Unable to use functional interface as a SAM target type

[retronym]

% cat sandbox/Test.java && javac -d . sandbox/Test.java && dbg qscala -Ydebug -Ytyper-debug -nc -Xexperimental -e 'val x: p1.Test[AnyRef, AnyRef] = {(a: Object, b: Object) => new Object}; x.invokeWithHeaders("x", "y")'
package p1;

@FunctionalInterface
public interface Test<Request, Response> {
    Object invokeWithHeaders(Object var1, Object var2);

    default Object invoke(Object var1) {
        throw new UnsupportedOperationException("ServerServiceCalls should be invoked by using the invokeWithHeaders method.");
    }

    default <Request, Response> Object of(Object var0) {
        return var0;
    }
}
Listening for transport dt_socket at address: 5006
[running phase parser on scalacmd4317652137244637493.scala]
[running phase namer on scalacmd4317652137244637493.scala]
[running phase packageobjects on scalacmd4317652137244637493.scala]
[running phase typer on scalacmd4317652137244637493.scala]
...
|    |    |    |    |    |-- p1.this.Test[scala.this.AnyRef,scala.this.AnyRef] BYVALmode-EXPRmode (site: anonymous class $anon, a ClassSymbol with flags final)
|    |    |    |    |    |    |-- p1.this.Test[Request,Response] TYPEmode (site: value x  in $anon, a TermSymbol with flags private[this])
|    |    |    |    |    |    |    |-- AnyRef TYPEmode (site: value x  in $anon, a TermSymbol with flags private[this])
|    |    |    |    |    |    |    |    [adapt] scala.this.AnyRef is now a TypeTree(scala.this.AnyRef)
|    |    |    |    |    |    |    |    \-> scala.this.AnyRef
|    |    |    |    |    |    |    |-- AnyRef TYPEmode (site: value x  in $anon, a TermSymbol with flags private[this])
|    |    |    |    |    |    |    |    [adapt] scala.this.AnyRef is now a TypeTree(scala.this.AnyRef)
|    |    |    |    |    |    |    |    \-> scala.this.AnyRef
|    |    |    |    |    |    |    \-> p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]
|    |    |    |    |    |    |-- ((a: Object, b: Object) => new Object.<init>()) : pt=p1.this.Test[scala.this.AnyRef,scala.this.AnyRef] BYVALmode-EXPRmode (site: value x  in $anon, a TermSymbol with flags private[this])
|    |    |    |    |    |    |    |-- Object TYPEmode (site: value a in $anon, a TermSymbol with flags <param> <locked>)
|    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    |-- Object TYPEmode (site: value a in $anon, a TermSymbol with flags <param>)
|    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    |-- Object TYPEmode (site: value b in $anon, a TermSymbol with flags <param> <locked>)
|    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    |-- Object TYPEmode (site: value b in $anon, a TermSymbol with flags <param>)
|    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    |-- new Object.<init>() : pt=lang.this.Object EXPRmode (site: value $anonfun in $anon, a TermSymbol with flags <synthetic>)
|    |    |    |    |    |    |    |    |-- new Object.<init> BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value $anonfun in $anon, a TermSymbol with flags <synthetic>)
|    |    |    |    |    |    |    |    |    |-- new Object EXPRmode-POLYmode-QUALmode (silent: value $anonfun in $anon, a TermSymbol with flags <synthetic>)
|    |    |    |    |    |    |    |    |    |    |-- Object FUNmode-TYPEmode (silent: value $anonfun in $anon, a TermSymbol with flags <synthetic>)
|    |    |    |    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    |    |    \-> ()lang.this.Object
|    |    |    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    |    |    solving for (Request: ?Request, Response: ?Response)
|    |    |    |    |    |    |    solving for (Request: ?Request, Response: ?Response)
|    |    |    |    |    |    |    solving for (Request: ?Request, Response: ?Response)
|    |    |    |    |    |    |    solving for (Request: ?Request, Response: ?Response)

      def fullyDefinedMeetsExpectedFunTp(pt: Type): Boolean = isFullyDefined(pt) && {
        val samMethType = pt memberInfo sam
        fun.tpe <:< functionType(samMethType.paramTypes, samMethType.resultType)
//      |                 |
//      |                 `--scala.this.Function2[scala.this.Any,scala.this.Any,lang.this.Object]
//      |
//      `---- scala.this.Function2[lang.this.Object,lang.this.Object,lang.this.Object]
      }

We miss out on the loose matching between Object and Any that usually occurs with Java method types:

  /** Are `syms1` and `syms2` parameter lists with pairwise equivalent types? */
  protected[internal] def matchingParams(syms1: List[Symbol], syms2: List[Symbol], syms1isJava: Boolean, syms2isJava: Boolean): Boolean = syms1 match {
    case Nil =>
      syms2.isEmpty
    case sym1 :: rest1 =>
      syms2 match {
        case Nil =>
          false
        case sym2 :: rest2 =>
          val tp1 = sym1.tpe
          val tp2 = sym2.tpe
          (tp1 =:= tp2 ||
           syms1isJava && tp2.typeSymbol == ObjectClass && tp1.typeSymbol == AnyClass ||
           syms2isJava && tp1.typeSymbol == ObjectClass && tp2.typeSymbol == AnyClass) &&
          matchingParams(rest1, rest2, syms1isJava, syms2isJava)
      }
  }

