testsuite: Add testcase for CWG3162 - Evaluation context of manifestly constant-evalu...

Another testcase for something we already implement the desired way.

2026-04-03  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/reflect/define_aggregate10.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

diff --git a/gcc/testsuite/g++.dg/reflect/define_aggregate10.C b/gcc/testsuite/g++.dg/reflect/define_aggregate10.C
new file mode 100644 (file)
index 0000000..59e63f7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/define_aggregate10.C
@@ -0,0 +1,21 @@
+// CWG3162 - Evaluation context of manifestly constant-evaluated expressions
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+
+#include <meta>
+
+struct S;
+
+consteval std::size_t
+foo (int p)
+{
+  constexpr std::size_t r = /* Q */ std::meta::is_complete_type (^^S) ? 1 : 2; // #1
+  if (!std::meta::is_complete_type (^^S))  // #2
+    std::meta::define_aggregate (^^S, {});
+  return p > 0 ? foo (p - 1) : r;
+}
+
+consteval {
+  if (foo (1) != 2)
+    throw;  // OK, not evaluated
+}