https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127232
--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-16 branch has been updated by Marek Polacek <[email protected]>: https://gcc.gnu.org/g:368f72c019d68fec8b496bb4c964cce0d5a0865d commit r16-9830-g368f72c019d68fec8b496bb4c964cce0d5a0865d Author: Marek Polacek <[email protected]> Date: Thu Oct 1 13:53:32 2026 -0400 c++/reflection: fix extract-ref [PR127232] A couple of problems here: [meta.reflection.extract]/5.3 says "If r represents a variable, then either that variable is usable in constant expressions or its lifetime began within the core constant expression currently under evaluation." but I was wrongly using is_constant_expression. That led to a crash because we passed a type to build_address. For "is usable..." we want decl_constant_var_p and for the "its lifetime began..." I'm adding a new function. Using cxx_eval_constant_expression for non-reference variables wouldn't work since evaluating a variable as a glvalue always succeeds, and as a prvalue fails for e.g. an uninitialized variable whose lifetime did begin. With this patch, a variable that isn't usable in constant expressions throws. Thus the extract2.C and extract3.C changes; one can use object_of to get the object instead. Also, DECL_INITIAL of a PARM_DECL is not an initializer but DECL_ARG_TYPE. So we need cxx_eval_constant_expression to handle e.g.: int g; consteval void h (const int &p) { std::meta::extract<const int &> (^^p); } consteval { h (g); } where we need to get g from the PARM_DECL p, and return "(const int &) &g". With cxx_eval_constant_expression with vc_glvalue, the maybe_get_reference_referent call is no longer necessary. The result should be wrapped in a NOP_EXPR otherwise e.g. template <const int &> struct X { }; same_type<X<g>, X<extract<const int &> (object_of (^^g))>> s1; fails: we returned a pointer rather than a value of reference type T. This adds a new XFAIL because we don't have P2686 yet. It seems that object_of will need similar tweaks but that's for another patch. PR c++/127232 gcc/cp/ChangeLog: * constexpr.cc (constexpr_global_ctx::in_eval_lifetime_p): New. (cxx_constexpr_in_eval_lifetime_p): New. * cp-tree.h (cxx_constexpr_in_eval_lifetime_p): Declare. * reflect.cc (extract_ref): Check decl_constant_var_p and cxx_constexpr_in_eval_lifetime_p instead of is_constant_expression. Use cxx_eval_constant_expression instead of DECL_INITIAL for references. Adjust the checking assert. Drop the maybe_get_reference_referent call. Build a NOP_EXPR around the result. gcc/testsuite/ChangeLog: * g++.dg/reflect/extract2.C: Adjust dg-error. * g++.dg/reflect/extract3.C: Add dg-error. Add tests using object_of. * g++.dg/reflect/extract13.C: New test. * g++.dg/reflect/extract14.C: New test. Reviewed-by: Jason Merrill <[email protected]> (cherry picked from commit 947ff08bff31425cdcb02b445f135a1ece869873)
