The following code results in a -Warray-bounds warning with GCC 13.1+ and -fsanitize=undefined. No diagnostics are produced by older compilers or if UBSan is not enabled.

template <unsigned inline_capacity> struct ByteBuffer {
  static ByteBuffer create_uninitialized();
  static void copy() {
      auto new_buf = create_uninitialized();
      new_buf.data();
  }
  char *data() { return m_inline ? m_inline_buffer : m_outline_buffer; }
  union {
    char m_inline_buffer[inline_capacity];
    char *m_outline_buffer;
  };
  bool m_inline;
};

void test()  {
  ByteBuffer<56> buf1;
  buf1.data();
  ByteBuffer<2>::copy();
}

Compile with g++ -O2 -Warray-bounds -fsanitize=undefined:

In member function 'char* ByteBuffer<inline_capacity>::data() [with unsigned int inline_capacity = 56]',
    inlined from 'char* ByteBuffer<inline_capacity>::data() [with unsigned int inline_capacity = 2]' at <source>:7:9,
    inlined from 'static void ByteBuffer<inline_capacity>::copy() [with unsigned int inline_capacity = 2]' at <source>:5:19,
    inlined from 'void test()' at <source>:18:22:
<source>:7:34: warning: array subscript 'ByteBuffer<56>[0]' is partly outside array bounds of 'ByteBuffer<2> [1]' [-Warray-bounds=]
    7 |   char *data() { return m_inline ? m_inline_buffer : m_outline_buffer; }
      |                         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>: In function 'void test()':
<source>:4:12: note: object 'new_buf' of size 16
    4 |       auto new_buf = create_uninitialized();
      |            ^~~~~~~

It looks like something about the two SSO buffer lengths gets confused: the error trace incorrectly indicates that ByteBuffer<56>::data() is getting inlined into ByteBuffer<2>::data(). Removing either this template parameter, the m_outline_buffer union member, or calling ByteBuffer::copy with the same template parameter in test() fixes the diagnostic.

https://godbolt.org/z/6rqEnhP6q