[Bug libstdc++/104606] [11/12/13/14 Regression] comparison operator resolution with std::optional and -std=c++20

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104606

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #5)
> (In reply to Jakub Jelinek from comment #4)
> > Just wild guess, perhaps the PR98842 changes between 11.1 and 11.2?
> 
> That would mean it is a bug in GCC 10.4 also :(.

It's not, even though 10.4 has the library change. Compiling the test case with
10.4 works. But preprocessing with 10.4 and then compiling with 11.1 fails,
confirming a front end change is to blame.

Bisection shows it started to error with r11-2774

c++: Check satisfaction before non-dep convs. [CWG2369]

It's very hard to use concepts to protect a template from hard errors due to
unwanted instantiation if constraints aren't checked until after doing all
substitution and checking of non-dependent conversions.

It was pretty straightforward to insert the satisfaction check into the
logic, but I needed to make the 3-parameter version of
satisfy_declaration_constraints call push_tinst_level like the 2-parameter
version already does.  For simplicity, I also made it add any needed outer
template arguments from the TEMPLATE_DECL to the args.

The testsuite changes are mostly because this change causes unsatisfaction
to cause deduction to fail rather than reject the candidate later in
overload resolution.

gcc/cp/ChangeLog:

DR 2369
* cp-tree.h (push_tinst_level, push_tinst_level_loc): Declare.
* constraint.cc (satisfy_declaration_constraints):
Use add_outermost_template_args and push_tinst_level.
* pt.c (add_outermost_template_args): Handle getting
a TEMPLATE_DECL as the first argument.
(push_tinst_level, push_tinst_level_loc): No longer static.
(fn_type_unification): Check satisfaction before non-dependent
conversions.