[gcc r14-8904] c++: fix ICE with __type_pack_element [PR113834]

[gcc r14-8904] c++: fix ICE with __type_pack_element [PR113834]

[Marek Polacek]

https://gcc.gnu.org/g:f29f7f86935e29786bf9f976ec99d7639b381b14

commit r14-8904-gf29f7f86935e29786bf9f976ec99d7639b381b14
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Feb 9 12:03:50 2024 -0500

    c++: fix ICE with __type_pack_element [PR113834]
    
    Here we crash on this invalid code because we seem to infinitely recurse
    and end up with __type_pack_element with index that doesn't tree_fits_shwi_p
    which then crashes on tree_to_shwi.
    
    Thanks to Jakub for suggesting a nicer fix than my original one.
    
            PR c++/113834
    
    gcc/cp/ChangeLog:
    
            * semantics.cc (finish_type_pack_element): Perform range checking
            before tree_to_shwi.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/ext/type_pack_element4.C: New test.

Diff:
---
 gcc/cp/semantics.cc                           |  7 +++----
 gcc/testsuite/g++.dg/ext/type_pack_element4.C | 17 +++++++++++++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 3299e2704465..57840176863b 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -4650,20 +4650,19 @@ finish_type_pack_element (tree idx, tree types, tsubst_flags_t complain)
 	error ("%<__type_pack_element%> index is not an integral constant");
       return error_mark_node;
     }
-  HOST_WIDE_INT val = tree_to_shwi (idx);
-  if (val < 0)
+  if (tree_int_cst_sgn (idx) < 0)
     {
       if (complain & tf_error)
 	error ("%<__type_pack_element%> index is negative");
       return error_mark_node;
     }
-  if (val >= TREE_VEC_LENGTH (types))
+  if (wi::to_widest (idx) >= TREE_VEC_LENGTH (types))
     {
       if (complain & tf_error)
 	error ("%<__type_pack_element%> index is out of range");
       return error_mark_node;
     }
-  return TREE_VEC_ELT (types, val);
+  return TREE_VEC_ELT (types, tree_to_shwi (idx));
 }
 
 /* Implement the __direct_bases keyword: Return the direct base classes
diff --git a/gcc/testsuite/g++.dg/ext/type_pack_element4.C b/gcc/testsuite/g++.dg/ext/type_pack_element4.C
new file mode 100644
index 000000000000..aa508c79090b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/type_pack_element4.C
@@ -0,0 +1,17 @@
+// PR c++/113834
+// { dg-do compile { target c++17 } }
+
+template <typename... _Elements> class tuple{};
+template <unsigned long __i, typename... _Elements>
+__type_pack_element<__i, _Elements...> &get(tuple<_Elements...> &__t) noexcept; // { dg-error "index is out of range" }
+tuple<int,int> data;
+template <unsigned long Level>
+unsigned take_impl(unsigned idx) {
+  if constexpr (Level != -1){
+    return take_impl<Level - 1>(get<Level - 1>(data)); // { dg-error "" }
+  }
+  return 0;
+}
+int main() {
+  take_impl<2>(0);
+}