traits: add trivial type_traits tests

CMakeLists.txt

@@ -507,6 +507,7 @@ if (TESTS)
         stringid
         strongdef
         tuple
+        traits
         typeidx
         uri
         vector

test/traits.cpp

@@ -0,0 +1,58 @@
+#include <cruft/util/tap.hpp>
+#include <cruft/util/types/traits.hpp>
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename FuncT>
+constexpr bool
+returns_int (FuncT)
+{
+    return std::is_same_v<
+        int,
+        typename func_traits<FuncT>::return_type
+    >;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename, class = std::void_t<>>
+struct has_return_type : std::false_type {};
+
+
+//-----------------------------------------------------------------------------
+template <typename T>
+struct has_return_type<
+    T, std::void_t<typename func_traits<T>::return_type>
+> : std::true_type {};
+
+
+///////////////////////////////////////////////////////////////////////////////
+int plain (char, float);
+int with_noexcept (char, float) noexcept;
+struct foo {
+    int bar (double, char) const;
+    int with_noexcept (double, char) noexcept;
+};
+
+
+///////////////////////////////////////////////////////////////////////////////
+int
+main (void)
+{
+    util::TAP::logger tap;
+
+    tap.expect (returns_int ( plain), "free-function by-reference");
+    tap.expect (returns_int (&plain), "free-function by-pointer");
+
+    tap.expect (returns_int ( with_noexcept), "free-function with noexcept by-reference");
+    tap.expect (returns_int (&with_noexcept), "free-function with noexcept by-pointer");
+
+    tap.expect (returns_int (&foo::bar), "member-function by reference");
+    tap.expect (returns_int (&foo::with_noexcept), "member-function with noexcept by reference");
+
+    using all_t = void (*const &)(void *, unsigned int, unsigned int, void **) noexcept;
+    tap.expect (std::is_same_v<void, typename func_traits<all_t>::return_type>,
+                "complex function type");
+
+    tap.expect (!has_return_type<int>::value, "integer is not applicable to func_traits");
+}