Fix opaque types check

Author: odersky

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

@@ -888,8 +888,8 @@ class Namer { typer: Typer =>
       val unsafeInfo = if (isDerived) rhsBodyType else abstracted(rhsBodyType)
 
       def opaqueToBounds(info: Type): Type =
-        if sym.isOpaqueAlias && info.typeParams.nonEmpty then
-          report.error(em"opaque type alias must be fully applied", rhs.srcPos)
+        if sym.isOpaqueAlias && info.typeParams.nonEmpty && info.hkResult.typeParams.nonEmpty then
+          report.error(em"opaque type alias cannot have multiple type parameter lists", rhs.srcPos)
         sym.opaqueToBounds(info, rhs1, tparamSyms)
 
       if (isDerived) sym.info = unsafeInfo

tests/neg/i9735.scala

@@ -1,6 +1,7 @@
 trait Two[A, B]
 
-opaque type U[A] = [B] =>> Two[A, B] // error: opaque type alias must be fully applied // error: cannot instantiate
-opaque type T[A] = [B] =>> String // error: opaque type alias must be fully applied
-opaque type S = [B] =>> String // error: opaque type alias must be fully applied
-
+opaque type U[A] = [B] =>> Two[A, B] // error: opaque type alias cannot have multiple type parameter lists // error: cannot instantiate
+opaque type T = [A] =>> [B] =>> String // error: opaque type alias cannot have multiple type parameter lists
+opaque type S = [B] =>> String // ok
+opaque type IArray[+T] = Array[? <: T] // ok
+opaque type S2[B] = String // ok