Backport #15679: Don't ignore expected types of New
#15754
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A problem arises if we typecheck an expression like `new C()` with expected type `C[U]` where `C` is defined like this ```scala class C[X](using X)() ``` In this case, we'd like to propagate `U` as the type instance of `X` before we resolve the using clause. To do this, we have to keep the expected result type `C[X]` for typechecking the function part `new C`. Previously, that type was wrapped in an IgnoredProto. The problem was detected now since a class C with just the using clause and no empty parameter clause was previously expanded to ```scala class C[X]()(using X) ``` but is now expanded to ```scala class C[X](using X)() ``` Under the previous expansion, we type checked and `new C()` before looking for an argument of the using clause, so the problem did not arise.
It turns out an sjsJUnitTest fails otherwise, since that one _does_ insert an implicit conversion after a `new`. This is all goes to show that what we do to deal with implicit conversions is completely crazy. It's a black art when to propagate exected types. There's often no better or worse. Almost any change we do can heal some code and break some other code. I believe the only way to get our of this swamp is to get rid of unrestricted implicit conversions.
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Backports #15679