[PATCH] c++: Fix error-recovery with requires expression [PR99500]

[Marek Polacek via Gcc-patches]

This fixes an ICE on invalid code where one of the parameters was
error_mark_node and thus resetting its DECL_CONTEXT crashed.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

gcc/cp/ChangeLog:

        PR c++/99500
        * parser.c (cp_parser_requirement_parameter_list): Handle
        error_mark_node.

gcc/testsuite/ChangeLog:

        PR c++/99500
        * g++.dg/cpp2a/concepts-err3.C: New test.
---
 gcc/cp/parser.c                            | 7 +++++--
 gcc/testsuite/g++.dg/cpp2a/concepts-err3.C | 4 ++++
 2 files changed, 9 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-err3.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 0a7d18af98b..dbc44ed0765 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -28835,8 +28835,11 @@ cp_parser_requirement_parameter_list (cp_parser 
*parser)
       if (parm == void_list_node || parm == explicit_void_list_node)
        break;
       tree decl = TREE_VALUE (parm);
-      DECL_CONTEXT (decl) = NULL_TREE;
-      CONSTRAINT_VAR_P (decl) = true;
+      if (decl != error_mark_node)
+       {
+         DECL_CONTEXT (decl) = NULL_TREE;
+         CONSTRAINT_VAR_P (decl) = true;
+       }
     }
 
   return parms;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-err3.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-err3.C
new file mode 100644
index 00000000000..9427fd5c5a6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-err3.C
@@ -0,0 +1,4 @@
+// PR c++/99500
+// { dg-do compile { target c++20 } }
+
+bool b = requires (bool a, int a) { requires true; }; // { dg-error 
"conflicting declaration" }

base-commit: 8dc225d311ed87633fa970164bdda19bf228b8a3
-- 
2.29.2