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

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

[Marek Polacek]

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.

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);

	PR c++/112410

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_simple_type_specifier): Disambiguate
	between a variable and function declaration with auto.
	(cp_parser_constructor_declarator_p): Use cp_parser_starts_param_decl_p.
	(cp_parser_starts_param_decl_p): New, factored out of
	cp_parser_constructor_declarator_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-fncast13.C: New test.
---
 gcc/cp/parser.cc                           | 79 ++++++++++++++++++----
 gcc/testsuite/g++.dg/cpp23/auto-fncast13.C | 61 +++++++++++++++++
 2 files changed, 125 insertions(+), 15 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast13.C

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 5116bcb78f6..3edee092e56 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -2887,6 +2887,8 @@ static bool cp_parser_next_token_ends_template_argument_p
   (cp_parser *);
 static bool cp_parser_nth_token_starts_template_argument_list_p
   (cp_parser *, size_t);
+static bool cp_parser_starts_param_decl_p
+  (cp_parser *);
 static enum tag_types cp_parser_token_is_class_key
   (cp_token *);
 static enum tag_types cp_parser_token_is_type_parameter_key
@@ -19991,6 +19993,8 @@ 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);
@@ -20002,12 +20006,43 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 	      if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_PAREN))
 		{
 		  cp_lexer_consume_token (parser->lexer);
+		  /* An auto specifier that appears in a parameter declaration
+		     might be the placeholder for a late return type, or it
+		     can be an implicit template parameter.  But it can also
+		     be a prvalue cast, rendering the current construct not
+		     a function declaration at all.  Check if it decidedly
+		     cannot be a valid function-style cast first...  */
+		  if (!cp_parser_starts_param_decl_p (parser))
+		    {
+		      /* Ug, we couldn't tell.  Try to parse whatever follows
+			 as a declarator; this should detect cases like
+			 auto(i), auto(*), auto(f[]), auto(f)(int).  */
+		      cp_parser_declarator (parser, CP_PARSER_DECLARATOR_EITHER,
+					    CP_PARSER_FLAGS_NONE,
+					    /*ctor_dtor_or_conv_p=*/nullptr,
+					    /*parenthesized_p=*/NULL,
+					    /*member_p=*/true,
+					    /*friend_p=*/false,
+					    /*static_p=*/true);
+		      /* OK, if we now see a ')', it looks like a valid
+			 function declaration.  Otherwise, let's go with
+			 auto(x).  */
+		      decay_copy
+			= cp_lexer_next_token_is_not (parser->lexer,
+						      CPP_CLOSE_PAREN);
+		    }
 		  cp_parser_skip_to_closing_parenthesis (parser,
 							 /*recovering*/false,
 							 /*or_comma*/false,
 							 /*consume_paren*/true);
 		  continue;
 		}
+	      /* The easy case: it has got to be C++23 auto(x).  */
+	      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 +20054,12 @@ 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 ();
@@ -32066,21 +32107,7 @@ cp_parser_constructor_declarator_p (cp_parser *parser, cp_parser_flags flags,
 	  && !cp_parser_require (parser, CPP_OPEN_PAREN, RT_OPEN_PAREN))
 	constructor_p = false;
 
-      if (constructor_p
-	  && cp_lexer_next_token_is_not (parser->lexer, CPP_CLOSE_PAREN)
-	  && cp_lexer_next_token_is_not (parser->lexer, CPP_ELLIPSIS)
-	  /* A parameter declaration begins with a decl-specifier,
-	     which is either the "attribute" keyword, a storage class
-	     specifier, or (usually) a type-specifier.  */
-	  && !cp_lexer_next_token_is_decl_specifier_keyword (parser->lexer)
-	  /* GNU attributes can actually appear both at the start of
-	     a parameter and parenthesized declarator.
-	     S (__attribute__((unused)) int);
-	     is a constructor, but
-	     S (__attribute__((unused)) foo) (int);
-	     is a function declaration. [[attribute]] can appear in the
-	     first form too, but not in the second form.  */
-	  && !cp_next_tokens_can_be_std_attribute_p (parser))
+      if (constructor_p && !cp_parser_starts_param_decl_p (parser))
 	{
 	  tree type;
 	  tree pushed_scope = NULL_TREE;
@@ -34334,6 +34361,28 @@ cp_parser_nth_token_starts_template_argument_list_p (cp_parser * parser,
   return false;
 }
 
+/* We've consumed a '(' and now we're asking if what follows starts
+   a parameter declaration.  Return true if it does.  */
+
+static bool
+cp_parser_starts_param_decl_p (cp_parser *parser)
+{
+  return (cp_lexer_next_token_is (parser->lexer, CPP_CLOSE_PAREN)
+	  || cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS)
+	  /* A parameter declaration begins with a decl-specifier,
+	     which is either the "attribute" keyword, a storage class
+	     specifier, or (usually) a type-specifier.  */
+	  || cp_lexer_next_token_is_decl_specifier_keyword (parser->lexer)
+	  /* GNU attributes can actually appear both at the start of
+	     a parameter and parenthesized declarator.
+	     S (__attribute__((unused)) int);
+	     is a constructor, but
+	     S (__attribute__((unused)) foo) (int);
+	     is a function declaration.  [[attribute]] can appear in the
+	     first form too, but not in the second form.  */
+	  || cp_next_tokens_can_be_std_attribute_p (parser));
+}
+
 /* Returns the kind of tag indicated by TOKEN, if it is a class-key,
    or none_type otherwise.  */
 
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;
+}

base-commit: 016b3002e13acefb5773da78659c050187d3a96f
-- 
2.41.0

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

[Jason Merrill]

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);
}

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.  :/

Jason

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

[Marek Polacek]

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

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

[Jason Merrill]

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?

> 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

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

[Marek Polacek]

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.  :/

Marek

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

[Jason Merrill]

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

[PATCH v3] c++: fix parsing with auto(x) [PR112410]

[Marek Polacek]

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

Re: [PATCH v3] c++: fix parsing with auto(x) [PR112410]

[Jason Merrill]

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 **&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?

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 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