Allow inferred parameter types always, when eta-expanding (#18771)

Author: dwijnand

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

@@ -4178,11 +4178,7 @@ 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))

tests/semanticdb/metac.expect

@@ -1082,7 +1082,7 @@ Language => Scala
 Symbols => 181 entries
 Occurrences => 159 entries
 Diagnostics => 1 entries
-Synthetics => 6 entries
+Synthetics => 5 entries
 
 Symbols:
 _empty_/Enums. => final object Enums extends Object { self: Enums.type => +30 decls }
@@ -1265,7 +1265,7 @@ _empty_/Enums.unwrap().(ev) => implicit given param ev: <:<[A, Option[B]]
 _empty_/Enums.unwrap().(opt) => param opt: Option[A]
 _empty_/Enums.unwrap().[A] => typeparam A 
 _empty_/Enums.unwrap().[B] => typeparam B 
-local0 => param x: Option[B]
+local0 => param x: A
 
 Occurrences:
 [0:7..0:12): Enums <- _empty_/Enums.
@@ -1433,7 +1433,6 @@ Diagnostics:
 
 Synthetics:
 [52:9..52:13):Refl => *.unapply[Option[B]]
-[52:31..52:50):identity[Option[B]] => *[Function1[A, Option[B]]]
 [54:14..54:18):Some => *.apply[Some[Int]]
 [54:14..54:34):Some(Some(1)).unwrap => *(given_<:<_T_T[Option[Int]])
 [54:19..54:23):Some => *.apply[Int]