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From: "federico at kircheis dot it" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/101557] the value of '<temporary>' is not usable in a
 constant expression
Date: Sun, 16 Apr 2023 09:31:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101557

--- Comment #5 from Federico Kircheis <federico at kircheis dot it> ---
Today I just found a possible workaround that involves macros and lambdas...

----
struct node {
    const char* d;
    const node& left;
};

#define LEAF(a) []()-> const node&{ constexpr static auto a =3D node{"a",Nu=
ll};
return a; }()
#define NODE(a,b) []()-> const node&{constexpr static auto a =3D node{"a",b=
};
return a; }()

constexpr node Null =3D node{"",Null};

constexpr auto a =3D node{"a",NODE(a,LEAF(b))};
constexpr auto a2 =3D node{"a",node{"a", node{"a", Null}}};


constexpr int mysize(const node& n) noexcept{
    return (&n =3D=3D &Null) ? 0 : 1 + mysize(n.left);=20
}
constexpr auto size0 =3D mysize(Null);
static_assert(size0 =3D=3D 0, "");

constexpr auto size3 =3D mysize(a);
static_assert(size3 =3D=3D 3, "");
----

"mysize(a)" compiles, while "mysize(a2)" does not, even if the two
datastructures are  equivalent


Unfortunately clang does not accept it=