Avoid eta-reduction on f => f(f)

Fixes #19962

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/EtaReduce.scala

@@ -40,7 +40,7 @@ class EtaReduce extends MiniPhase:
       case Apply(Select(fn, name), args)
       if (name == nme.apply || defn.FunctionSpecializedApplyNames.contains(name))
           && mdef.paramss.head.corresponds(args)((param, arg) =>
-              arg.isInstanceOf[Ident] && arg.symbol == param.symbol)
+              arg.isInstanceOf[Ident] && arg.symbol == param.symbol && arg.symbol != fn.symbol)
           && isPurePath(fn)
           && fn.tpe <:< tree.tpe
           && defn.isFunctionClass(fn.tpe.widen.typeSymbol) =>

tests/pos/i19962.scala

@@ -0,0 +1,2 @@
+def selfie0: (AnyRef => AnyRef) => AnyRef = (f:AnyRef => AnyRef) => f(f)
+def selfie1: Any = (f: Any => Any) => f(f)