[gcc r16-7145] c++/reflection: add fixed test [PR123871]

[Marek Polacek]

https://gcc.gnu.org/g:6f7c038eee71e519571df5741413f0547fbe85a7

commit r16-7145-g6f7c038eee71e519571df5741413f0547fbe85a7
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Jan 29 09:39:12 2026 -0500

    c++/reflection: add fixed test [PR123871]
    
    Incredibly, a patch that I just pushed (r16-7140-g101f2970adc0a7) fixed
    this ICE.
    
    I notice that we don't emit the diag_array_subscript diagnostic because
    slice calls fold_non_dependent_expr which calls _eval_outermost with
    allow_non_constant=true.  I wonder if we want to change this.
    
            PR c++/123871
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/reflect/error11.C: New test.

Diff:
---
 gcc/testsuite/g++.dg/reflect/error11.C | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/gcc/testsuite/g++.dg/reflect/error11.C b/gcc/testsuite/g++.dg/reflect/error11.C
new file mode 100644
index 000000000000..d6c94fb276e4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/error11.C
@@ -0,0 +1,22 @@
+// PR c++/123871
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+
+#include <meta>
+
+template<class T>
+consteval auto
+test ()
+{
+  auto lambda = [](this auto){
+      constexpr std::meta::info array[1] = {^^T};
+      using X = [: array[100] :];  // { dg-error "splice argument must be" }
+      sizeof(X);
+  };
+}
+
+void
+g ()
+{
+  test<int>();
+}