[concepts] Update to match the working draft (and bit more)

[Andrew Sutton]

Well,,that's unfortunate. Please forgive the alternative patch submission.

https://github.com/asutton/gcc/blob/master/concepts.patch


> Attached is a rework of the Concepts TS implementation to match the
> Working Draft. It's a big patch -- I'd loved to make it smaller, but
> it didn't work out that way.
>
> Here's a brief summary of changes:
>
> - Make concepts work with -std=c++2a; warn if -fconcepts is also supplied.
> - Add a new flag -fconcepts-ts to enable TS syntax* when -std=c++2a is
used.
> - No more bool for concepts. They are their own kind of declaration.
> - New grammar for requires clauses (unfortunately). This can be
> overriden with -fconcepts-ts.
> - Support "concept bool" with -fconcepts-ts. This includes both
> variable and function concepts.
> - Constraints are instantiated only at the point of use and properly
> interleave substitution and evaluation. This should fix any issues
> with "premature substitution" errors.
> - Implement semantic comparison of atomic constraints (P0717). This
> may be buggy. More testing with complex refinement hierarchies is
> needed.
> - Completely rewrite the subsumption algorithm in logic. The WD broke
> a number of assumptions the previous version relied on, so a simple
> fix wasn't possible. We haven't seen the performance issues related to
> subsumption that showed up in the past. They're still there, but other
> core changes minimize the likelihood of achieving worst case.
> - Declaration matching is syntactic (P0716).
> - Warnings are emitted for the use of TS syntax unless -fconcepts-ts
> is specified. And if you do use -fconcepts, the same-type rule for
> abbreviated function templates is dead.
> - And just because... make template introduction semantics actually
> conform to the TS. We weren't allowing the introduction of a fixed
> series of template parameters for an introduced pack. We do now.
>
> This is not a perfect patch.
>
> - It somehow breaks partial template specializations of variable
> templates (cpp2a/concepts pr80471.C). I have no idea how that
> happened. It almost looks like a GC bug.
> - There's a new regression in cpp2a/concepts-ts2.C)
> - This breaks a lot of concepts TS support (the g++.dg/concepts dir).
> - We've seen other errors in parts of GCC not even remotely related to
> concepts**.
>
> My goals over the next few weeks are to clean up the regressions and
> start working through the backlog of concepts issues. That includes
> fixing new issues as they arise.
>
> * This patch does not preserve the Concepts TS semantics. Anybody
> relying on e.g., subtleties of the partial ordering rules in the TS
> will find themselves with broken code. This was a conscious choice.
> there are serious design issues in the TS.
>
> ** Unfortunately, my testing effort before sending this patch is a bit
> hampered by the fact that a clean bootstrap build ICEs here (bootstrap
> build on Mac OS -- can give more details if needed).
>
> ../../isl/isl_tab_pip.c: In function
‘isl_tab_basic_set_non_trivial_lexmin’:
> ../../isl/isl_tab_pip.c:5087:21: internal compiler error: in check, at
> tree-vrp.c:155
>  5087 | __isl_give isl_vec *isl_tab_basic_set_non_trivial_lexmin(
>       |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> This one isn't my fault :)
>
> Enjoy,
>

>> Hi. This is the qmail-send program at sourceware.org.
>> I'm afraid I wasn't able to deliver your message to the following
addresses.
>> This is a permanent error; I've given up. Sorry it didn't work out.
>>
>> <gcc-patches@gcc.gnu.org>:
>> ezmlm-reject: fatal: Sorry, I don't accept messages larger than 400000
bytes (#5.2.3)


> Andrew Sutton