From a44260b8b4ee2382394675330e5a6da3f74daf3e Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Tue, 24 Sep 2024 14:01:57 +1000
Subject: [PATCH] fixed_string: add constexpr equality operators

This fixes some errors seen under UBSan when comparing fixed_string
objects in std::integral_constant under GCC.

In particular the M != N case appears to be important.
---
 cruft/util/fixed_string.hpp | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/cruft/util/fixed_string.hpp b/cruft/util/fixed_string.hpp
index 18122a23..4f5a007d 100644
--- a/cruft/util/fixed_string.hpp
+++ b/cruft/util/fixed_string.hpp
@@ -43,9 +43,18 @@ namespace cruft {
 
         auto begin (void) const& noexcept { return std::begin (value); }
         auto end   (void) const& noexcept { return std::end   (value); }
-    };
 
 
+        constexpr bool
+        operator== (fixed_string<N> const&) const noexcept = default;
+
+        template <std::size_t M>
+        requires (M != N)
+        constexpr bool
+        operator== (fixed_string<M> const&) const noexcept
+        { return false; }
+    };
+
     template <std::size_t N>
     fixed_string (char const (&)[N]) -> fixed_string<N - 1>;
 }