[Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression

["federico.kircheis at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557

            Bug ID: 101557
           Summary: the value of '<temporary>' is not usable in a constant
                    expression
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: federico.kircheis at gmail dot com
  Target Milestone: ---

Consider following snippet


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

constexpr const node Null = node{"",Null,Null};

constexpr const node a 
{
    "a",
    node
    {
        "b",
        node{"e",Null,Null},
        node{"d",Null,Null},
    },
    node{"c",Null,Null},
};

constexpr int mysize(const node& n) noexcept{
    return (&n == &Null) ? 0 : 1 + mysize(n.left) + mysize(n.right); 
}

constexpr auto size0 = mysize(Null);
static_assert(size0 == 0, "");
constexpr auto size1 = mysize(node{"c", Null, Null});
static_assert(size1 == 1, "");

constexpr auto size2 = mysize(node
    {
        "b",
        node{"e",Null,Null},
        node{"d",Null,Null},
    });
static_assert(size2 == 3, "");


// this line does not compile
constexpr auto size3 = mysize(a);
static_assert(size3 == 5, "");
----


The expression `constexpr auto size3 = mysize(a);` does not compile with the
error message

----
<source>:41:30:   in 'constexpr' expansion of 'mysize(a)'
<source>:25:42:   in 'constexpr' expansion of 'mysize((* & n.node::left))'
<source>:41:32: error: the value of '<temporary>' is not usable in a constant
expression
   41 | constexpr auto size3 = mysize(a);
      |                                ^
<source>:22:1: note: '<temporary>' was not declared 'constexpr'
   22 | };
      | ^
Compiler returned: 1
----



A similar effect can be achieved with

----
constexpr node b = a.left;
----

with following error

----
<source>:45:23: error: the value of '<temporary>' is not usable in a constant
expression
   45 | constexpr node b2 = a.left;
      |                       ^~~~
<source>:22:1: note: '<temporary>' was not declared 'constexpr'
   22 | };
      | ^
Compiler returned: 1
----

even if


----
constexpr node b = a;
----

compiles without warnings.


Generally accessing subobjects should not be an issue, for example
----
struct sub{};

struct node2{
    const sub& s;
};
constexpr const node2 n2{
    sub{}
};
constexpr auto s = n2.s;
----

also compiles without issues