[C++ PATCH] PR c++/96437: ICE-on-invalid-code error recovery.

[Roger Sayle]

This patch fixes PR c++/96437 which is an ICE-on-invalid-code regression
affecting mainline.

This patch has been tested on x86_64-pc-linux-gnu, enabling languages
c and c++, with make bootstrap and make -k check with no new failures.
Ok for mainline?

2022-03-07  Roger Sayle  <ro...@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/pr96437.C: New test case.


Thanks in advance,
Roger
--


diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 03d99ab..992f839 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -48217,7 +48217,8 @@ 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/pr96437.C b/gcc/testsuite/g++.dg/pr96437.C
new file mode 100644
index 0000000..00c4141
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr96437.C
@@ -0,0 +1,5 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-excess-errors "" } */
+
+template <()> void A(auto){}