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

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

[federico.kircheis at gmail dot com]

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

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
clang++ rejects it similarly.
pr101557.C:40:16: error: constexpr variable 'size3' must be initialized by a
constant expression
constexpr auto size3 = mysize(a);
               ^       ~~~~~~~~~
pr101557.C:22:45: note: read of temporary is not allowed in a constant
expression outside the expression that created the temporary
Wonder if this isn't related to the recently posted PR100976 patch.

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

[federico.kircheis at gmail dot com]

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

--- Comment #2 from Federico Kircheis <federico.kircheis at gmail dot com> ---
clang does not reject it:

https://godbolt.org/z/8Mq1e3o3j

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

[pinskia at gcc dot gnu.org]

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Federico Kircheis from comment #2)
> clang does not reject it:
> 
> https://godbolt.org/z/8Mq1e3o3j

clang 11 does reject it but clang 12 does NOT reject it.

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

[federico.kircheis at gmail dot com]

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

--- Comment #4 from Federico Kircheis <federico.kircheis at gmail dot com> ---
Indeed.
I just checked the latest versions.

I wonder if there is a common cause that makes this recursive data structure
harder to evaluate at compile time.

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

[federico at kircheis dot it]

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

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

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

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


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

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

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


Unfortunately clang does not accept it