Adapt Namer.checkFlags to type defdefs

Converted flags to termflags before.

Author: odersky

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

@@ -284,10 +284,14 @@ class Namer { typer: Typer =>
     def checkFlags(flags: FlagSet) =
       if (flags.isEmpty) flags
       else {
-        val (ok, adapted, kind) = tree match {
-          case tree: TypeDef => (flags.isTypeFlags, flags.toTypeFlags, "type")
-          case _ => (flags.isTermFlags, flags.toTermFlags, "value")
+        val isType = tree match {
+          case tree: TypeDef => true
+          case tree: DefDef => tree.name.isTypeName
+          case _ => false
         }
+        val (ok, adapted, kind) =
+          if (isType) (flags.isTypeFlags, flags.toTypeFlags, "type")
+          else (flags.isTermFlags, flags.toTermFlags, "value")
         if (!ok)
           ctx.error(i"modifier(s) `$flags' incompatible with $kind definition", tree.pos)
         adapted

tests/neg/typemethods.scala

@@ -0,0 +1,20 @@
+trait Nat {
+  def toInt: Int
+}
+
+case object Z extends Nat {
+  transparent def toInt = 0
+}
+
+case class S[N <: Nat](n: N) extends Nat {
+  transparent def toInt = n.toInt + 1
+}
+
+object Test extends App {
+
+  type ToNat(n: Int) <: Nat =
+    if n == 0 then Z.type
+    else S[ToNat(n - 1)]
+
+  type ToNat = Int  // error: double definition
+}

tests/neg/typemethods2.scala

@@ -0,0 +1,28 @@
+trait Nat {
+  def toInt: Int
+}
+
+case object Z extends Nat {
+  transparent def toInt = 0
+}
+
+case class S[N <: Nat](n: N) extends Nat {
+  transparent def toInt = n.toInt + 1
+}
+
+class C {
+  type ToNat = Int
+
+  type ToNat2(n: Int) <: Nat =
+    if n == 0 then Z.type
+    else S[ToNat2(n - 1)]}
+
+object Test extends C {
+
+  override type ToNat(n: Int) <: Nat = // error: illegal override
+    if n == 0 then Z.type
+    else S[ToNat(n - 1)]
+
+  override type ToNat2[X] = X    // error: illegal override
+
+}