Fix underlyingClassRef for hk types

This gives us the right error message if we try to instantiate
an hk opaque type with `new`.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1166,7 +1166,8 @@ object Types {
         else if (tp.symbol.isAliasType) tp.underlying.underlyingClassRef(refinementOK)
         else NoType
       case tp: AppliedType =>
-        tp.superType.underlyingClassRef(refinementOK)
+        if (tp.tycon.isLambdaSub) NoType
+        else tp.superType.underlyingClassRef(refinementOK)
       case tp: AnnotatedType =>
         tp.underlying.underlyingClassRef(refinementOK)
       case tp: RefinedType =>

tests/neg/alloc-abstract.scala

@@ -0,0 +1,15 @@
+class Test[T] {
+  type U <: T
+
+  type Foo[T] = Array[T]
+
+  new T                   // error: not a class type
+  new T()                 // error: not a class type
+  new U                   // error: not a class type
+  new U()                 // error: not a class type
+  new IArray[String]      // error: not a class type
+  new IArray[String]()    // error: not a class type
+  new IArray[String](10)  // error: not a class type // error: too mamy arguments
+
+  new Foo[String](10)     // ok
+}

tests/neg/parser-stability-21.scala

@@ -1,3 +1,3 @@
 class x0[x1[]]    // error
-    extends x1[   // error
+    extends x1[
 // error