There are two Types.returnTypeSubstitutable methods. They follow slightly different protocols for their inputs, but ultimately compute the same thing*. This is far from obvious, since their bodies do not look alike and pass through a few layers before sharing the same code.

The second one is called by Check.checkOverride, and the first one is used by everything else.

*I _think_ they compute the same thing; they're certainly meant to. Here's a case analysis:

primitives and void:
    isSameType(R1, R2)
reference types, methods have same sig:
    subtypeUnchecked(R1, adapt(R2))
reference types, methods have different sigs:
    first method: subtypeUnchecked(R1, erase(R2))
    second method: subtypeUnchecked(R1, R2) || subtype(R1, erase(R2))