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List: gcc-patches
Subject: [PATCH v2] c++: fix parsing with auto(x) [PR112410]
From: Marek Polacek <polacek () redhat ! com>
Date: 2023-11-11 1:13:30
Message-ID: ZU7VOhuVUvCPnqqG () redhat ! com
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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.
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_simple_type_specifier): Disambiguate
between a variable and function declaration with auto.
(cp_parser_parameter_declaration_clause): 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 | 27 +++++++++-
gcc/testsuite/g++.dg/cpp23/auto-fncast13.C | 61 ++++++++++++++++++++++
gcc/testsuite/g++.dg/cpp23/auto-fncast14.C | 9 ++++
3 files changed, 96 insertions(+), 1 deletion(-)
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..947351b09b8 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -19991,6 +19991,8 @@ cp_parser_simple_type_specifier (cp_parser* parser,
/* The 'auto' might be the placeholder return type for a function decl
with trailing return type. */
bool have_trailing_return_fn_decl = false;
+ /* Or it might be auto(x) or auto {x}. */
+ bool decay_copy = false;
cp_parser_parse_tentatively (parser);
cp_lexer_consume_token (parser->lexer);
@@ -20008,6 +20010,11 @@ cp_parser_simple_type_specifier (cp_parser* parser,
/*consume_paren*/true);
continue;
}
+ else if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
+ {
+ decay_copy = true;
+ break;
+ }
if (cp_lexer_next_token_is (parser->lexer, CPP_DEREF))
{
@@ -20019,6 +20026,11 @@ cp_parser_simple_type_specifier (cp_parser* parser,
}
cp_parser_abort_tentative_parse (parser);
+ if (decay_copy)
+ {
+ type = error_mark_node;
+ break;
+ }
if (have_trailing_return_fn_decl)
{
type = make_auto ();
@@ -24973,7 +24985,20 @@ cp_parser_parameter_declaration_clause (cp_parser* parser,
parameter-declaration-list, then the entire
parameter-declaration-clause is erroneous. */
if (parameters == error_mark_node)
- return NULL_TREE;
+ {
+ /* 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 (parser->fully_implicit_function_template_p
+ /* Don't do this for the inner (). */
+ && parser->default_arg_ok_p)
+ abort_fully_implicit_template (parser);
+ return NULL_TREE;
+ }
/* Peek at the next token. */
token = cp_lexer_peek_token (parser->lexer);
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: e0c1476d5d7c450b1b16a40364cea4e91237ea93
--
2.41.0
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