For a span with statically empty extent, we currently model the
preconditions of front(), back(), and operator[] as if they are
mandates, by using a static_assert to verify that extent != 0. This
causes us to reject valid programs that would instantiate these member
functions and at runtime never call them.
Since they are already followed by more general runtime asserts, this
patch just removes these static_asserts altogether,
libstdc++-v3/ChangeLog:
* include/std/span (span::front): Remove static_assert.
(span::back): Likewise.
(span::operator[]): Likewise.
* testsuite/23_containers/span/back_neg.cc: Rewrite to verify
that we check the preconditions of back() only when it's called.
* testsuite/23_containers/span/front_neg.cc: Likewise for
front().
* testsuite/23_containers/span/index_op_neg.cc: Likewise for
operator[].
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