Bogus type error with constructor params and anonymous classes

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The following code causes bogus type error, when anonymous classes are used as parameters:

class B
object C extends B

class F[T <: B](cons: => T)
class F2[T <: B](cons: => T) extends F(cons)

object D extends F2(C) // works
object E extends F2(new B {})

test.scala:8: error: illegal inheritance;
 self-type ESI-8849.type does not conform to F2SI-8845[BSI-11]'s selftype F2SI-8845[BSI-11]
object E extends F2((new B {}))
                 ^
test.scala:8: error: type mismatch;
 found   : BSI-11
 required: BSI-11
object E extends F2((new B {}))
                     ^
two errors found

Possible workaround at the moment is to provide an explicit type ascription:

object E extends F2(new B {}:B)

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Imported From: https://issues.scala-lang.org/browse/SI-3048?orig=1
Reporter: miasma

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@adriaanm said:
looks like the underlying cause is type inference

minimal test case:

class A
class B[T <: A](cons: T)

object C extends B/*[A]*/(new A{})

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@adriaanm said:
Martin, re-assigning to you as I can't figure out what's going wrong. Note that the anonymous class A{} and the absence of type argument to B are essential to reproduce.

First, I thought it was because case (3) of adapt creates a TypeApply tree like [T <: A] B[T] [T]
I [http://github.com/adriaanm/scala/commit/952245ef4a6385443564fccb5aaf25f012d05908#L2R796 changed] this to result in simply B[T]', but that does not fix much. (as an aside, that fix does prevent a very weird substitution. Before this change, the TreeTypeSubstituterininferMethodInstancewould substituteT -> A, which would change the tree new [T <: A] B[T] [T]' -> `new [T <: A] B[T] [anonymous class $$anon]')

Then, I figure typedTypeConstructor' might be at fault for using normalize instead of dealias, which would turn the type constructor Binto[T<:A]B[T]`, so I [http://github.com/adriaanm/scala/commit/952245ef4a6385443564fccb5aaf25f012d05908#L2R4161 changed that to use dealias]

the full patch I cobbled together so far is at:
http://github.com/adriaanm/scala/commit/952245ef4a6385443564fccb5aaf25f012d05908

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@hubertp said:
I will try to track it down using events system.

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Commit Message Bot (anonymous) said:
(extempore in r25161) Don't infer anonymous classes. The type is widened as little as
possible, just far enough to avoid all kinds of undesirable consequences
which accompany the preservation of too much type information. (The
problems are akin to inferring the singleton type too freely.)

// Example of code which did not compile, but now does
class A
class B[T <: A](cons: T)
object C extends B(new A {})

Closes #4110, #3048. I already ran this by moors, so review by odersky.