Re: [PATCH] c++: fix infinite looping with arr[arr] [PR125454]

[Jason Merrill]

On 5/28/26 1:54 PM, Marek Polacek wrote:

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/16?

OK.

-- >8 --
Here r16-3466 moved the canonicalization step that transforms
idx[array] to array[idx] to the beginning of cp_build_array_ref.
When we have array[array], we'll be swapping till we blow the stack.

Previously, we'd give the !INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P
error so there was no problem.

        PR c++/125454

gcc/cp/ChangeLog:

        * typeck.cc (cp_build_array_ref): Don't recurse for array[array].

gcc/testsuite/ChangeLog:

        * g++.dg/other/array8.C: New test.
---
  gcc/cp/typeck.cc                    |  3 ++-
  gcc/testsuite/g++.dg/other/array8.C | 15 +++++++++++++++
  2 files changed, 17 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/other/array8.C

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 483e664397b..37a0c3cfb7a 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -4162,7 +4162,8 @@ cp_build_array_ref (location_t loc, tree array, tree idx,
      return error_mark_node;

/* 0[array] */

-  if (TREE_CODE (TREE_TYPE (idx)) == ARRAY_TYPE)
+  if (TREE_CODE (TREE_TYPE (idx)) == ARRAY_TYPE
+      && TREE_CODE (TREE_TYPE (array)) != ARRAY_TYPE)
      {
        std::swap (array, idx);

diff --git a/gcc/testsuite/g++.dg/other/array8.C b/gcc/testsuite/g++.dg/other/array8.C

new file mode 100644
index 00000000000..d238e3ba589
--- /dev/null
+++ b/gcc/testsuite/g++.dg/other/array8.C
@@ -0,0 +1,15 @@
+// PR c++/125454
+// { dg-do compile { target c++11 } }
+
+template <typename T>
+void
+foo()
+{
+  T a = a[a]; // { dg-error "array subscript is not an integer" }
+}
+
+void
+bar ()
+{
+  foo<int[]>;
+}

base-commit: 51a122bf9bf8f01ddfd53fe9e527e93b2ef99ffb