Feat/modsup#1750
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GCC 15+ (and Clang in strict module mode) reject headers included in a
named module's global module fragment when those headers declare CPO
variables in anonymous namespaces — the variables have internal linkage
(TU-local), which violates the C++20 module rules for exported entities
that reference them transitively through template constraints.
Introduce ASIO_CPO_INLINE_CONSTEXPR (in detail/config.hpp) as an opt-in
flag that replaces the anonymous-namespace singleton pattern in the four
affected CPO headers with `inline constexpr impl cpo{}`. This is
semantically equivalent: the impl types are stateless function objects
with constexpr constructors, and inline variables have the same ODR
guarantee (one definition across all TUs) as the static_instance<> scheme.
The flag is compiler-neutral. GCC named-module builds activate it via
-DASIO_GCC_NAMED_MODULES; other compilers may set ASIO_CPO_INLINE_CONSTEXPR
directly. Existing builds are unaffected: the flag defaults to off and
the original anonymous-namespace path is preserved under #else.
Files changed:
asio/include/asio/detail/config.hpp — macro definition and detection
asio/include/asio/prefer.hpp — prefer CPO
asio/include/asio/require.hpp — require CPO
asio/include/asio/query.hpp — query CPO
asio/include/asio/require_concept.hpp — require_concept CPO
Namespace-scope `const` variables in C++ have internal linkage by
default. When asio/detail/socket_types.hpp is included in a C++20
named module's global module fragment, these variables become TU-local
entities; GCC 15+ rejects module functions that reference them
(e.g. reactive_socket_service::accept, basic_socket_streambuf::
connect_to_endpoints, reactive_socket_accept_op::do_perform).
Introduce ASIO_DETAIL_CONSTEXPR_VAR in detail/config.hpp:
- When ASIO_CPO_INLINE_CONSTEXPR is set: expands to `inline constexpr`,
giving these variables external linkage (C++17 inline variable).
- Otherwise: expands to `const`, preserving the existing behaviour.
Apply the macro to all namespace-scope integer constants in
socket_types.hpp across all platform branches (Windows Runtime,
Windows/Cygwin, and POSIX), plus the three cross-platform constants
(custom_socket_option_level, enable_connection_aborted_option,
always_fail_option).
…SIO_CPO_INLINE_CONSTEXPR The ASIO_DETAIL_CONSTEXPR_VAR macro selects between 'inline constexpr' and 'const' via a #if on ASIO_CPO_INLINE_CONSTEXPR. The C preprocessor evaluates that #if when the macro is defined, not when it is expanded, so ASIO_DETAIL_CONSTEXPR_VAR must be defined after the ASIO_GCC_NAMED_MODULES → ASIO_CPO_INLINE_CONSTEXPR detection block in the same file.
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I've had some challenges in using ASIO with a C++23 project that heavily uses modules when compiling with GCC. These PR addresses the problems I've seen. Happy to hear your comments.