That push recompiles a clause of an extended metafunction by
replacing the LHS with a recompiled form of the `lhs-pat' field of the
`metafunc-case' struct. But that field (used by the test generator)
also incorporates the clause's `where' and `side-condition' clauses,
causing them to executed twice when applying the recompiled clause (once
while matching its LHS, and once while evaluating its RHS).