The success checker (RFC 0006) used to lose differing if-branches to :any
and accept the use. (inc (if c "a" :k)) typed the if as :any — sound but
imprecise, since the value is provably {:str | :kw}, every member of which
is in inc's error domain.
Adds {:union #{T...}} to the lattice: join-t forms a scalar union of
differing branches instead of collapsing to :any, capped at 4 distinct
scalars (the member space is the five scalar tags, so the lattice stays
finite and the inter-procedural fixpoint still terminates). The checker's
not-number?/not-seqable? report a union only when EVERY member is in the
error domain — any valid member accepts the call, so still no false
positives. type-name renders "a string or a keyword".
Unions are scalar-only and carry no :struct/:vec/:set key, so every
structural predicate already treats them as opaque — specialization sees
them exactly as :any and codegen is unchanged. Gate green, suite 4718,
conformance 335/335, bench even.
9f076937af