https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104230
--- Comment #2 from anton at socialhacker dot com --- Hmm, sorry, still learning the bugzilla UI. I thought that patch would be attached to my next comment. I don't believe that patch is a solution to the problem. It was just a way I was able to make progress debugging the issue. I followed that patch with one that allowed GCC to mangle the resulting POINTER_PLUS_EXPR. But the end result was that the two template instantiations were not considered the same (as far as __is_same was concerned). I'm pretty sure that the problem is upstream instead of downstream from the location where the diagnostic is generated (in invalid_tparm_referent_p). And I've been working my way backwards into the parser, but it's much slower going for me. I've found where the two different parse trees are generated for the two different version of the template parameters. But they happen in such different portions of the parser I'm not sure at all how to have them generate the same tree eventually, which is I'm guessing the right answer to the problem. That is, instead of generating a COMPONENT_REF for the accepted form, and a POINTER_PLUS_EXPR for the rejected form, GCC should probably be generating COMPONENT_REF for both. But I think it's forgotten the required information to do that conversion from POINTER_PLUS_EXPR by the time it's needed. I'm guessing someone who knows the parser side well might have a better chance at seeing how that should be done.
