[gcc r15-8869] c++: pack indexing and if consteval

[Jason Merrill via Gcc-cvs]

https://gcc.gnu.org/g:94d1dacf350c3c463aead7e4d5c172f138045eb1

commit r15-8869-g94d1dacf350c3c463aead7e4d5c172f138045eb1
Author: Jason Merrill <ja...@redhat.com>
Date:   Sat Mar 22 10:52:14 2025 -0400

    c++: pack indexing and if consteval
    
    The pack index is manifestly constant-evaluated, and the call to
    maybe_constant_value needs to reflect that or we wrongly complain about
    non-constant index if the evaluation uses if consteval.
    
    gcc/cp/ChangeLog:
    
            * semantics.cc (finish_type_pack_element): Pass mce_true to
            maybe_constant_value.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp26/pack-indexing16.C: New test.

Diff:
---
 gcc/cp/semantics.cc                          |  2 +-
 gcc/testsuite/g++.dg/cpp26/pack-indexing16.C | 16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index b0a5f9ad6607..b390d663e243 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -5086,7 +5086,7 @@ finish_underlying_type (tree type)
 static tree
 finish_type_pack_element (tree idx, tree types, tsubst_flags_t complain)
 {
-  idx = maybe_constant_value (idx);
+  idx = maybe_constant_value (idx, NULL_TREE, mce_true);
   if (TREE_CODE (idx) != INTEGER_CST || !INTEGRAL_TYPE_P (TREE_TYPE (idx)))
     {
       if (complain & tf_error)
diff --git a/gcc/testsuite/g++.dg/cpp26/pack-indexing16.C 
b/gcc/testsuite/g++.dg/cpp26/pack-indexing16.C
new file mode 100644
index 000000000000..92ade86d06b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/pack-indexing16.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++26 } }
+
+int i;
+constexpr int idx()
+{
+  if consteval { return 0; }
+  else { return i; }
+}
+
+template <int... Ns>
+int first () { return Ns...[idx()]; }
+
+int main()
+{
+  return first<0,1,2>();
+}