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From: "carlosgalvezp at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug gdb/31331] Wenum-constexpr-conversion should be fixed, soon
 treated as a hard error
Date: Fri, 10 May 2024 20:35:01 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31331

--- Comment #36 from Carlos Galvez <carlosgalvezp at gmail dot com> ---
Applying the following diff I believe I can maintain the current behavior
without changing any tests. It also serves as documentation of what exactly=
 it
does. I've run the test suite a few times and don't notice a change in the
numbers, but of course it'd be great if someone more experienced tests this.

I also confirm that we still get a compiler error as requested in comment #=
4.

With this in place one can look into refactoring/redesigning to avoid having
this at all, hopefully it should be easier to understand.

Let me know what you think!

--- a/gdbsupport/enum-flags.h
+++ b/gdbsupport/enum-flags.h
@@ -88,15 +88,26 @@ template<> struct integer_for_size<2, 1> { typedef int1=
6_t
type; };
 template<> struct integer_for_size<4, 1> { typedef int32_t type; };
 template<> struct integer_for_size<8, 1> { typedef int64_t type; };

+template<typename T>
+struct enum_will_be_promoted_to_signed_integer
+{
+  static constexpr bool value =3D std::is_signed<decltype(~T())>::value;
+};
+
+template <typename T>
+struct enum_has_underlying_small_integer
+{
+  static constexpr bool value =3D sizeof(T) < sizeof(int);
+};
+
 template<typename T>
 struct enum_underlying_type
 {
-  DIAGNOSTIC_PUSH
-  DIAGNOSTIC_IGNORE_ENUM_CONSTEXPR_CONVERSION
   typedef typename
-    integer_for_size<sizeof (T), static_cast<bool>(T (-1) < T (0))>::type
+    integer_for_size<sizeof (T),
+                     enum_will_be_promoted_to_signed_integer<T>::value &&
+                     !enum_has_underlying_small_integer<T>::value>::type
     type;
-  DIAGNOSTIC_POP
 };

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