c++: lambda in non-type template parm type [PR99478]

In this testcase, the non-type template parameter has the type of a
lambda-expression.  This makes no sense because a lambda in template context
is specified to be distinct between different specializations of the
template, even if the lambda is non-dependent, but here which specialization
we are dealing with depends on which lambda we have, and vice versa.

gcc/cp/ChangeLog:

	PR c++/99478
	* parser.c (cp_parser_lambda_expression): Reject lambda
	in template parameter type.

gcc/testsuite/ChangeLog:

	PR c++/99478
	* g++.dg/cpp2a/lambda-uneval14.C: New test.

1 files changed, 15 insertions, 1 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index aec3aa3..3a10720 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -10796,7 +10796,21 @@ cp_parser_lambda_expression (cp_parser* parser)
   LAMBDA_EXPR_LOCATION (lambda_expr) = token->location;
 
   if (cxx_dialect >= cxx20)
-    /* C++20 allows lambdas in unevaluated context.  */;
+    {
+      /* C++20 allows lambdas in unevaluated context, but one in the type of a
+	 non-type parameter is nonsensical.
+
+	 Distinguish a lambda in the parameter type from a lambda in the
+	 default argument by looking at local_variables_forbidden_p, which is
+	 only set in default arguments.  */
+      if (processing_template_parmlist && !parser->local_variables_forbidden_p)
+	{
+	  error_at (token->location,
+		    "lambda-expression in template parameter type");
+	  token->error_reported = true;
+	  ok = false;
+	}
+    }
   else if (cp_unevaluated_operand)
     {
       if (!token->error_reported)