[PATCH] c++: concept in default argument [PR109859]

[Marek Polacek]

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

-- >8 --
1) We're hitting the assert in cp_parser_placeholder_type_specifier.
It says that if it turns out to be false, we should do error() instead.
Do so, then.

2) lambda-targ8.C should compile fine, though.  The problem was that
local_variables_forbidden_p wasn't cleared when we're about to parse
the optional template-parameter-list for a lambda in a default argument.

        PR c++/109859

gcc/cp/ChangeLog:

        * parser.cc (cp_parser_lambda_declarator_opt): Temporarily clear
        local_variables_forbidden_p.
        (cp_parser_placeholder_type_specifier): Turn an assert into an error.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-defarg3.C: New test.
        * g++.dg/cpp2a/lambda-targ8.C: New test.
---
 gcc/cp/parser.cc                              |  9 +++++++--
 gcc/testsuite/g++.dg/cpp2a/concepts-defarg3.C |  8 ++++++++
 gcc/testsuite/g++.dg/cpp2a/lambda-targ8.C     | 10 ++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-defarg3.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-targ8.C

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 4dd9474cf60..bdc4fef243a 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -11891,6 +11891,11 @@ cp_parser_lambda_declarator_opt (cp_parser* parser, 
tree lambda_expr)
                 "lambda templates are only available with "
                 "%<-std=c++20%> or %<-std=gnu++20%>");
 
+      /* Even though the whole lambda may be a default argument, its
+        template-parameter-list is a context where it's OK to create
+        new parameters.  */
+      auto lvf = make_temp_override (parser->local_variables_forbidden_p, 0u);
+
       cp_lexer_consume_token (parser->lexer);
 
       template_param_list = cp_parser_template_parameter_list (parser);
@@ -20978,8 +20983,8 @@ cp_parser_placeholder_type_specifier (cp_parser 
*parser, location_t loc,
       /* 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);
+         if (!tentative)
+           error_at (loc, "local variables may not appear in this context");
          return error_mark_node;
        }
       return build_constrained_parameter (con, proto, args);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-defarg3.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-defarg3.C
new file mode 100644
index 00000000000..1d689dc8f30
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-defarg3.C
@@ -0,0 +1,8 @@
+// PR c++/109859
+// { dg-do compile { target c++20 } }
+
+template<class>
+concept C = true;
+
+template <class = C> // { dg-error "may not appear in this context" }
+int f();
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ8.C 
b/gcc/testsuite/g++.dg/cpp2a/lambda-targ8.C
new file mode 100644
index 00000000000..3685b0ef880
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ8.C
@@ -0,0 +1,10 @@
+// PR c++/109859
+// { dg-do compile { target c++20 } }
+
+template<typename>
+concept A = true;
+
+template<auto = []<A a> {}>
+int x;
+
+void g() { (void) x<>; }

base-commit: cc62b2c3da118f08f71d2ae9c08bafb55b35767a
-- 
2.46.0