Elementary question about complete_type vs tsubst_flags_t

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

Hi,

I have an old question about an issue which I noticed a while ago, and 
for example clearly shows up in c++/62085: in a few places in pt.c we 
call complete_type from functions getting a tsubst_flags_t. Clearly, 
complete_type often calls instantiate_class_template_1, which, in turn, 
often calls tsubst with an hard-coded tf_error. Thus possible errors 
coming from an initial tf_none. As I said, c++/62085 would be a 
straightforward example: tsubst_flags_t is tf_none when tsubst calls 
complete_type at line #12527 (case TYPENAME_TYPE) and then things start 
going wrong until an hard error from abstract_virtuals_error_sfinae.

Is there a general lesson about such uses of complete_type? Seem at 
least brittle to me...

Thanks!
Paolo.