Summary

Building charlock_holmes 0.7.9 fails on systems with ICU 77+ because the native extension is compiled without an explicit C++ standard, and modern ICU headers now require C++17 features (std::u16string_view, std::enable_if_t, std::is_same_v, auto non-type template parameters, etc.).

Environment

• OS: Fedora 44
• Compiler: GCC 15
• ICU: 77.1 (system libicu-devel)
• Ruby: 4.0.3
• charlock_holmes: 0.7.9
• Bundler: 2.7.2

The same issue is reproducible on any distro that has shipped libicu 76+ where the compiler / Ruby CXXFLAGS does not already default to -std=c++17 or newer.

Steps to reproduce

gem install charlock_holmes -v 0.7.9

Actual behavior

Compilation of transliterator.cpp fails with hundreds of errors originating in ICU's own headers, e.g.:

/usr/include/unicode/char16ptr.h:271:38: error: 'enable_if_t' in namespace 'std' does not name a template type
  271 | template<typename T, typename = std::enable_if_t<std::is_same_v<T, UChar>>>
/usr/include/unicode/char16ptr.h:271:33: note: 'std::enable_if_t' is only available from C++14 onwards

/usr/include/unicode/unistr.h:3035:19: error: expected type-specifier
 3035 |   inline operator std::u16string_view() const {

/usr/include/unicode/localpointer.h:561:26: error: parameter declared 'auto'
  561 | template <typename Type, auto closeFunction>

Full failure mode is make exiting with Error 1 while compiling transliterator.o.

Root cause

ICU 76 (Oct 2024) and especially ICU 77 (Mar 2025) updated their public headers to use C++17 features as part of their normal API surface — std::u16string_view, std::enable_if_t, std::is_same_v, and auto non-type template parameters. ICU's own build documentation now states it requires a C++17-capable compiler.

charlock_holmes's extconf.rb does not set -std=c++17 (or newer) in $CXXFLAGS. Because Ruby's mkmf-generated Makefile typically inherits whatever RbConfig::CONFIG['CXXFLAGS'] was set to at Ruby build time, the compiler often ends up using C++11 (or no explicit standard at all), which then can't parse the modern ICU headers.

Workaround

Setting the C++ standard explicitly via Bundler works:

bundle config build.charlock_holmes --with-cxxflags=-std=c++17
gem uninstall charlock_holmes -v 0.7.9 -I
bundle install

Or directly via gem:

gem install charlock_holmes -v 0.7.9 -- --with-cxxflags=-std=c++17

Suggested fix

Append -std=c++17 to $CXXFLAGS in ext/charlock_holmes/extconf.rb, ideally guarded so it only applies when the compiler is GCC/Clang. Something like:

$CXXFLAGS << " -std=c++17" if RbConfig::CONFIG["CXX"] =~ /g\+\+|clang\+\+/

This matches what other gems wrapping modern C++ libraries have already done.

Happy to send a PR if that approach sounds right.