[committed] libstdc++: Revert changes to std::unique_ptr<T[]>::operator[] [PR 101271]

[Jonathan Wakely]

On 28/06/21 14:24 +0100, Jonathan Wakely wrote:
>PR libstdc++/101236 shows that LLVM depends on being able to use
>unique_ptr<T[]>::operator[] when T is incomplete. This is undefined, but
>previously worked with libstdc++. When I added the conditional noexcept
>to that operator we started to diagnose the incomplete type.
>
>This change restores support for that case, by making the noexcept
>condition check that the type is complete before checking whether
>indexing on the pointer can throw.  A workaround for PR c++/101239 is
>needed to avoid a bogus error where G++ fails to do SFINAE on the
>ill-formed p[n] expression and gets an ICE. Instead of checking that the
>p[n] expression is valid in the trailing-return-type, we only check that
>the element_type is complete.
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/101236
>	* include/bits/unique_ptr.h (unique_ptr<T[], D>::operator[]):
>	Fail gracefully if element_type is incomplete.
>	* testsuite/20_util/unique_ptr/cons/incomplete.cc: Clarify that
>	the standard doesn't require this test to work for array types.
>	* testsuite/20_util/unique_ptr/lwg2762.cc: Check that incomplete
>	types can be used with array specialization.
>	* testsuite/20_util/unique_ptr/101236.cc: New test.

This (and the previous change that it tried to fix) needs to be
reverted, as attached.

Tested powerpc64le-linux. Committed to trunk.

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