The equivalence of the four hydrogen atoms of methane rested on indirect evidence, e.g.
This is usually regarded as being, like (ii.), a statement of equivalence.
However, equivalence questions for the deterministic case, whether two grammars or pushdown automata generate the same language or tree are open.
If we adopt a conception of meaning where truth-conditional equivalence is sufficient for synonymy, we have an apparent counterexample to compositionality.