The class `Lattice` should automatically delegate invocations of the
meet operator to the meet operation of the associated lattice value
class if that class provides a static function called `meet`. This
process fails for two reasons:
1. `Lattice::has_meet` checks for a member function `meet` without
arguments of the lattice value class, although it should check for a
static member function.
2. The function template `Lattice::meet<VT>()` implementing the default
meet operation directly in the lattice is always present and takes
precedence over the delegating function template `Lattice::meet<VT,
std::integral_constant<bool, true>>()`.
This change fixes the automatic delegation of the meet operation of a
lattice to the lattice value class in the presence of a static `meet`
function by conditionally enabling either the delegating function
template or the non-delegating function template and by changing
`Lattice::has_meet` so that it checks for a static `meet` member
function in the lattice value type.
The test from `TestSparseBackwardDataFlowAnalysis.cpp` is changed, such
that the `meet` function is not provided directly in the `WrittenTo`
lattice, but by the `Lattice` base class in order to trigger delegation
to a lattice value class.
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