[cfe-dev] Diagnostic proposal

[Arthur O'Dwyer via cfe-dev]

On Tue, Jan 28, 2020 at 10:40 AM via cfe-dev <cfe-dev at lists.llvm.org> wrote:

> Hi @clang,
>
> I have a small proposal for an improved error message. Consider
>
> https://godbolt.org/z/j3i9jy
>
> There the function foo for the template class is instantiated for int,
> because foo is virtual. A non-virtual function wouldn't get instantiated
> (since it is not used) and no error would be generated. Currently the
> diagnostic only hints to the constructor as the final instantiation
> context. An extended message, that the function is instantiated, because it
> is virtual, would be helpful.
> Otherwise, with a larger c'tor and a more complex class hierarchy it is
> not so easy to tell, what is going on.
>

I don't think that posting "proposed new error message" ideas to cfe-dev@
is likely to be useful, unless you are posting to say that you are working
on a patch.

If anyone *does* work on a patch to improve Clang's notes on why a template
function is being instantiated: I offer this other example, which came up
on the Cpplang Slack workspace <https://cppalliance.org/slack/> last week.
https://godbolt.org/z/9E8FKL
The destructor of Indestructible<true> is instantiated because it's needed
by A<Indestructible<true>>'s implicitly defaulted destructor. The compiler
error message doesn't indicate this relationship.  I believe the root cause
is the same as Olaf's case.  A patch that improves the handling of one
should improve the other as well, I would hope.

–Arthur
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