[PATCH] c++: bogus error w/ variadic concept-id as if cond [PR98394]

[Patrick Palka via Gcc-patches]

Here when tentatively parsing the if condition as a declaration, we try
to treat C<1> as the start of a constrained placeholder type, which we
quickly reject because C doesn't accept a type as its first argument.
But since we're parsing tentatively, we shouldn't emit an error in this
case.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/11?

        PR c++/98394

gcc/cp/ChangeLog:

        * parser.c (cp_parser_placeholder_type_specifier): Don't emit
        a "does not constrain a type" error when parsing tentatively.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-pr98394.C: New test.
---
 gcc/cp/parser.c                               |  7 +++++--
 gcc/testsuite/g++.dg/cpp2a/concepts-pr98394.C | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pr98394.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 4c2075742d6..f1498e28da4 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -19909,8 +19909,11 @@ cp_parser_placeholder_type_specifier (cp_parser 
*parser, location_t loc,
       if (!flag_concepts_ts
          || !processing_template_parmlist)
        {
-         error_at (loc, "%qE does not constrain a type", DECL_NAME (con));
-         inform (DECL_SOURCE_LOCATION (con), "concept defined here");
+         if (!tentative)
+           {
+             error_at (loc, "%qE does not constrain a type", DECL_NAME (con));
+             inform (DECL_SOURCE_LOCATION (con), "concept defined here");
+           }
          return error_mark_node;
        }
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr98394.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-pr98394.C
new file mode 100644
index 00000000000..c8407cdf7cd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr98394.C
@@ -0,0 +1,14 @@
+// PR c++/98394
+// { dg-do compile { target c++20 } }
+
+template<int...>
+concept C = true;
+
+template<int, int>
+concept D = true;
+
+int main() {
+  if (C<1>); // { dg-bogus "does not constrain a type" }
+  if (D<1>); // { dg-error "wrong number of template arguments" }
+            // { dg-bogus "does not constrain a type" "" { target *-*-* } .-1 }
+}
-- 
2.34.0.rc1.14.g88d915a634