SyntheticAccessorLowering was initially implemented under the assumption
that any access to an invisible declaration will cause an accessor to be
generated _in the same file_. Moreover, it's declared in the group of
phases that are performed by file.
But this assumption is incorrect for constructors which need to be
hidden (those which take parameters of inline class types), since such
constructor is public and can be called from anywhere. In this case,
SyntheticAccessorLowering actually generated a new accessor for the
hidden constructor for each (!) source file where that constructor is
called, which led to ClassFormatError because of the class file having
multiple methods with the same signature. The internal `functionMap`
cache didn't help because it's not shared among phase instances for
different files (well, it helped to generate not more than one accessor
per usage-file).
In this change, we use the global cache, stored in JvmBackendContext,
for accessors to hidden constructors. Note that after this change, calls
to hidden constructors are always transformed to the corresponding
accessor in SyntheticAccessorLowering right away, but that accessor
might be orphaned for a while (not declared in any parent's
declarations). Only when SyntheticAccessorLowering encounters the
original constructor which needs to be hidden, it adds the accessor
beside it.
The test is sensitive to the file order, so both variants are added.
Steven Schäfer