Refine isFunction criterion

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -329,6 +329,11 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
   def isFunctionWithUnknownParamType(tree: Tree): Boolean =
     functionWithUnknownParamType(tree).isDefined
 
+  def isFunction(tree: Tree): Boolean = tree match
+    case Function(_, _) | Match(EmptyTree, _) => true
+    case Block(Nil, expr) => isFunction(expr)
+    case _ => false
+
   /** Is `tree` an context function or closure, possibly nested in a block? */
   def isContextualClosure(tree: Tree)(using Context): Boolean = unsplice(tree) match {
     case tree: FunctionWithMods => tree.mods.is(Given)

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

@@ -3831,8 +3831,7 @@ class Typer extends Namer
       && !isSelfOrSuperConstrCall(tree)
     then tree match
       case closureDef(meth)
-      if meth.span == meth.rhs.span.toSynthetic
-          && !original.isInstanceOf[untpd.Function] =>
+      if meth.span == meth.rhs.span.toSynthetic && !untpd.isFunction(original) =>
         // It's a synthesized lambda, for instance via an eta expansion: report a hard error
         // There are two tests for synthetic lambdas which both have to be true.
         // The first test compares spans of closure definition with the closure's right hand