The final 8 specification of most-specific testing includes the following rules for when two functional interface types may be compared structurally.

When there are no inference variables (15.12.2.5):
"T is not a subtype of S"

When there are inference variables (18.5.4):
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Si is a functional interface type and Ti is a parameterization of functional interface, I, and none of the following is true:
- Si is a superinterface of I, or a parameterization of a superinterface of I.
- Si is subinterface of I, or a parameterization of a subinterface of I.
- Si is an intersection type and each element of the intersection is a superinterface of I, or a parameterization of a superinterface of I.
- Si is an intersection type and some element of the intersection is a subinterface of I, or a parameterization of a subinterface of I.
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javac should be updated to conform to these rules. (Current behavior is to test whether either type has a supertype that is a parameterization of the other type's symbol.)