Re: [C++ RFC / Patch] PR 54080, PR 52875 and more (aka SFINAE vs template recursion depth)

[Paolo Carlini <paolo dot carlini at oracle dot com>]

.. another thought I had, less esoteric ;) is the following: we use 
tf_none for two rather different reasons: for SFINAE and to avoid 
recursive Error routines calls, when we call tsubst (... tf_none, ...) 
from dump_template_bindings.

I understand, given your reply, that in general in the first case, thus 
SFINAE, we should avoid all hard errors *but* the one about too deep 
recursions (barring some sort of powerful infinite recursion detector). 
What about the second case, however? Should it be different? An error 
message is being produced in any case, for a reason or another, it 
shouldn't be prevented or made more difficult only because there is deep 
recursion somewhere. I think that in that second case we should suppress 
the error message about too deep recursion too. But how to tell it 
apart? Looks like we want some sort of separate tf_*, a 
tf_in_diagnostic, or something, very similar to tf_none, but truly with 
no exceptions. Actually this vague idea occured to me a number of times, 
I think having that would help in a number of situations.

What do you think?

Thanks,
Paolo.

(*) Maybe there is third one, like in some recent tweaks Jakub did, but 
let's leave it alone here.