[Bug c++/96749] [coroutines] unexpected 'warning: statement has no effect [-Wunused-value]'

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96749

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Iain D Sandoe
<iains@gcc.gnu.org>:

https://gcc.gnu.org/g:8fba8178ac49493ed7998fb9008810aac8138c19

commit r10-9515-g8fba8178ac49493ed7998fb9008810aac8138c19
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Tue Mar 2 10:12:58 2021 +0000

    coroutines : Avoid generating empty statements [PR96749].

    In the compiler-only idiom:
    " a = (target expr creats temp, op uses temp) "
    the target expression variable needs to be promoted to a frame one
    (if the expression has a suspend point).  However, the only uses of
    the var are in the second part of the compound expression - and we
    were creating an empty statement corresponding to the (now unused)
    first arm.  This then produces the spurious warnings noted.

    Fixed by avoiding generation of a separate variable nest for
    isolated target expressions (or similarly isolated co_awaits used
    in a function call).

    gcc/cp/ChangeLog:

            PR c++/96749
            * coroutines.cc (flatten_await_stmt): Allow for the case
            where a target expression variable only has uses in the
            second part of a compound expression.
            (maybe_promote_temps): Avoid emiting empty statements.

    gcc/testsuite/ChangeLog:

            PR c++/96749
            * g++.dg/coroutines/pr96749-1.C: New test.
            * g++.dg/coroutines/pr96749-2.C: New test.

    (cherry picked from commit ed8198461735f9b5b3c2cbe50f9913690ce4b4ca)