This PR shares common functionality relate to equalities between same
constructors, and when these are type-correct. In particular it uses the
more complete logic from `mkInjectivityThm` also in other places, such
as `CasesOnSameCtor` and the deriving code for `BEq`, `DecidableEq`,
`Ord`, for more consistency and better error messages.
Closes#3386
Currently, when generating the signature of an injectivity lemma for a
certain constructor `c : forall xs, Foo a_1 ... a_n`,
`mkInjectiveTheoremTypeCore?` will differentiate between variables which
are bound to stay the same between the two equal values (i.e inductive
indices), and non-fixed ones. To do that, the function currently checks
whether a variable `x ∈ xs` appears in the final co-domain `Foo a_1 ...
a_n` of the constructor. This condition isn't enough however. As shown
in the linked issue, the codomain may also depend on variables which
appears in the type of free vars contained in `Foo a_1 ... a_n`, but not
in the term itself. This PR fixes the issue by also checking the types
of any free variable occuring in the final codomain, so as to ensure
injectivity lemmas are well-typed.
