Function rustc_trait_selection::traits::coherence::overlap

fn overlap<'tcx>(
    tcx: TyCtxt<'tcx>,
    track_ambiguity_causes: TrackAmbiguityCauses,
    skip_leak_check: SkipLeakCheck,
    impl1_def_id: DefId,
    impl2_def_id: DefId,
    overlap_mode: OverlapMode
) -> Option<OverlapResult<'tcx>>

Can both impl a and impl b be satisfied by a common type (including where-clauses)? If so, returns an ImplHeader that unifies the two impls.