Invalid `PolyFunction` refinements 

## Compiler version

3.4.0-RC1-bin-20230726-97677cc-NIGHTLY


## Issue

We are not restricting the signatures of the user-written (in source or macro) refinements of `PolyFunction`. We should restrict and check that those refinements are valid

## Example 1

```scala
def test = polyFun(1)

def polyFun: PolyFunction { def apply(x: Int): Int } =
  new PolyFunction { def apply(x: Int): Int = x + 1 }
```

Works but is outside of the current spec of `PolyFunction`. These should be supported at some point. We should probably emit an error now and allow it once we update the spec.

## Example 2

```Scala
def test = polyFun(1)(2)

def polyFun: PolyFunction { def apply(x: Int)(y: Int): Int } =
  new PolyFunction:
    def apply(x: Int)(y: Int): Int = x + y
```

This is not supported by the spec and is not intended to be supported. It currently crashes the compiler with 

<details>
<summary>java.util.NoSuchElementException: head of empty list</summary>

```
Exception in thread "main" java.lang.AssertionError: assertion failed
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:11)
        at dotty.tools.dotc.core.TypeErasure$.functionType$1(TypeErasure.scala:569)
        at dotty.tools.dotc.core.TypeErasure$.eraseRefinedFunctionApply(TypeErasure.scala:572)
        at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$apply(TypeErasure.scala:658)
        at dotty.tools.dotc.core.TypeErasure.eraseResult(TypeErasure.scala:881)
        at dotty.tools.dotc.core.TypeErasure.eraseInfo(TypeErasure.scala:815)
        at dotty.tools.dotc.core.TypeErasure$.transformInfo(TypeErasure.scala:275)
        at dotty.tools.dotc.transform.Erasure.transform(Erasure.scala:97)

  exception while retyping @SourceFile("tests/run-custom-args/erased/erased-15.scala") final module class 
  erased-15$package() extends Object() {
  private def writeReplace(): AnyRef =
    new scala.runtime.ModuleSerializationProxy(classOf[erased-15$package.type])
  def test: Int = polyFun.apply(1)(2)
  def polyFun: (x: Int) => (y: Int) => Int =
    {
      final class $anon() extends Object(), PolyFunction {
        def apply(x: Int)(y: Int): Int = x.+(y)
        private val $outer: erased-15$package
        final def erased-15$package$_$$anon$$$outer: erased-15$package = $outer
      }
      new Object with PolyFunction {...}():((x: Int) => (y: Int) => Int)
    }
} of class TypeDef # -1

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/lampepfl/dotty/issues/new/choose

     while compiling: tests/run-custom-args/erased/erased-15.scala
        during phase: MegaPhase{elimErasedValueType, pureStats, vcElideAllocations, etaReduce, arrayApply, elimPolyFunction, tailrec, completeJavaEnums, mixin, lazyVals, memoize, nonLocalReturns, capturedVars}
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.10
    compiler version: version 3.4.0-RC1-bin-SNAPSHOT-nonbootstrapped-git-c04d2db
            settings: -classpath /Users/nicolasstucki/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.10/scala-library-2.13.10.jar:/Users/nicolasstucki/GitHub/dotty/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.4.0-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.4.0-RC1-bin-SNAPSHOT.jar -d t/out

                tree: PackageDef(object <empty>)
       tree position: tests/run-custom-args/erased/erased-15.scala:1:0
           tree type: <empty>.type
              symbol: package <empty>
   symbol definition: final lazy module object <empty>: <empty> (a Symbol)
      symbol package: <root>
       symbol owners: 
           call site: package <empty> in module class <empty>

  == Source file context for tree position ==

-- Error: tests/run-custom-args/erased/erased-15.scala:1:0 ---------------------
1 |def test = polyFun(1)(2)
  |^

2 |def polyFun: PolyFunction { def apply(x: Int)(y: Int): Int } =
3 |  new PolyFunction:
4 |    def apply(x: Int)(y: Int): Int = x + y
        at dotty.tools.dotc.core.Denotations$SingleDenotation.goForward$1(Denotations.scala:830)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:876)
        at dotty.tools.dotc.core.Symbols$Symbol.recomputeDenot(Symbols.scala:122)
        at dotty.tools.dotc.core.Symbols$Symbol.computeDenot(Symbols.scala:116)
        at dotty.tools.dotc.core.Symbols$Symbol.denot(Symbols.scala:109)
        at dotty.tools.dotc.core.Symbols$.toDenot(Symbols.scala:496)
        at dotty.tools.dotc.core.SymDenotations$ClassDenotation.paramAccessors$$anonfun$1(SymDenotations.scala:2392)
        at dotty.tools.dotc.core.Scopes$Scope.filter(Scopes.scala:102)
        at dotty.tools.dotc.core.SymDenotations$ClassDenotation.paramAccessors(SymDenotations.scala:2392)
        at dotty.tools.dotc.core.Contexts$Context.superCallContext(Contexts.scala:408)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2626)
        at dotty.tools.dotc.transform.Erasure$Typer.typedClassDef(Erasure.scala:1043)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3087)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3091)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3166)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:174)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3243)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3247)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3269)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3315)
        at dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1054)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2861)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3133)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3167)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:174)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3243)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3247)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3359)
        at dotty.tools.dotc.transform.Erasure.run(Erasure.scala:144)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:327)
        at scala.collection.immutable.List.map(List.scala:246)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:331)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:246)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:262)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:270)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:279)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:279)
        at dotty.tools.dotc.Run.compileSources(Run.scala:194)
        at dotty.tools.dotc.Run.compile(Run.scala:179)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
        at dotty.tools.dotc.Driver.process(Driver.scala:197)
        at dotty.tools.dotc.Driver.process(Driver.scala:165)
        at dotty.tools.dotc.Driver.process(Driver.scala:177)
        at dotty.tools.dotc.Driver.main(Driver.scala:207)
        at dotty.tools.dotc.Main.main(Main.scala)
```
</details>

