https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118944
--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
> As far as I know, these functions shouldn't even be being instantiated yet,
> yet they appear to be.
It's not required by the standard, but GCC checks non-dependent expressions
ahead of time in order to proactively diagnose IFNDR template definitions that
could never produce a valid specialization ([temp.res.general]/6). So I guess
it boils down to whether f<char> is considered type-dependent? I don't think
it is, at least according to [temp.res.expr]/3, so we should in theory be able
to check the call ahead of time, but it seems something is going wrong after we
partially substitute the explicit template arguments {char} into the template
during template argument deduction.
