C++17 and "Alice in Wonderland"

From: Edward Diener (eldiener_at_[hidden])
Date: 2021-03-23 20:13:04

I apologize for this non-Boost related post, but I ran into what I 
thought was a C++17 bug in clang ( also in vc++14.2 C++17 ), whose 
explanation to me of why it is not a bug really sounds like the 
non-logic of "Alice in Wonderland". I am not putting down the person who 
explains to me why it is not a bug in any way, as I know he is a C++ 
expert, but if this is really C++17 something must be wrong with my 
admittedly limited understanding of C++ and where it is going. My bug 
report is at https://bugs.llvm.org/show_bug.cgi?id=49684 and the 
explanation given there just eludes me as to what C++17 is doing, as my 
further comments makes apparent. Does anyone actually understand how 
this can possibly be ? How does C++17 apparently deduce a template 
argument different from what the actual type is and than declare 
therefore a mismatch ? This is really something from another world. My 
apologies for bringing this up in Boost, as it is purely a C++ issue, 
but my original problem comes from something I have been working on in 
the hopes it might eventually be a Boost library, so I decided to see if 
any of the expert Boost contributors might know what is going on in 
C++17. I realize that if everyone did as I have done here the mailing 
list would be a mass of irrelevant posts not Boost related.