lattice: Don't short-circuit layers between PartialStruct and Type (#47988)
This catches a corner case I missed in #46881. In particular, when
both arguments are `Const`, it is legal for intermediate lattice
layers between the PartialsLattice and the ConstLattice to do
something other than give the merged type. The current code
in the PartialsLattice was short-circuiting this, leading to
inference imprecision. I think technically, we'd be ok to do
the short-circuiting when both are known to be PartialStruct
(since we know that they widenlattice to a type), but it doesn't
seem worth the extra effort to check.
julia
3add4cd3 - lattice: Don't short-circuit layers between PartialStruct and Type (#47988)
