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

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");