Which leads to:

|    |    |    |    |    |    |    [search #1] start `<notype>`, searching for adaptation to pt=scala.this.Function1[scala.this.Function2[lang.this.Object,lang.this.Object,lang.this.Object],p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]] (silent: value x  in $anon, a TermSymbol with flags private[this]) implicits disabled
|    |    |    |    |    |    |    [search #2] start `<notype>`, searching for adaptation to pt=scala.this.Function1[scala.this.<byname>[scala.this.Function2[lang.this.Object,lang.this.Object,lang.this.Object]],p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]] (silent: value x  in $anon, a TermSymbol with flags private[this]) implicits disabled
/var/folders/b7/xcc2k0ln6ldcv247ffpy2d1w0000gp/T/scalacmd4317652137244637493.scala:1: error: type mismatch;
 found   : scala.this.Function2[lang.this.Object,lang.this.Object,lang.this.Object]
 required: p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]
val x: p1.Test[AnyRef, AnyRef] = {(a: Object, b: Object) => new Object}; x.invokeWithHeaders("x", "y")
                                                         ^
|    |    |    |    |    |    |    \-> <error>
|    |    |    |    |    |    \-> [val x ] p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]
|    |    |    |    |    |-- def x BYVALmode-EXPRmode (site: anonymous class $anon, a ClassSymbol with flags final)
|    |    |    |    |    |    |-- p1.this.Test[scala.this.AnyRef,scala.this.AnyRef] : pt=p1.this.Test[scala.this.AnyRef,scala.this.AnyRef] EXPRmode (site: getter x in $anon, a MethodSymbol with flags <method> <stable> <accessor> <trans_flag> private)
|    |    |    |    |    |    |    |-- scala.this.AnyRef{} EXPRmode-POLYmode-QUALmode (site: getter x in $anon, a MethodSymbol with flags <method> <stable> <accessor> <trans_flag> private)
|    |    |    |    |    |    |    |    \-> Main.$anon.type (with underlying type scala.this.AnyRef{})
|    |    |    |    |    |    |    \-> p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]
|    |    |    |    |    |    \-> [def x] => p1.this.Test[scala.this.AnyRef,scala.this.AnyRef]
|    |    |    |    |    |-- x.invokeWithHeaders("x", "y") BYVALmode-EXPRmode (site: value <local $anon> in $anon, a TermSymbol)
|    |    |    |    |    |    |-- x.invokeWithHeaders BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local $anon> in $anon, a TermSymbol)
|    |    |    |    |    |    |    |-- x EXPRmode-POLYmode-QUALmode (silent: value <local $anon> in $anon, a TermSymbol)
|    |    |    |    |    |    |    |    \-> $anon.this.x.type (with underlying type p1.this.Test[scala.this.AnyRef,scala.this.AnyRef])
|    |    |    |    |    |    |    \-> (<param> <synthetic> x$1: scala.this.Any, <param> <synthetic> x$2: scala.this.Any)lang.this.Object
|    |    |    |    |    |    |-- "x" : pt=scala.this.Any BYVALmode-EXPRmode (silent: value <local $anon> in $anon, a TermSymbol)
|    |    |    |    |    |    |    \-> lang.this.String("x")
|    |    |    |    |    |    |-- "y" : pt=scala.this.Any BYVALmode-EXPRmode (silent: value <local $anon> in $anon, a TermSymbol)
|    |    |    |    |    |    |    \-> lang.this.String("y")
|    |    |    |    |    |    \-> lang.this.Object
|    |    |    |    |    \-> [class $anon] scala.this.AnyRef{}
|    |    |    |    |-- new $anon.<init>() : pt=scala.this.Unit EXPRmode (site: method main in Main, a MethodSymbol with flags <method>)
|    |    |    |    |    |-- new $anon.<init> BYVALmode-EXPRmode-FUNmode-POLYmode (silent: method main in Main, a MethodSymbol with flags <method>)
|    |    |    |    |    |    |-- new $anon EXPRmode-POLYmode-QUALmode (silent: method main in Main, a MethodSymbol with flags <method>)
|    |    |    |    |    |    |    |-- $anon FUNmode-TYPEmode (silent: method main in Main, a MethodSymbol with flags <method>)
|    |    |    |    |    |    |    |    \-> scala.this.AnyRef{}
|    |    |    |    |    |    |    \-> scala.this.AnyRef{}
|    |    |    |    |    |    \-> ()scala.this.AnyRef{}
|    |    |    |    |    |-- { new $anon.<init>(); () } : pt=scala.this.Unit EXPRmode (site: method main in Main, a MethodSymbol with flags <method>)
|    |    |    |    |    |    |-- () : pt=scala.this.Unit EXPRmode (site: method main in Main, a MethodSymbol with flags <method>)
|    |    |    |    |    |    |    \-> scala.this.Unit
|    |    |    |    |    |    \-> scala.this.Unit
|    |    |    |    |    [adapt] ()scala.this.AnyRef{} adapted to { new $anon.<init>(); () } based on pt scala.this.Unit
|    |    |    |    |    \-> scala.this.Unit
|    |    |    |    \-> scala.this.Unit
|    |    |    \-> [def main] (<param> args: scala.this.Array[String])scala.this.Unit
|    |    \-> [object Main] <empty>.this.Main.type
|    \-> [package <empty>] type

[adriaanm]

fixed by scala/scala#7966