This makes the deduction guides for std::function and std::packaged_task
work for explicit object member functions, i.e. "deducing this", as per
LWG 3617.
libstdc++-v3/ChangeLog:
PR libstdc++/113335
* include/bits/std_function.h (__function_guide_helper): Add
partial specialization for explicit object member functions, as
per LWG 3617.
* testsuite/20_util/function/cons/deduction_c++23.cc: Check
explicit object member functions.
* testsuite/30_threads/packaged_task/cons/deduction_c++23.cc:
Likewise.
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