Fix gcc 8.1 - 8.3 bug and compilation

Fixes issue #4129

include/fmt/base.h

@@ -1580,8 +1580,19 @@ template <> struct is_output_iterator<appender, char> : std::true_type {};
 
 template <typename It, typename T>
 struct is_output_iterator<
+#if FMT_GCC_VERSION >= 800 && FMT_GCC_VERSION <= 803
+    // Workaround old gcc bug, raised in issue 4129
+    It, T,
+    void_t<decltype(*std::declval<conditional_t<
+                        std::is_array<It>::value,
+                        typename std::add_pointer<
+                            typename std::remove_extent<It>::type>::type,
+                        It>&>()++ = std::declval<T>())>>
+#else
     It, T, void_t<decltype(*std::declval<It&>()++ = std::declval<T>())>>
-    : std::true_type {};
+#endif
+    : std::true_type {
+};
 
 // A type-erased reference to an std::locale to avoid a heavy <locale> include.
 class locale_ref {