[FIX] clang-18: char_traits for non-character-types are deprecated

Author: eseiler

build_system/seqan3-config.cmake

@@ -197,6 +197,15 @@ if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION V
     message (FATAL_ERROR "Clang < 17 is not supported. The detected compiler version is ${CMAKE_CXX_COMPILER_VERSION}.")
 endif ()
 
+# char_traits are deprecated in such a way that we get a deprecation error with -Werror while checking some traits
+# and concepts that should result in SFINAE. It seems like deprecation errors are ignored in requires clauses.
+# The easiest workaround seems to be to just remove the deprecation.
+if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang"
+    AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 18
+    AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19)
+    set (SEQAN3_DEFINITIONS ${SEQAN3_DEFINITIONS} "-D_LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION")
+endif ()
+
 # ----------------------------------------------------------------------------
 # Require C++20
 # ----------------------------------------------------------------------------