At the beginning of section 4.5, the spec states that
The info string may not contain any backtick characters.
However, just before example 85, it states
Info strings for backtick code blocks cannot contain backticks
Which implies that it may be permissible for a tilde code fence to have an info string containing backticks. This needs to be clarified to answer the question: what does
~~~ aa ```
foo
compile to?