Assert that index is valid in wxArray::operator[]

Restore compatibility with all the previous wxWidgets versions that had
this assert and make sure that errors in the code using this operator
don't go unnoticed, as it happened with #26148.

Closes #26149.

include/wx/dynarray.h

@@ -122,6 +122,18 @@ public:
     bool IsEmpty() const { return this->empty(); }
     size_t Count() const { return this->size(); }
 
+    T& operator[](size_t uiIndex)
+    {
+        wxASSERT( uiIndex < this->size() );
+        return base_vec::operator[](uiIndex);
+    }
+
+    const T& operator[](size_t uiIndex) const
+    {
+        wxASSERT( uiIndex < this->size() );
+        return base_vec::operator[](uiIndex);
+    }
+
     T& Item(size_t uiIndex) const
     {
         wxASSERT( uiIndex < this->size() );

tests/arrays/arrays.cpp

@@ -321,6 +321,11 @@ TEST_CASE("wxArrayString", "[dynarray]")
     CHECK( a5.size() == 3 );
     CHECK( a5[2] == "Foo" );
 
+    // This is undefined behaviour but because the array has just been resized
+    // down, its memory hopefully hasn't been reallocated yet, so we shouldn't
+    // crash accessing it. But we must assert due to the index being invalid.
+    WX_ASSERT_FAILS_WITH_ASSERT( a5[3].clear() );
+
     wxArrayString a6;
     a6.Add("Foo");
     a6.Insert(a6[0], 1, 100);