Spurious type error in super constructor call under -Xexperimental (since sammy merge)

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class A(stream: java.io.InputStream)
class B(url: java.net.URL) extends A(url.openStream)

Distilled from ProcessBuilderImpl.scala in the stdlib.

Regressed in scala/scala@9c36a7617

% scalac-hash 9c36a7617~1 -Xexperimental sandbox/test.scala

% scalac-hash 9c36a7617 -Xexperimental sandbox/test.scala
sandbox/test.scala:2: error: type mismatch;
 found   : java.io.InputStream
 required: Int
class B(url: java.net.URL) extends A(url.openStream)
                                         ^
one error found

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Imported From: https://issues.scala-lang.org/browse/SI-9141?orig=1
Reporter: @retronym
Affected Versions: 2.11.5
See #9178

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@adriaanm said:
Interesting, under -Xexperimental:

scala> samOf(typeOf[java.io.InputStream])
res1: $r.intp.global.Symbol = method read

and -Ytyper-debug on the example says:

|    |    |-- { super.<init>(url.openStream); () } EXPRmode (site: <init> in B) 
|    |    |    |-- super.<init>(url.openStream) BYVALmode-EXPRmode (site: <init> in B) 
|    |    |    |    |-- super EXPRmode-POLYmode-QUALmode (silent: <init> in B) 
|    |    |    |    |    |-- this EXPRmode (silent: <init> in B) 
|    |    |    |    |    |    \-> B
|    |    |    |    |    \-> B.super.type (with underlying type A)
|    |    |    |    |-- url.openStream : pt=java.io.InputStream BYVALmode-EXPRmode (site: <init> in <empty>) 
|    |    |    |    |    |-- url EXPRmode-POLYmode-QUALmode (site: <init> in <empty>) 
|    |    |    |    |    |    \-> url.type (with underlying type java.net.URL)
|    |    |    |    |    |-- { (() => url.openStream()) } : pt=java.io.InputStream BYVALmode-EXPRmode (site: <init> in <empty>) 
|    |    |    |    |    |    |-- (() => url.openStream()) : pt=java.io.InputStream BYVALmode-EXPRmode (site: <init> in <empty>) 
|    |    |    |    |    |    |    |-- { def read$body(): Int = url.openStream(); final class $a... : pt=java.io.InputStream BYVALmode-EXPRmode (site: <init> in <empty>) 
|    |    |    |    |    |    |    |    |-- def read$body BYVALmode-EXPRmode (site: <init> in <empty>) 
|    |    |    |    |    |    |    |    |    |-- url.openStream() : pt=Int EXPRmode (site: method read$body in <empty>) 
|    |    |    |    |    |    |    |    |    |    |-- url.openStream BYVALmode-EXPRmode-FUNmode-POLYmode (silent: method read$body in <empty>) 
|    |    |    |    |    |    |    |    |    |    |    \-> ()java.io.InputStream
/Users/adriaan/Desktop/bugs/t9141.scala:2: error: type mismatch;
 found   : java.io.InputStream
 required: Int
class B(url: java.net.URL) extends A(url.openStream)
                                         ^

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@adriaanm said:
This raises an interesting question about the order of implicit conversions. Particularly, eta-expansion and empty application. I seem to remember someone bringing this up recently.

The reason this doesn't type-check under -Xexperimental is because it meets the criteria for eta-expansion as defined at http://scala-lang.org/files/archive/spec/2.11/06-expressions.html#eta-expansion (where "function type" is generalized to include sam types). Without -Xexperimental, it went straight to "empty application".

[scabug]

@adriaanm said:
And by "recently" I mean something like 1964 bugs ago: #7187

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@retronym said:
See also the SAM-version of that classic ticket: #9178.

[magnolia-k]

The code in this example can be compiled.

Also, the -Xexperimental option has been deprecated since Scala 2.13.0, so the issue itself can be closed.