c++/reflection: consteval-only type in splice-spec [PR125900]

In this test in the template for the

  std::println("{}", static_cast<int>(s.[:member:]));

line provokes a bogus "consteval-only expressions are only allowed
in a constant-evaluated context" error.  (Not the line before
because there we have an OVERLOAD because the second argument's
type isn't obvious, and so the whole thing is considered type-dep.)

A splice-specifier is [: constant-expression :], and [expr.const.defns]:
an expression or conversion is manifestly constant-evaluated if it is
a constant-expression.

So I think we shouldn't walk SPLICE_EXPRs in
check_out_of_consteval_use_r.

	PR c++/125900

gcc/cp/ChangeLog:

	* reflect.cc (check_out_of_consteval_use_r): Don't walk
	SPLICE_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/reflect/expr18.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 4e56ca07b9eeb9dd765893b34c147ca398a57f71)

gcc/cp/reflect.cc

diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
index 9a1dded..74b3a8e 100644
--- a/gcc/cp/reflect.cc
+++ b/gcc/cp/reflect.cc
@@ -8752,6 +8752,10 @@
       || TREE_CODE (t) == DECL_EXPR
       /* Neither into USING_STMT.  */
       || TREE_CODE (t) == USING_STMT
+      /* The operand of a splice is a constant-expression, thus
+	 manifestly constant-evaluated, so consteval-only types are permitted
+	 here.  */
+      || TREE_CODE (t) == SPLICE_EXPR
       /* Blocks can appear in the TREE_VEC operand of OpenMP
 	 depend/affinity/map/to/from OMP_CLAUSEs when using iterators.  */
       || TREE_CODE (t) == BLOCK)

gcc/testsuite/g++.dg/reflect/expr18.C

diff --git a/gcc/testsuite/g++.dg/reflect/expr18.C b/gcc/testsuite/g++.dg/reflect/expr18.C
new file mode 100644
index 0000000..f2ceefc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/expr18.C
@@ -0,0 +1,25 @@
+// PR c++/125900
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+
+#include <meta>
+#include <print>
+
+struct S {
+    float f;
+};
+
+int main() {
+    S s{.f = 1.5f};
+
+    constexpr auto member{^^S::f};
+    std::println("{}", s.[:member:]);
+    std::println("{}", static_cast<int>(s.[:member:]));
+
+    constexpr auto access_context{std::meta::access_context::current()};
+    template for (constexpr auto member :
+                  std::define_static_array(nonstatic_data_members_of(^^S, access_context))) {
+        std::println("{}", s.[:member:]);
+        std::println("{}", static_cast<int>(s.[:member:]));
+    }
+}