[gcc r12-7550] PR c++/96437: ICE-on-invalid-code error recovery.

[gcc r12-7550] PR c++/96437: ICE-on-invalid-code error recovery.

[Roger Sayle]

https://gcc.gnu.org/g:3093f8a18e7a9c5a9bba976d58a2f91033e7f35a

commit r12-7550-g3093f8a18e7a9c5a9bba976d58a2f91033e7f35a
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue Mar 8 23:23:19 2022 +0000

    PR c++/96437: ICE-on-invalid-code error recovery.
    
    This patch fixes PR c++/96437 which is an ICE-on-invalid-code regression
    affecting mainline.
    
    2022-03-08  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/cp/ChangeLog
            PR c++/96437
            * parser.cc (synthesize_implicit_template_parm): Check that
            TREE_VALUE (new_parm) isn't error_mark_node before setting its
            DECL_VIRTUAL_P.
    
    gcc/testsuite/ChangeLog
            PR c++/96437
            * g++.dg/cpp2a/pr96437.C: New test case.

Diff:
---
 gcc/cp/parser.cc                     | 3 ++-
 gcc/testsuite/g++.dg/cpp2a/pr96437.C | 4 ++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index dc2d39a8751..9153c74f069 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -48218,7 +48218,8 @@ synthesize_implicit_template_parm  (cp_parser *parser, tree constr)
      function template is equivalent to an explicit template.
 
      Note that DECL_ARTIFICIAL is used elsewhere for template parameters.  */
-  DECL_VIRTUAL_P (TREE_VALUE (new_parm)) = true;
+  if (TREE_VALUE (new_parm) != error_mark_node)
+    DECL_VIRTUAL_P (TREE_VALUE (new_parm)) = true;
 
   // Chain the new parameter to the list of implicit parameters.
   if (parser->implicit_template_parms)
diff --git a/gcc/testsuite/g++.dg/cpp2a/pr96437.C b/gcc/testsuite/g++.dg/cpp2a/pr96437.C
new file mode 100644
index 00000000000..79d481e7002
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/pr96437.C
@@ -0,0 +1,4 @@
+/* { dg-do compile } */
+/* { dg-excess-errors "" } */
+
+template <()> void A(auto){}