[COMMITTED] testsuite: fix escaping of square brackets

[Sam James]

This fails for me as:
+ERROR: g++.dg/cpp26/constexpr-new3.C -std=c++26: unknown dg option: 2 for "
  dg-error 40 "accessing value of 'int [2]' object through a 'long int' glvalue 
in a constant expression" "
otherwise.

Escape '[' and ']'.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp26/constexpr-new3.C: Escape '[' and ']'.
---
 gcc/testsuite/g++.dg/cpp26/constexpr-new3.C | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C 
b/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C
index 74661994eb1b..c79060fc9b6c 100644
--- a/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-new3.C
@@ -37,7 +37,7 @@ baz ()
 {
   std::allocator<int> a;
   auto b = a.allocate (2);
-  new (b) long (42);           // { dg-error "accessing value of 'int [2]' 
object through a 'long int' glvalue in a constant expression" }
+  new (b) long (42);           // { dg-error "accessing value of 'int 
\\\[2\\\]' object through a 'long int' glvalue in a constant expression" }
   a.deallocate (b, 2);
   return true;
 }

base-commit: 612194c064c2ca279191712bcc01f8fcef65ade9
-- 
2.50.1