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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/103901] A lambda with a new type in its body cannot be
 defined inside template parameter list
Date: Tue, 04 Jan 2022 20:15:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103901

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
If it is valid, I think e.g.
--- gcc/cp/parser.c.jj  2022-01-04 09:59:37.297641779 +0100
+++ gcc/cp/parser.c     2022-01-04 20:56:19.065070397 +0100
@@ -11046,6 +11046,9 @@ cp_parser_lambda_expression (cp_parser*
     bool save_in_consteval_if_p =3D in_consteval_if_p;
     in_consteval_if_p =3D false;

+    int saved_processing_template_parmlist =3D processing_template_parmlis=
t;
+    processing_template_parmlist =3D 0;
+
     /* By virtue of defining a local class, a lambda expression has access=
 to
        the private variables of enclosing classes.  */

@@ -11078,6 +11081,7 @@ cp_parser_lambda_expression (cp_parser*

     in_consteval_if_p =3D save_in_consteval_if_p;
     in_discarded_stmt =3D discarded;
+    processing_template_parmlist =3D saved_processing_template_parmlist;

     parser->num_template_parameter_lists =3D saved_num_template_parameter_=
lists;
     parser->in_statement =3D in_statement;
should fix that.  But as the stack-overflow link says, it is unclear.
Also, we still reject even with the above patch
decltype ([]{ struct A {} ; return 1; }) a;
(guess that is PR101911).=