Fix type aliases in beta-reduction of polyfunctions

Fixes #17052

Author: nicolasstucki

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

@@ -117,7 +117,7 @@ object BetaReduce:
           case ref @ TypeRef(NoPrefix, _) =>
             ref.symbol
           case _ =>
-            val binding = TypeDef(newSymbol(ctx.owner, tparam.name, EmptyFlags, targ.tpe, coord = targ.span)).withSpan(targ.span)
+            val binding = TypeDef(newSymbol(ctx.owner, tparam.name, EmptyFlags, TypeAlias(targ.tpe), coord = targ.span)).withSpan(targ.span)
             bindings += binding
             binding.symbol
 

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

@@ -533,6 +533,11 @@ object TreeChecker {
         i"owner chain = ${tree.symbol.ownersIterator.toList}%, %, ctxOwners = ${ctx.outersIterator.map(_.owner).toList}%, %")
     }
 
+    override def typedTypeDef(tdef: untpd.TypeDef, sym: Symbol)(using Context): Tree = {
+      assert(sym.info.isInstanceOf[ClassInfo | TypeBounds], i"wrong type, expect a template or type bounds for ${sym.fullName}, but found: ${sym.info}")
+      super.typedTypeDef(tdef, sym)
+    }
+
     override def typedClassDef(cdef: untpd.TypeDef, cls: ClassSymbol)(using Context): Tree = {
       val TypeDef(_, impl @ Template(constr, _, _, _)) = cdef: @unchecked
       assert(cdef.symbol == cls)

compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala

@@ -1345,18 +1345,22 @@ object SourceCode {
     }
 
     private def printBoundsTree(bounds: TypeBoundsTree)(using elideThis: Option[Symbol]): this.type = {
-      bounds.low match {
-        case Inferred() =>
-        case low =>
-          this += " >: "
-          printTypeTree(low)
-      }
-      bounds.hi match {
-        case Inferred() => this
-        case hi =>
-          this += " <: "
-          printTypeTree(hi)
-      }
+      if bounds.low.tpe == bounds.hi.tpe then
+        this += " = "
+          printTypeTree(bounds.low)
+      else
+        bounds.low match {
+          case Inferred() =>
+          case low =>
+            this += " >: "
+            printTypeTree(low)
+        }
+        bounds.hi match {
+          case Inferred() => this
+          case hi =>
+            this += " <: "
+            printTypeTree(hi)
+        }
     }
 
     private def printBounds(bounds: TypeBounds)(using elideThis: Option[Symbol]): this.type = {

tests/pos/i17052.scala

@@ -0,0 +1,2 @@
+def test[F[_]](fAny: F[Any]) =
+  { [X] => (fx: F[X]) => { val fx2: F[X] = fx; () } }.apply[Any](fAny)