Function rustc_trait_selection::traits::coherence::overlap

fn overlap<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>, 
    skip_leak_check: SkipLeakCheck, 
    a_def_id: DefId, 
    b_def_id: DefId
) -> 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.