Re: [PATCH v2] c++: fix parsing with auto(x) [PR112410]

[Marek Polacek <polacek@redhat.com>]

On Mon, Nov 13, 2023 at 09:26:41PM -0500, Jason Merrill wrote:
> On 11/10/23 20:13, Marek Polacek wrote:
> > On Thu, Nov 09, 2023 at 07:07:03PM -0500, Jason Merrill wrote:
> > > On 11/9/23 14:58, Marek Polacek wrote:
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > > 
> > > > -- >8 --
> > > > Here we are wrongly parsing
> > > > 
> > > >     int y(auto(42));
> > > > 
> > > > which uses the C++23 cast-to-prvalue feature, and initializes y to 42.
> > > > However, we were treating the auto as an implicit template parameter.
> > > > 
> > > > Fixing the auto{42} case is easy, but when auto is followed by a (,
> > > > I found the fix to be much more involved.  For instance, we cannot
> > > > use cp_parser_expression, because that can give hard errors.  It's
> > > > also necessary to disambiguate 'auto(i)' as 'auto i', not a cast.
> > > > auto(), auto(int), auto(f)(int), auto(*), auto(i[]), auto(...), etc.
> > > > are all function declarations.  We have to look at more than one
> > > > token to decide.
> > > 
> > > Yeah, this is a most vexing parse problem.  The code is synthesizing
> > > template parameters before we've resolved whether the auto is a
> > > decl-specifier or not.
> > > 
> > > > In this fix, I'm (ab)using cp_parser_declarator, with member_p=false
> > > > so that it doesn't commit.  But it handles even more complicated
> > > > cases as
> > > > 
> > > >     int fn (auto (*const **&f)(int) -> char);
> > > 
> > > But it doesn't seem to handle the extremely vexing
> > > 
> > > struct A {
> > >    A(int,int);
> > > };
> > > 
> > > int main()
> > > {
> > >    int a;
> > >    A b(auto(a), 42);
> > > }
> > 
> > Argh.  This test should indeed be accepted and is currently rejected,
> > but it's a different problem: 'b' is at block scope and you can't
> > have a template there.  But when I put it into a namespace scope,
> > it shows that my patch doesn't work correctly.  I've added auto-fncast14.C
> > for the latter and opened c++/112482 for the block-scope problem.
> > > I think we need to stop synthesizing immediately when we see RID_AUTO, and
> > > instead go back after we successfully parse a declaration and synthesize for
> > > any autos we saw along the way.  :/
> > 
> > That seems very complicated :(.  I had a different idea though; how
> > about the following patch?  The idea is that if we see that parsing
> > the parameter-declaration-list didn't work, we undo what synthesize_
> > did, and let cp_parser_initializer parse "(auto(42))", which should
> > succeed.  I checked that after cp_finish_decl y is initialized to 42.
> 
> Nice, that's much simpler.  Do you also still need the changes to
> cp_parser_simple_type_specifier?

I do, otherwise we parse

  int f (auto{42});

just as if it had been

  int f (auto);

because the {42} is consumed in the cp_parser_simple_type_specifier/RID_AUTO
loop.  :/

Marek