[PATCH] Add a new type attribute always_alias (PR79671)

[Bernd Edlinger]

Hi,

this is related to the P1 codegen bug PR79671:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671

Therefore I would like to add this now although
the trunk is already at in stage 4.

I propose to add a new type attribute always_alias that
works like may_alias but unlike may_alias it should
also make instances alias anything.  It is also exposed
in C/C++ as __attribute__((always_alias)).

For C++17 the FE automatically introduces this attribute
at std::byte and all class/struct/union which contain
either a std::byte array or an unsigned char array.

Note that the patch [1] needs to be applied as well,
because otherwise the new attribute will trip several
failures in the libstdc++ testsuite.


Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
Is it OK for trunk?


Thanks
Bernd.


[1] https://gcc.gnu.org/ml/gcc-patches/2017-04/msg00189.html
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