Allow [T] => { (x: T) => x } syntax for polymorphic functions

Fixes #12323

Author: jrudolph

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

@@ -1611,6 +1611,8 @@ object desugar {
     def makePolyFunction(targs: List[Tree], body: Tree): Tree = body match {
       case Parens(body1) =>
         makePolyFunction(targs, body1)
+      case Block(Nil, body1) =>
+        makePolyFunction(targs, body1)
       case Function(vargs, res) =>
         assert(targs.nonEmpty)
         // TODO: Figure out if we need a `PolyFunctionWithMods` instead.

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1312,6 +1312,7 @@ object Parsers {
 
     private def isFunction(tree: Tree): Boolean = tree match {
       case Parens(tree1) => isFunction(tree1)
+      case Block(Nil, tree1) => isFunction(tree1)
       case _: Function => true
       case _ => false
     }

tests/run/polymorphic-functions.scala

@@ -95,4 +95,6 @@ object Test extends App {
   // Parens handling
   val tt1: [T] => (T => T) = [T] =>  (x: T) => x
   val tt2: [T] =>  T => T =  [T] => ((x: T) => x)
+  val tt3: [T] =>  T => T =  [T] => { (x: T) => x }
+  val tt4: [T] =>  T => T =  [T] => (x: T) => { x }
 }