Re: C++17 and "Alice in Wonderland"

From: Daniela Engert (dani_at_[hidden])
Date: 2021-03-24 08:41:38

Am 23.03.2021 um 21:13 schrieb Edward Diener via Boost:
> 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 )...

In a template of form

template<class T, bool (*)(T)> whatever

up until C++14, the function argument type in the non-type template
parameter was never looked at in the deduction of type T because NTTPs
weren't deemed dependent on placeholder types in their function
arguments. Beginning with C++17, they are. Therefore you may get
conflicting type deduction because the second, additional deduction path
from the function argument type in the NTTP will never deduce a
top-level cv-qualified T in case of non-reference-like arguments. But
the first, formerly only deduction path, may. And therein lies the
conflict in your example where T = const char in the first path and T =
char is in the second.

Well, at least this is my understanding of P0172.

Ciao
  Dani

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