=Release
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0ab5e44..e96fb64 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -14,20 +14,31 @@ if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
   set(GSL_STANDALONE_PROJECT ON)
 endif ()
 
+
+include(CheckCXXCompilerFlag)
+if (NOT MSVC)
+    CHECK_CXX_COMPILER_FLAG("-std=c++14" COMPILER_SUPPORTS_CXX14)
+else()
+    CHECK_CXX_COMPILER_FLAG("-std:c++14" COMPILER_SUPPORTS_CXX14)
+endif()
+
 # when minimum version required is 3.8.0 remove if below
 # both branches do exactly the same thing
 if (CMAKE_VERSION VERSION_LESS 3.7.9)
     if (NOT MSVC)
-        include(CheckCXXCompilerFlag)
-        CHECK_CXX_COMPILER_FLAG("-std=c++14" COMPILER_SUPPORTS_CXX14)
         if(COMPILER_SUPPORTS_CXX14)
             target_compile_options(GSL INTERFACE "-std=c++14")
         else()
             message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++14 support. Please use a different C++ compiler.")
         endif()
+    else()
+        if(COMPILER_SUPPORTS_CXX14)
+            target_compile_options(GSL INTERFACE "-std:c++14")
+        else()
+            message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++14 support. Please use a different C++ compiler.")
+        endif()
     endif()
 else ()
-    # set the GSL library to be compiled only with c++14
     target_compile_features(GSL INTERFACE cxx_std_14)
     # on *nix systems force the use of -std=c++XX instead of -std=gnu++XX (default)
     set(CMAKE_CXX_EXTENSIONS OFF)
diff --git a/include/gsl/pointers b/include/gsl/pointers
index ad15ce3..69499d6 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -77,6 +77,12 @@ public:
         Expects(ptr_ != nullptr);
     }
 
+    template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
+    constexpr explicit not_null(T u) : ptr_(u)
+    {
+        Expects(ptr_ != nullptr);
+    }
+
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(const not_null<U>& other) : not_null(other.get())
     {
diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index fa99bb7..1cb9c10 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -112,6 +112,7 @@ struct NonCopyableNonMovable
 };
 
 bool helper(not_null<int*> p) { return *p == 12; }
+bool helper_const(not_null<const int*> p) { return *p == 12; }
 
 TEST_CASE("TestNotNullConstructors")
 {
@@ -328,4 +329,62 @@ TEST_CASE("TestNotNullCustomPtrComparison")
     CHECK((NotNull2(p2) >= NotNull1(p1)) == (p2 >= p1));
 }
 
+
+#if defined(__cplusplus) && (__cplusplus >= 201703L)
+TEST_CASE("TestNotNullConstructorTypeDeduction")
+{
+    {
+        int i = 42;
+
+        not_null x{&i};
+        helper(not_null{&i});
+        helper_const(not_null{&i});
+
+        CHECK(*x == 42);
+    }
+
+    {
+        int i = 42;
+        int* p = &i;
+
+        not_null x{p};
+        helper(not_null{p});
+        helper_const(not_null{p});
+
+        CHECK(*x == 42);
+    }
+
+    {
+        auto workaround_macro = []() {
+            int* p1 = nullptr;
+            not_null x{p1};
+        };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    }
+
+    {
+        auto workaround_macro = []() {
+            const int* p1 = nullptr;
+            not_null x{p1};
+        };
+        CHECK_THROWS_AS(workaround_macro(), fail_fast);
+    }
+
+    {
+        int* p = nullptr;
+
+        CHECK_THROWS_AS(helper(not_null{p}), fail_fast);
+        CHECK_THROWS_AS(helper_const(not_null{p}), fail_fast);
+    }
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    {
+        not_null x{nullptr};
+        helper(not_null{nullptr});
+        helper_const(not_null{nullptr});
+    }
+#endif
+}
+#endif // #if defined(__cplusplus) && (__cplusplus >= 201703L)
+
 static_assert(std::is_nothrow_move_constructible<not_null<void *>>::value, "not_null must be no-throw move constructible");