Pre-rendered to be … fragment of the mismatch message.
Whether the slot was marked optional with a trailing ?.
Boolean predicates for the type names up to the first unresolvable one.
The type names, in declaration order, with whitespace trimmed.
First unresolvable type name, or null when every name resolved.
Everything the hot path needs from a type-string slot (
"string","string?","a|b","a|b?"), resolved once at compile time.