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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Date: Fri, 24 Mar 2023 20:43:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109277

--- 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. =E2=80=94 end note] Access checking is performed in a context unrela=
ted 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
=E2=80=9Cimmediate context=E2=80=9D and can result in
the program being ill-formed. =E2=80=94 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 =
...=