https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109277
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Though the code might be undefined ...
20.15.6/5 says:
```
The predicate condition for a template specialization is_convertible<From, To>
shall be satisfied if and
only if the return expression in the following code would be well-formed,
including any implicit conversions
to the return type of the function:
To test() {
return declval<From>();
}
[Note: This requirement gives well-defined results for reference types, void
types, array types, and function
types. — end note] Access checking is performed in a context unrelated to To
and From. Only the validity
of the immediate context of the expression of the return statement (8.7.3)
(including initialization of the
returned object or reference) is considered. [Note: The initialization can
result in side effects such as
the instantiation of class template specializations and function template
specializations, the generation of
implicitly-defined functions, and so on. Such side effects are not in the
“immediate context” and can result in
the program being ill-formed. — end note]
```
I am not 100% sure but I suspect this is just ill-formed code which was not
rejected before but is now using the builtin rather than the template form ...
