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Previously, the expected result type of a FunProto type was ignored and taken into
account only in case of ambiguities. arrayclone-new.scala shows that this is not enough.
In a case like
val x: Array[Byte] = Array(1, 2)
we typed 1, 2 to be Int, so overloading resulution would give the Array.apply of
type (Int, Int*)Array[Int]. But that's a dead end, since Array[Int] is not a subtype
of Array[Byte].
This commit proposes the following modified rule for overloading resulution:
A method alternative is applicable if ... (as before), and if its result type
is copmpatible with the expected type of the method application.
The commit does not pre-select alternatives based on comparing with the expected
result type. I tried that but it slowed down typechecking by a factor of at least 4.
Instead, we proceed as usual, ignoring the result type except in case of
ambiguities, but check whether the result of overloading resolution has a
compatible result type. If that's not the case, we filter all alternatives
for result type compatibility and try again.
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