[libcxx-commits] [PATCH] D58987: Make underlying_type SFINAE-friendly

[Marshall Clow via Phabricator via libcxx-commits]

mclow.lists added a comment.

Here's the whole wording (from P0340):

>   template <class T>
>   struct underlying_type;	
>    
> 
> If `T` is an enumeration type, the member typedef `type` names the underlying type of T (9.6 [dcl.enum]); otherwise, there is no member `type`.
> 
> Mandates: `T` shall be a is not an incomplete enumeration type. (9.6)

So you'll need a test of an incomplete enumeration type.



================
Comment at: include/type_traits:4714
 
-template <class _Tp>
+template <class _Tp, bool = is_enum<_Tp>::value>
 struct underlying_type
----------------
I think I'd rather bury the boolean in an `__impl` class.


================
Comment at: test/std/utilities/meta/meta.trans/meta.trans.other/underlying_type.pass.cpp:66
+#if TEST_STD_VER > 17
+	test_sfinae(0);
+#endif // TEST_STD_VER > 17
----------------
I prefer a failing test; so that we can look at the error message: `no type named type in std::underlying_type<int>` blah blah.


Repository:
  rCXX libc++

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  https://reviews.llvm.org/D58987/new/

https://reviews.llvm.org/D58987