## Example 3

```scala
import scala.reflect.Selectable.reflectiveSelectable

def test = polyFun.foo(1)
def polyFun: PolyFunction { def foo(x: Int): Int } =
  new PolyFunction { def foo(x: Int): Int = x + 1 }
```

Not in the spec. Crashes the compiler with 

<details>
<summary>java.util.NoSuchElementException: head of empty list</summary>

```
Exception in thread "main" java.util.NoSuchElementException: head of empty list

  unhandled exception while running MegaPhase{elimErasedValueType, pureStats, vcElideAllocations, etaReduce, arrayApply, elimPolyFunction, tailrec, completeJavaEnums, mixin, lazyVals, memoize, nonLocalReturns, capturedVars} on Test.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/lampepfl/dotty/issues/new/choose

     while compiling: Test.scala
        during phase: MegaPhase{elimErasedValueType, pureStats, vcElideAllocations, etaReduce, arrayApply, elimPolyFunction, tailrec, completeJavaEnums, mixin, lazyVals, memoize, nonLocalReturns, capturedVars}
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.10
    compiler version: version 3.4.0-RC1-bin-SNAPSHOT-nonbootstrapped-git-c04d2db
            settings: -classpath /Users/nicolasstucki/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.10/scala-library-2.13.10.jar:/Users/nicolasstucki/GitHub/dotty/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.4.0-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.4.0-RC1-bin-SNAPSHOT.jar -d t/out

                tree: EmptyTree
       tree position: :<unknown>
           tree type: <notype>
              symbol: val <none>
           call site: package <root> in module class <root>

  == Source file context for tree position ==


        at scala.collection.immutable.Nil$.head(List.scala:662)
        at scala.collection.immutable.Nil$.head(List.scala:661)
        at dotty.tools.dotc.transform.ElimPolyFunction.functionTypeOfPoly(ElimPolyFunction.scala:50)
        at dotty.tools.dotc.transform.ElimPolyFunction.transform(ElimPolyFunction.scala:36)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.goForward$1(Denotations.scala:830)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:876)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.goForward$1(Denotations.scala:847)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:876)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.goForward$1(Denotations.scala:847)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:876)
        at dotty.tools.dotc.core.Types$NamedType.computeDenot(Types.scala:2425)
        at dotty.tools.dotc.core.Types$NamedType.denot(Types.scala:2388)
        at dotty.tools.dotc.ast.Trees$DenotingTree.denot(Trees.scala:256)
        at dotty.tools.dotc.ast.Trees$Tree.symbol(Trees.scala:145)
        at dotty.tools.dotc.ast.tpd$.localOwner(tpd.scala:609)
        at dotty.tools.dotc.ast.tpd$.localCtx(tpd.scala:613)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1745)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1791)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.traverse(CapturedVars.scala:74)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.fold$1(Trees.scala:1654)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1656)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1687)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1791)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.traverse(CapturedVars.scala:74)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1742)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1791)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.traverse(CapturedVars.scala:74)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.fold$1(Trees.scala:1654)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1656)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1749)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1791)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.traverse(CapturedVars.scala:74)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1746)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1791)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.traverse(CapturedVars.scala:74)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1790)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.fold$1(Trees.scala:1654)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.apply(Trees.scala:1656)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1755)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1791)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.traverse(CapturedVars.scala:74)
        at dotty.tools.dotc.transform.CapturedVars$CollectCaptured.runOver(CapturedVars.scala:77)
        at dotty.tools.dotc.transform.CapturedVars.prepareForUnit(CapturedVars.scala:84)
        at dotty.tools.dotc.transform.MegaPhase.prepUnit(MegaPhase.scala:1095)
        at dotty.tools.dotc.transform.MegaPhase.prepUnit(MegaPhase.scala:1095)
        at dotty.tools.dotc.transform.MegaPhase.prepUnit(MegaPhase.scala:1095)
        at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:468)
        at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:481)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:327)
        at scala.collection.immutable.List.map(List.scala:246)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:331)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:246)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:262)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:270)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:279)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:279)
        at dotty.tools.dotc.Run.compileSources(Run.scala:194)
        at dotty.tools.dotc.Run.compile(Run.scala:179)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
        at dotty.tools.dotc.Driver.process(Driver.scala:197)
        at dotty.tools.dotc.Driver.process(Driver.scala:165)
        at dotty.tools.dotc.Driver.process(Driver.scala:177)
        at dotty.tools.dotc.Driver.main(Driver.scala:207)
        at dotty.tools.dotc.Main.main(Main.scala)
```
</details>



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Invalid `PolyFunction` refinements #18302

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Invalid PolyFunction refinements #18302

Description

Compiler version

Issue

Example 1

Example 2

Example 3

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Invalid `PolyFunction` refinements #18302