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This implements P3948R1: constant_wrapper is the only tool needed for passing constant expressions via function arguments. This changes function_ref from nontype_t to constant_wrapper and implements the ambiguity check (static_asert in function_ref from constant_wrapper constructor). In addition to P3948R1 this also includes the (forgotten) deduction guide changes suggested in the draft PR [1]. [1] https://github.com/cplusplus/draft/pull/8878 libstdc++-v3/ChangeLog: * include/bits/funcref_impl.h (function_ref::function_ref): Change nontype_t parameter to constant_wrapper, and adjust accordingly. Add static_assert detecting ambigous semantics. (function_ref::operator=): Detect constant_wrapper rather than nontype_t. * include/bits/funcwrap.h (function_ref): Change nontype_t parameter to constant_wrapper in deduction guides. * include/bits/utility.h (std::nontype_t, std::nontype) (std::__is_nontype_v): Remove. (std::__is_constant_wrapper_v): Define. * src/c++23/std.cc.in (std::nontype_t, std::nontype): Remove exports. * testsuite/20_util/function_ref/cw_cons_neg.cc: New tests for ambiguity check. * testsuite/20_util/function_ref/assign.cc: Replace nontype_t with constant_wrapper and nontype with std::cw. * testsuite/20_util/function_ref/call.cc: Likewise. * testsuite/20_util/function_ref/cons.cc: Likewise. * testsuite/20_util/function_ref/cons_neg.cc: Likewise. * testsuite/20_util/function_ref/dangling.cc: Likewise. * testsuite/20_util/function_ref/deduction.cc: Likewise. * testsuite/20_util/function_ref/mutation.cc: Likewise. Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com> Reviewed-by: Jonathan Wakely <jwakely@redhat.com> Signed-off-by: Matthias Kretz <m.kretz@gsi.de> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>