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 **&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:
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 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 gcc/testsuite/g++.dg/cpp23/auto-fncast13.C
create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast14.C
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 5116bcb78f6..d1104336215 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -23594,6 +23594,19 @@ cp_parser_direct_declarator (cp_parser* parser,
/* Consume the `)'. */
parens.require_close (parser);
+ /* For code like
+ int x(auto(42));
+ A a(auto(i), 42);
+ we have synthesized an implicit template parameter and marked
+ what we thought was a function as an implicit function
template.
+ But now, having seen the whole parameter list, we know it's not
+ a function declaration, so undo that. */
+ if (cp_parser_error_occurred (parser)
+ && parser->fully_implicit_function_template_p
+ /* Don't do this for the inner (). */
+ && parser->default_arg_ok_p)
+ abort_fully_implicit_template (parser);
+
/* If all went well, parse the cv-qualifier-seq,
ref-qualifier and the exception-specification. */
if (member_p || cp_parser_parse_definitely (parser))
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast13.C
b/gcc/testsuite/g++.dg/cpp23/auto-fncast13.C
new file mode 100644
index 00000000000..1bceffb70cf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast13.C
@@ -0,0 +1,61 @@
+// PR c++/112410
+// { dg-do compile { target c++23 } }
+
+int f1 (auto(int) -> char);
+int f2 (auto x);
+int f3 (auto);
+int f4 (auto(i));
+
+int v1 (auto(42));
+int v2 (auto{42});
+int e1 (auto{i}); // { dg-error "not declared" }
+int i;
+int v3 (auto{i});
+int v4 (auto(i + 1));
+int v5 (auto(+i));
+int v6 (auto(i = 4));
+
+int f5 (auto(i));
+int f6 (auto());
+int f7 (auto(int));
+int f8 (auto(f)(int));
+int f9 (auto(...) -> char);
+// FIXME: ICEs (PR c++/89867)
+//int f10 (auto(__attribute__((unused)) i));
+int f11 (auto((i)));
+int f12 (auto(i[]));
+int f13 (auto(*i));
+int f14 (auto(*));
+
+int e2 (auto{}); // { dg-error "invalid use of .auto." }
+int e3 (auto(i, i)); // { dg-error "invalid use of .auto." }
+
+char bar (int);
+char baz ();
+char qux (...);
+
+void
+g (int i)
+{
+ f1 (bar);
+ f2 (42);
+ f3 (42);
+ f4 (42);
+ f5 (42);
+ f6 (baz);
+ f7 (bar);
+ f8 (bar);
+ f9 (qux);
+// f10 (42);
+ f11 (42);
+ f12 (&i);
+ f13 (&i);
+ f14 (&i);
+
+ v1 = 1;
+ v2 = 2;
+ v3 = 3;
+ v4 = 4;
+ v5 = 5;
+ v6 = 6;
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast14.C
b/gcc/testsuite/g++.dg/cpp23/auto-fncast14.C
new file mode 100644
index 00000000000..9e7a06c87d5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast14.C
@@ -0,0 +1,9 @@
+// PR c++/112410
+// { dg-do compile { target c++23 } }
+
+struct A {
+ A(int,int);
+};
+
+int a;
+A b1(auto(a), 42);
base-commit: 01bc30b222a9d2ff0269325d9e367f8f1fcef942
--
2.41.0
