[C++ Patch] PR 71140 ("[concepts] ill-formed nested-requirement lacking a semicolon not rejected")

[Jason Merrill]

On 10/12/18 9:32 AM, Andrew Sutton wrote:
>>
>> BTW, I would discourage you from messing much with the concepts code
>> at this point, as a major overhaul should be coming soon.
> 
> Major overhaul:
> 
> https://github.com/asutton/gcc (branch is concepts; we're about 2 weeks
> back from trunk).

Awesome!

> Unfortunately, I we haven't been following GCC commit discipline, and
> there's a bunch of dead/legacy code that I need to clean up. And of course
> some regressions. I wanted to spend the weekend on this and forward a
> cleaner version next week. But no time is as good as the present it seems.
> 
> This fork reimplements concepts as currently specified in the WD (for the
> most part). It also preserves TS syntax (but not behavior), although there
> are certainly going to be some new bugs.
> 
> -std=c++2a turns on concepts by default (sets -fconcepts)
> -fconcepts-ts can be additionally specified to enable TS extensions
> (abbreviated fn templates, etc).
> -fconcepts on its own gives you (should give you) TS syntax with C++20
> semantics and no C++20 features.
> 
> Here's what's changed:
> - new requires clause syntax as required in the WD (-fconcepts-ts will
> change this back to the TS syntax)
> - concept bool is now a warning, although (IIRC) disabled with
> -fconcepts-ts. Function and variable concepts live on.
> - concepts are now their own kind of declaration (CONCEPT_DECL). That was a
> big change.

Can you say a bit about why that was better than continuing to use VAR_DECL?

> - now only 3 kinds of constraints: conj, disj, and pred (should be
> atomic, also needs a dead code cleanup).
> - constraints on declarations are represented as expressions -- no
> normalization until later
> - associated constraints are only instantiated when checked -- no premature
> substitution
> - new implementaiton of satisfaction, does not require ahead-of-time
> normalization
> - new implementation of normalization (fewer nodes, smaller impl)
> - atomic constraint comparison based on expr identity/parameter mapping
> - complete rewrite of subsumption (new comparison model invalidated some
> assumptions in the old impl)
> - constrained decls differentiated by syntax of constraints (not
> equivalence)
> - moved the concepts testsuite into c++2a directory as a vetting/curating
> process, new 2a tests, new ts-compatability tests
> 
> There are some bugs and regressions. I know for a fact that we've broken
> partial specialization of variable templats, but I'm really not sure how.
> There's also probably a bug in the constraint comparison implementation
> that affects partial ordering. More testing is needed. I mostly ignored the
> TS support while updating to the WD semantics, so that's been a little
> buggy when I brought it back online. Also, sometimes diagnostics aren't
> emitted correctly.
> 
> I'm not quite sure how to proceed with submitting this patch. Once I made
> the decision to make concepts their own kind of declaration, the idea of
> sending small patches went right out the window.

Yeah, don't worry about trying to send small patches.  I don't mind 
reviewing what's on the branch, though at least the final patch should 
be sent to the list for archival.

What feedback are you looking for at this point?

Jason