public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer,
                                                                           IEnumerable<TInner> inner,
                                                                           Func<TOuter, TKey> outerKeySelector,
                                                                           Func<TInner, TKey> innerKeySelector,
                                                                           Func<TOuter, TInner, TResult> resultSelector,
                                                                           IEqualityComparer<TKey> comparer)
    {
      return JoinImpl(outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer);
    }

Create JoinImpl from usage. None of type parameters are created and all of them become "UNRESOLVED" in method signature.

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