Types that have a custom == almost surely also require a custom Hashable conformance (i.e. the synthesized one will be wrong). Ideally, we wouldn’t synthesize if there’s a custom equatable, but that’s not source-stable. The compiler should emit a warning if it synthesizes a Hashable conformance for a type whose ==(Self, Self) -> Bool implementation is not also synthesized.
This should be relatively simple to do: in the "full Hashable derivation" part of DerivedConformanceEquatableHashable.cpp, after we have decided that we are definitely going to derive an implementation, find the == operator in the type's Equatable conformance and see if it isImplicit(). If it is, emit a warning.