Here is an inline diff to unbreak openmw with libc++19. I backported
the fix from the new version 0.49.0, so it can be removed once
openmw is updated. upstream decided to use span instead of
basic_string_view.

some links of interest:
https://gitlab.com/OpenMW/openmw/-/commit/e5ad1cd2144cfedf0418a30a125bf057734150f0
https://reviews.llvm.org/D157058
https://reviews.llvm.org/D138307

I was able to play. I did have to uninstall, remove the global
defaults.bin and reinstall for it to launch successfully.

--8<---------------cut here---------------start------------->8---
--- -openmw-0.48.0p2 -------------------
You should also remove /etc/openmw/defaults.bin (which was modified)

Loading settings file: /etc/openmw/defaults.bin
[18:46:01.116 E] Error: Trying to retrieve a non-existing setting: log buffer size.
[18:46:01.116 E] Make sure the defaults.bin file was properly installed.
--8<---------------cut here---------------end--------------->8---

Feedback and tests are welcome. OK?

Index: Makefile
===================================================================
RCS file: /cvs/ports/games/openmw/Makefile,v
diff -u -p -r1.35 Makefile
--- Makefile	2 Sep 2025 12:22:24 -0000	1.35
+++ Makefile	3 Sep 2025 01:40:26 -0000
@@ -1,4 +1,3 @@
-BROKEN = implicit instantiation of undefined template 'std::char_traits<signed char>'
 ONLY_FOR_ARCHS =	amd64 i386
 
 COMMENT =		open source implementation of TES III: Morrowind
@@ -8,7 +7,7 @@ GH_ACCOUNT =		OpenMW
 GH_PROJECT =		openmw
 GH_TAGNAME =		openmw-$V
 DISTNAME =		openmw-$V
-REVISION =		1
+REVISION =		2
 
 DISTFILES.a = 		e75adf86f91eb3082220085e42dda62679f9a3ea.zip
 DIST_SUBDIR =		openmw
Index: patches/patch-components_to_utf8_to_utf8_cpp
===================================================================
RCS file: patches/patch-components_to_utf8_to_utf8_cpp
diff -N patches/patch-components_to_utf8_to_utf8_cpp
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ patches/patch-components_to_utf8_to_utf8_cpp	3 Sep 2025 01:40:26 -0000
@@ -0,0 +1,14 @@
+Fix build with libc++19; backported from openmw-0.49.0
+
+Index: components/to_utf8/to_utf8.cpp
+--- components/to_utf8/to_utf8.cpp.orig
++++ components/to_utf8/to_utf8.cpp
+@@ -52,7 +52,7 @@ namespace
+         return std::find_if(input.begin(), input.end(), [] (unsigned char v) { return v == 0 || v >= 128; });
+     }
+ 
+-    std::basic_string_view<signed char> getTranslationArray(FromType sourceEncoding)
++    std::span<const signed char> getTranslationArray(FromType sourceEncoding)
+     {
+         switch (sourceEncoding)
+         {
Index: patches/patch-components_to_utf8_to_utf8_hpp
===================================================================
RCS file: patches/patch-components_to_utf8_to_utf8_hpp
diff -N patches/patch-components_to_utf8_to_utf8_hpp
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ patches/patch-components_to_utf8_to_utf8_hpp	3 Sep 2025 01:40:26 -0000
@@ -0,0 +1,22 @@
+Fix build with libc++19; backported from openmw-0.49.0
+
+Index: components/to_utf8/to_utf8.hpp
+--- components/to_utf8/to_utf8.hpp.orig
++++ components/to_utf8/to_utf8.hpp
+@@ -3,6 +3,7 @@
+ 
+ #include <string>
+ #include <cstring>
++#include <span>
+ #include <vector>
+ #include <string_view>
+ 
+@@ -46,7 +47,7 @@ namespace ToUTF8
+             inline std::pair<std::size_t, bool> getLengthLegacyEnc(std::string_view input) const;
+             inline void copyFromArrayLegacyEnc(std::string_view::iterator& chp, std::string_view::iterator end, char* &out) const;
+ 
+-            const std::basic_string_view<signed char> mTranslationArray;
++            const std::span<const signed char> mTranslationArray;
+     };
+ 
+     class Utf8Encoder