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

[Marek Polacek <polacek@redhat.com>]

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