Re: Apparent deeply-nested missing error bug with gcc 7.3

[Jason Merrill <jason@redhat.com>]

On Tue, Jun 19, 2018 at 6:04 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On Tue, 19 Jun 2018 at 01:30, Soul Studios wrote:
>>
>> >
>> > It's never called.
>> >
>> > I added a call to abort() to that function, and the tests all pass. So
>> > the function is never used, so GCC never compiles it and doesn't
>> > notice that the return type is invalid. That's allowed by the
>> > standard. The compiler is not required to diagnose ill-formed code in
>> > uninstantiated templates.
>> >
>>
>>
>> UPDATE: My bad.
>> The original compiler feature detection on the test suite was broken/not
>> matching the correct libstdc++ versions.
>> Hence the emplace_back/emplace_front tests were not running.
>
> Told you so :-P
>
>
>> However, it does surprise me that GCC doesn't check this code.
>
> It's a dependent expression so can't be fully checked until
> instantiated -- and as you've discovered, it wasn't being
> instantiated. There's a trade-off between compilation speed and doing
> additional work to check uninstantiated templates with arbitrarily
> complex expressions in them.

And specifically, &<type-dependent expression> might use an overloaded
operator& that returns a reference, so it might be possible to have a
valid instantiation, so the compiler must not reject the template.

Jason