Backport #15658: Harden erasure of TermRefs #15751
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If a getter TermRef has a MethodType, erase it to the erasure of the method result type. Fixes scala#15649 Since the fault in scala#15649 is highly non-deterministic, I can only guess what the reason was. I think what happened was that the TermRef was created very late, when the TermRef was already a getter. I.e. after phase `Getters`, which is itself after `Erasure`. The whole thing was launched from the code that generates static forwarders in the backend. Type erasure is run at erasure phase, but if there is no previous denotation of the `TermRef`, it will keep the first one it read, which would have the `MethodType` as underlying type. The end result was that the `TermRef` was erased to a method type, where it should have been erased to the result type. Consequently, we ended up with a MethodType as the parameter type of another method, which is unexpected. But all that is just a guess. I am not even sure this PR will fix the problem since it comes up so rarely.
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Backports #15658