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.

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index aec3aa3587ff09926a259962288d7fa7fb063ab4..3a10720631831c0d212101cf39595689e2bb179e 100644 (file)
--- 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)

diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C
new file mode 100644 (file)
index 0000000..a180359
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C
@@ -0,0 +1,6 @@
+// PR c++/99478
+// { dg-do compile { target c++20 } }
+
+template <decltype ([] {})> auto f() {} // { dg-error "lambda" }
+
+int main() { f<{}>(); }                // { dg-prune-output "no match" }