Re: [PATCH v3] c++: fix parsing with auto(x) [PR112410]
On 11/15/23 17:24, Marek Polacek wrote:
On Tue, Nov 14, 2023 at 05:27:03PM -0500, Jason Merrill wrote:
On 11/14/23 10:58, Marek Polacek wrote:
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 **)(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. :/
It isn't consumed there, that loop is just scanning forward to see if
there's a ->. The { is still the next token when we expect it to be a
closing ) in cp_parser_direct_declarator:
Ok, the tokens are rolled back after consuming so we can...
/* Parse the parameter-declaration-clause. */
params
= cp_parser_parameter_declaration_clause (parser, flags);
const location_t parens_end
= cp_lexer_peek_token (parser->lexer)->location;
/* Consume the `)'. */
parens.require_close (parser);
Maybe we want to abort_fully_implicit_template here rather than in
cp_parser_parameter_declaration_clause?
...do this instead. Much better.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK, thanks.
-- >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.
This patch rectifies that by undoing the implicit function template
modification. In the test above, we should notice that the parameter
list is ill-formed, and since we've synthesized an implicit template
parameter, we undo it by calling abort_fully_implicit_template. Then,
we'll parse the "(auto(42))" as an initializer.
PR c++/112410
gcc/cp/ChangeLog:
* parser.cc (cp_parser_direct_declarator): Maybe call
abort_fully_implicit_template if it turned out the parameter list was
ill-formed.
gcc/testsuite/ChangeLog:
* g++.dg/cpp23/auto-fncast13.C: New test.
* g++.dg/cpp23/auto-fncast14.C: New test.
---
gcc/cp/parser.cc | 13 +
gcc/testsuite/g++.dg/cpp23/auto-fncast13.C | 61 ++
gcc/testsuite/g++.dg/cpp23/auto-fncast14.C | 9
3 files changed, 83 insertions(+)
create mode 100644
[PATCH v3] c++: fix parsing with auto(x) [PR112410]
On Tue, Nov 14, 2023 at 05:27:03PM -0500, Jason Merrill wrote:
> On 11/14/23 10:58, Marek Polacek wrote:
> > 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 **)(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. :/
>
> It isn't consumed there, that loop is just scanning forward to see if
> there's a ->. The { is still the next token when we expect it to be a
> closing ) in cp_parser_direct_declarator:
Ok, the tokens are rolled back after consuming so we can...
> > /* Parse the parameter-declaration-clause. */
> > params
> > = cp_parser_parameter_declaration_clause (parser, flags);
> > const location_t parens_end
> > = cp_lexer_peek_token (parser->lexer)->location;
> >
> > /* Consume the `)'. */
> > parens.require_close (parser);
>
> Maybe we want to abort_fully_implicit_template here rather than in
> cp_parser_parameter_declaration_clause?
...do this instead. Much better.
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.
This patch rectifies that by undoing the implicit function template
modification. In the test above, we should notice that the parameter
list is ill-formed, and since we've synthesized an implicit template
parameter, we undo it by calling
