Two fixture helpers in surface.spec.ts returned ': SessionEvent', a
declaration-merge-extensible mapped union. Spreading such a value (the
rejection cases build '{ ...event, data: ... }') materialises an anonymous
union per fixture, and the following 'as SessionEvent' casts then compare
union against union, which dominates the root type-check program.
Return the single variant each helper actually builds, spelled as the
canonical indexed access SessionEvent<'system/message'> /
SessionEvent<'user/message'>, drop the casts that only existed because data
was a union, and trim the comment of the member count that drifts with every
declaration merge.
Measured on the host build: check 110.9 s -> 84.6 s (median, -23.7 %),
wall -17 % to -24 %; fixture types alone (clean buildinfo, root program):
check 53.4/45.8 s (Extract alias) vs 45.3/49.1 s (indexed access).
imccyu
3b2f7aaf7c