Follow-up on TyperState exception

[odersky]

Compiler version

3.8

Minimized example

#23609 triggered an assertion error in TyperState. The relevant explanation seems to be in ProtoTypes.scala:

            // To respect the pre-condition of `mergeConstraintWith` and keep
            // `protoTyperState` committable we must ensure that it does not
            // contain any type variable which don't already exist in the passed
            // TyperState. This is achieved by instantiating any such type
            // variable. NOTE: this does not suffice to discard type variables
            // in ancestors of `protoTyperState`, if this situation ever
            // comes up, an assertion in TyperState will trigger and this code
            // will need to be generalized.

The error was avoided by turning it into a handled exception in #23665. But that's really only a workaround. We should get to the bottom of this and prevent the exception being thrown.