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Infer missing parameter types for function literals passed
to higher-order overloaded methods by deriving the
expected argument type from the function types in the
overloaded method type's argument types.
This eases the pain caused by methods becoming overloaded
because SAM types and function types are compatible,
which used to disable parameter type inference because
for overload resolution arguments are typed without
expected type, while typedFunction needs the expected
type to infer missing parameter types for function literals.
It also aligns us with dotty. The special case for
function literals seems reasonable, as it has precedent,
and it just enables the special case in typing function
literals (derive the param types from the expected type).
Since this does change type inference, you can opt out
using the Scala 2.11 source level.
Fix scala/scala-dev#157
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Crucially, the fully-defined expected type must be checked for
conformance to the original expected type!!
The logic in adaptToSam that checks whether pt is fully defined
probably needs some more thought. See pos/t8310 for a good test
case. Argument type checking is a challenge, as we first check
against a lenient pt (this lenient expected type has wildcards,
and thus is not fully defined, but we should still consider sam
adaptation a success even if we end up with wildcards for some
unknown type parameters, they should be determined later).
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