[gcc r17-2909] c++/reflection: ^^ rejects pack-index-specifier [PR126546]

https://gcc.gnu.org/g:78b03e2296722df3b69f28677cbeed03c1642dbf

commit r17-2909-g78b03e2296722df3b69f28677cbeed03c1642dbf
Author: Marek Polacek <[email protected]>
Date:   Mon Aug 3 13:02:01 2026 -0400

    c++/reflection: ^^ rejects pack-index-specifier [PR126546]
    
    [expr.reflect] says that a reflection of a pack-index-expressions
    is ill-formed.  But that applies to the ^^id-expression production,
    not ^^type-id, in which a pack-index-specifier (for types) can be
    used.  So it's wrong for get_reflection to check PACK_INDEX_P, it
    should only reject PACK_INDEX_EXPR.
    
    I don't think that currently we can get there with a PACK_INDEX_EXPR
    though: for a pack-index-expression Xs...[0] cp_parser_reflection_name
    will consume Xs and then immediately call get_reflection which gives
    an error.  But leaving the PACK_INDEX_EXPR check in doesn't seem like
    a bad idea.
    
            PR c++/126546
    
    gcc/cp/ChangeLog:
    
            * reflect.cc (get_reflection): Check PACK_INDEX_EXPR instead of
            PACK_INDEX_P.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/reflect/pack-index1.C: Accept a reflection of a
            pack-index-specifier.  Reject a reflection of a
            pack-index-expression.
            * g++.dg/reflect/pack-index2.C: New test.
    
    Reviewed-by: Jason Merrill <[email protected]>

Diff:
---
 gcc/cp/reflect.cc                          | 2 +-
 gcc/testsuite/g++.dg/reflect/pack-index1.C | 9 ++++++++-
 gcc/testsuite/g++.dg/reflect/pack-index2.C | 8 ++++++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
index 989d7cebc77c..7b4142f2d4fb 100644
--- a/gcc/cp/reflect.cc
+++ b/gcc/cp/reflect.cc
@@ -141,7 +141,7 @@ get_reflection (location_t loc, tree t, reflect_kind kind/*=REFLECT_UNDEF*/)
 
   /* Constant template parameters and pack-index-expressions cannot
      appear as operands of the reflection operator.  */
-  if (PACK_INDEX_P (t))
+  if (TREE_CODE (t) == PACK_INDEX_EXPR)
     {
       error_at (loc, "%<^^%> cannot be applied to a pack index");
       return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/reflect/pack-index1.C b/gcc/testsuite/g++.dg/reflect/pack-index1.C
index f68705b12991..c2b27a8c133d 100644
--- a/gcc/testsuite/g++.dg/reflect/pack-index1.C
+++ b/gcc/testsuite/g++.dg/reflect/pack-index1.C
@@ -14,10 +14,16 @@ f ()
 {
   constexpr auto r1 = ^^Ts...[0]::sx;
   constexpr auto r2 = ^^typename Ts...[0]::type;
+  constexpr auto e = ^^Ts...[0];
+}
 
+template<int... Ns>
+void
+g ()
+{
   // NTTPs and pack-index-expressions cannot appear as operands
   // of the reflection operator.
-  constexpr auto e = ^^Ts...[0]; // { dg-error "cannot be applied" }
+  constexpr auto e = ^^Ns...[0];  // { dg-error "cannot be applied" }
 }
 
 
@@ -25,4 +31,5 @@ void
 g ()
 {
   f<S>();
+  g<1, 2, 3>();
 }
diff --git a/gcc/testsuite/g++.dg/reflect/pack-index2.C b/gcc/testsuite/g++.dg/reflect/pack-index2.C
new file mode 100644
index 000000000000..113629510f11
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/pack-index2.C
@@ -0,0 +1,8 @@
+// PR c++/126546
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+
+template <class... Ts>
+consteval auto first() { return ^^Ts...[0]; }
+
+static_assert(first<int, double>() == ^^int);