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 **&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. :/
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:
/* 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?
Jason
