[gcc r12-8176] c++: wrong error with variadic concept [PR105268]

[gcc r12-8176] c++: wrong error with variadic concept [PR105268]

[Marek Polacek]

https://gcc.gnu.org/g:ccae4443c8322d9d82a19a1e7c689413a122a478

commit r12-8176-gccae4443c8322d9d82a19a1e7c689413a122a478
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Apr 14 13:48:15 2022 -0400

    c++: wrong error with variadic concept [PR105268]
    
    Here we issue a wrong error for the
    
      template<typename T = S<C_many<int>>> void g();
    
    line in the testcase.  I surmise that's because we mistakenly parse
    C_many<int> as a placeholder-type-specifier, and things go wrong from
    there.  We are in a default argument so we should reject parsing C_many<int>
    as a placeholder-type-specifier, which would mean creating a new parameter.
    We want C_many<int> to be a concept-id instead.
    
    It's interesting to see why the same problem didn't occur for C_one<int>.
    In that case, cp_parser_placeholder_type_specifier -> finish_type_constraints
    -> build_type_constraint -> build_concept_check -> build_standard_check ->
    coerce_template_parms fails the parse here:
    
     8916   nargs = inner_args ? NUM_TMPL_ARGS (inner_args) : 0;
     8917   if ((nargs - variadic_args_p > nparms && !variadic_p)
     8918       || (nargs < nparms - variadic_p
     8919           && require_all_args
     8920           && !variadic_args_p
     8921           && (!use_default_args
     8922               || (TREE_VEC_ELT (parms, nargs) != error_mark_node
     8923                   && !TREE_PURPOSE (TREE_VEC_ELT (parms, nargs))))))
     8924     {
     8925     bad_nargs:
     ...
     8943       return error_mark_node;
    
    because nargs is 2 (the targs are <WILDCARD_DECL, int>) while nparms is
    1 (for the one 'typename' in the tparam list of C_one).  But for
    C_many<int> variadic_p is true so we don't return error_mark_node but
    <type_argument_pack>.
    
    This patch does not issue any error for the !tentative case because
    I didn't figure out how to trigger that.  So it adds an assert instead.
    
            PR c++/105268
    
    gcc/cp/ChangeLog:
    
            * parser.cc (cp_parser_placeholder_type_specifier): Return
            error_mark_node when trying to build up a constrained parameter in
            a default argument.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/concepts/variadic6.C: New test.

Diff:
---
 gcc/cp/parser.cc                          | 11 ++++++++++-
 gcc/testsuite/g++.dg/concepts/variadic6.C | 20 ++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index bfd16e1ef62..169e6a62f5f 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -20041,7 +20041,16 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
   /* In a template parameter list, a type-parameter can be introduced
      by type-constraints alone.  */
   if (processing_template_parmlist && !placeholder)
-    return build_constrained_parameter (con, proto, args);
+    {
+      /* In a default argument we may not be creating new parameters.  */
+      if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN)
+	{
+	  /* If this assert turns out to be false, do error() instead.  */
+	  gcc_assert (tentative);
+	  return error_mark_node;
+	}
+      return build_constrained_parameter (con, proto, args);
+    }
 
   /* Diagnose issues placeholder issues.  */
   if (!flag_concepts_ts
diff --git a/gcc/testsuite/g++.dg/concepts/variadic6.C b/gcc/testsuite/g++.dg/concepts/variadic6.C
new file mode 100644
index 00000000000..0b386b0cd6d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/variadic6.C
@@ -0,0 +1,20 @@
+// PR c++/105268
+// { dg-do compile { target concepts } }
+
+template<typename> concept C_one = true;
+template<typename...> concept C_many = true;
+
+template<bool B> struct S { };
+
+template<typename T = S<C_one<int>>> void f();
+template<typename T = S<C_many<int>>> void g();
+
+void
+fn (auto a = S<C_one<int>>{})
+{
+}
+
+void
+fn2 (auto a = S<C_many<int>>{})
+{
+}