Fix #9735: Tighten opaque types check

We already has a check that opaque type aliases must be fully applied, but
that check did not kick in when the opaque type has type parameters.

Author: odersky

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

@@ -888,7 +888,7 @@ class Namer { typer: Typer =>
       val unsafeInfo = if (isDerived) rhsBodyType else abstracted(rhsBodyType)
 
       def opaqueToBounds(info: Type): Type =
-        if sym.isOpaqueAlias && tparamSyms.isEmpty && info.typeParams.nonEmpty then
+        if sym.isOpaqueAlias && info.typeParams.nonEmpty then
           report.error(em"opaque type alias must be fully applied", rhs.srcPos)
         sym.opaqueToBounds(info, rhs1, tparamSyms)
 

tests/neg/i9735.scala

@@ -0,0 +1,6 @@
+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
+