Merge #19508: Work around memory-aliasing in descriptor ParsePubkey

fa2ae0ac8d43086430a29c73940ad6b1cd129e96 span: Add Span::empty() and use it in script/descriptor (MarcoFalke)
fa8a99258947a9ee3749fa472180542920cd471c Work around memory-aliasing in descriptor ParsePubkey (MarcoFalke)

Pull request description:

  While this is not undefined behaviour, the memory aliasing trick is confusing when reading the code. Having `a.size()==0` and then access `a[0]` works in this particular case, but should probably be avoided to harden the code for the future.

ACKs for top commit:
  theStack:
    re-ACK https://github.com/bitcoin/bitcoin/pull/19508/commits/fa2ae0ac8d43086430a29c73940ad6b1cd129e96
  elichai:
    ACK fa2ae0ac8d43086430a29c73940ad6b1cd129e96
  jonatack:
    ACK fa2ae0ac8d43086430a29c73940ad6b1cd129e96

Tree-SHA512: 0ec7b09eef45504973a195923cdf1aa8522117c8e2f69b453e5ce9aa8a7e327c71138518022c32d05133dc99cb861101ed0f60fa891814ee3e9dab3a6fa61a84

1 files changed, 1 insertions, 0 deletions

diff --git a/src/span.h b/src/span.h
index 841f1eadf7..79f13c9203 100644
--- a/src/span.h
+++ b/src/span.h
@@ -151,6 +151,7 @@ public:
         return m_data[m_size - 1];
     }
     constexpr std::size_t size() const noexcept { return m_size; }
+    constexpr bool empty() const noexcept { return size() == 0; }
     CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
     {
         ASSERT_IF_DEBUG(size() > pos);