[Bug c++/88174] Make __real__ += __val usable in constexpr context.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88174

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:190776773516955df480bfa75731c34c5aaf2306

commit r13-1983-g190776773516955df480bfa75731c34c5aaf2306
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sun Aug 7 10:07:38 2022 +0200

    c++: Add support for __real__/__imag__ modifications in constant
expressions [PR88174]

    We claim we support P0415R1 (constexpr complex), but e.g.
     #include <complex>

    constexpr bool
    foo ()
    {
      std::complex<double> a (1.0, 2.0);
      a += 3.0;
      a.real (6.0);
      return a.real () == 6.0 && a.imag () == 2.0;
    }

    static_assert (foo ());

    fails with
    test.C:12:20: error: non-constant condition for static assertion
       12 | static_assert (foo ());
          |                ~~~~^~
    test.C:12:20:   in âconstexprâ expansion of âfoo()â
    test.C:8:10:   in âconstexprâ expansion of
âa.std::complex<double>::real(6.0e+0)â
    test.C:12:20: error: modification of â__real__
a.std::complex<double>::_M_valueâ is not a constant expression

    The problem is we don't handle REALPART_EXPR and IMAGPART_EXPR
    in cxx_eval_store_expression.
    The following patch attempts to support it (with a requirement
    that those are the outermost expressions, ARRAY_REF/COMPONENT_REF
    etc. are just not possible on the result of these, BIT_FIELD_REF
    would be theoretically possible if trying to extract some bits
    from one part of a complex int, but I don't see how it could appear
    in the FE trees.

    For these references, the code handles value being COMPLEX_CST,
    COMPLEX_EXPR or CONSTRUCTOR_NO_CLEARING empty CONSTRUCTOR (what we use
    to represent uninitialized values for C++20 and later) and the
    code starts by rewriting it to COMPLEX_EXPR, so that we can freely
    adjust the individual parts and later on possibly optimize it back
    to COMPLEX_CST if both halves are constant.

    2022-08-07  Jakub Jelinek  <jakub@redhat.com>

            PR c++/88174
            * constexpr.cc (cxx_eval_store_expression): Handle REALPART_EXPR
            and IMAGPART_EXPR.  Change ctors from releasing_vec to
            auto_vec<tree *>, adjust all uses.  For !preeval, update ctors
            vector.

            * g++.dg/cpp1y/constexpr-complex1.C: New test.