Final bits for libstdc/71579

std::common_type assertions attempt to give a proper 'required from
here' hint for user code, do not bring many changes to the
implementation and check all the template parameters for completeness.
In some cases the type could be checked for completeness more than
once. This seems to be unsolvable due to the fact that
std::common_type could be specialized by the user, so we have to call
std::common_type recursively, potentially repeating the check for the
first type.

std::common_reference assertions make sure that we detect incomplete
types even if the user specialized the std::basic_common_reference.

Changelog:

2020-11-12  Antony Polukhin  <antoshkka@gmail.com>
    PR libstdc/71579
    * include/std/type_traits (is_convertible, is_nothrow_convertible)
    (common_type, common_reference): Add static_asserts
    to make sure that the arguments of the type traits are not misused
    with incomplete types.
    * testsuite/20_util/common_reference/incomplete_basic_common_neg.cc:
New test.
    * testsuite/20_util/common_reference/incomplete_neg.cc: New test.
    * testsuite/20_util/common_type/incomplete_neg.cc: New test.
    * testsuite/20_util/common_type/requirements/sfinae_friendly_1.cc: Remove
    SFINAE tests on incomplete types.
    * testsuite/20_util/is_convertible/incomplete_neg.cc: New test.
    * testsuite/20_util/is_nothrow_convertible/incomplete_neg.cc: New test.



--
Best regards,
Antony Polukhin