Properly handle finally(actual_function) (#1056)

`finally` needs to use `decay_t` instead of `remove_cvref_t` so it can properly accept non-object function arguments by decaying to function pointer type. Adds test coverage for this use case which was previously missing.

include/gsl/util

@@ -65,40 +65,29 @@ template <class F>
 class final_action
 {
 public:
-    static_assert(!std::is_reference<F>::value && !std::is_const<F>::value &&
-                      !std::is_volatile<F>::value,
-                  "Final_action should store its callable by value");
+    explicit final_action(const F& ff) noexcept : f{ff} { }
+    explicit final_action(F&& ff) noexcept : f{std::move(ff)} { }
 
-    explicit final_action(F f) noexcept : f_(std::move(f)) {}
+    ~final_action() noexcept { if (invoke) f(); }
 
     final_action(final_action&& other) noexcept
-        : f_(std::move(other.f_)), invoke_(std::exchange(other.invoke_, false))
-    {}
+        : f(std::move(other.f)), invoke(std::exchange(other.invoke, false))
+    { }
 
-    final_action(const final_action&) = delete;
-    final_action& operator=(const final_action&) = delete;
-    final_action& operator=(final_action&&) = delete;
-
-    // clang-format off
-    GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // terminate if throws
-    // clang-format on
-    ~final_action() noexcept
-    {
-        if (invoke_) f_();
-    }
+    final_action(const final_action&)   = delete;
+    void operator=(const final_action&) = delete;
+    void operator=(final_action&&)      = delete;
 
 private:
-    F f_;
-    bool invoke_{true};
+    F f;
+    bool invoke = true;
 };
 
 // finally() - convenience function to generate a final_action
 template <class F>
-GSL_NODISCARD final_action<typename std::remove_cv<typename std::remove_reference<F>::type>::type>
-finally(F&& f) noexcept
+GSL_NODISCARD auto finally(F&& f) noexcept
 {
-    return final_action<typename std::remove_cv<typename std::remove_reference<F>::type>::type>(
-        std::forward<F>(f));
+    return final_action<std::decay_t<F>>{std::forward<F>(f)};
 }
 
 // narrow_cast(): a searchable way to do narrowing casts of values

tests/utils_tests.cpp

@@ -112,6 +112,16 @@ TEST(utils_tests, finally_function_ptr)
     EXPECT_TRUE(j == 1);
 }
 
+TEST(utils_tests, finally_function)
+{
+    j = 0;
+    {
+        auto _ = finally(g);
+        EXPECT_TRUE(j == 0);
+    }
+    EXPECT_TRUE(j == 1);
+}
+
 TEST(utils_tests, narrow_cast)
 {
     int n = 120;