PR c++/93257 - consteval void function.

A prvalue can have void type, and if it doesn't do anything prohibited in a
constant expression, it's vacuously constant.

	* constexpr.c (verify_constant): Allow void_node.

gcc/cp/ChangeLog

 2020-01-15  Jason Merrill  <jason@redhat.com>
 
+	PR c++/93257 - consteval void function.
+	* constexpr.c (verify_constant): Allow void_node.
+
 	PR c++/92871 - bad code with xvalue and GNU ?: extension.
 	* call.c (prevent_lifetime_extension): New.
 	(build_conditional_expr_1): Use it.

gcc/cp/constexpr.c

@@ -2609,7 +2609,8 @@ static bool
 verify_constant (tree t, bool allow_non_constant, bool *non_constant_p,
 		 bool *overflow_p)
 {
-  if (!*non_constant_p && !reduced_constant_expression_p (t))
+  if (!*non_constant_p && !reduced_constant_expression_p (t)
+      && t != void_node)
     {
       if (!allow_non_constant)
 	error ("%q+E is not a constant expression", t);

gcc/testsuite/g++.dg/cpp2a/consteval-void1.C

+// PR c++/93257
+// { dg-do compile { target c++2a } }
+
+template <bool, typename>
+consteval void test() {}
+
+int main() {
+    test<false, int>();
+    return 0;
+}