Allow inferred parameter types always, when eta-expanding

Rather than drawing an exemption based on fully defined types, report
the right function arity.  This leads eta-expansion to leave off the
type, so then the parameter type is inferred.  In the test case
pos/i18453 this leads to the type var ?A being instantiated to X, rather
than ?A & ?B being constrained against X & Y, which leads to ?A being
instantiated to X & Y - and then failing to find a Box[X & Y] in scope.

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -4167,11 +4167,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val funExpected = functionExpected
           val arity =
             if funExpected then
-              if !isFullyDefined(pt, ForceDegree.none) && isFullyDefined(wtp, ForceDegree.none) then
-                // if method type is fully defined, but expected type is not,
-                // prioritize method parameter types as parameter types of the eta-expanded closure
-                0
-              else defn.functionArity(ptNorm)
+              defn.functionArity(ptNorm)
             else
               val nparams = wtp.paramInfos.length
               if nparams > 1

tests/neg/i5976.scala

@@ -1,7 +1,7 @@
 object Test {
   def f(i: => Int) = i + i
-  val res = List(42).map(f) // error
+  val res = List(42).map(f)
 
   val g: (=> Int) => Int = f
   val h: Int => Int = g // error
-}
+}

tests/pos/i18453.scala

@@ -0,0 +1,8 @@
+trait Box[T]
+
+class Test:
+  def f[A, B](c: A => A & B)(using ba: Box[A]): Unit = ???
+
+  def g[X, Y](using bx: Box[X]): Unit =
+    def d(t: X): X & Y = t.asInstanceOf[X & Y]
+    f(d)

tests/pos/i18453.workaround.scala

@@ -0,0 +1,8 @@
+trait Box[T]
+
+class Test:
+  def f[A, B](c: A => A & B)(using ba: Box[A]): Unit = ???
+
+  def g[X, Y](using bx: Box[X]): Unit =
+    def d(t: X): X & Y = t.asInstanceOf[X & Y]
+    f(u => d(u))