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
